health calculator

Gestational Age Calculator

Calculate gestational age in weeks and days from your last period, due date, or IVF transfer, with due date and trimester.

Gestational age is how far a pregnancy has progressed, counted from the first day of the last menstrual period rather than from conception, and reported as completed weeks plus days such as 22w4d. The estimated due date is the LMP plus 280 days. With an LMP of 1 January 2025, the due date is 8 October 2025, and on 15 August 2025 the gestational age is 226 days — 32w2d.

Formula Verified by Dr. Sarah Jenkins, PhD

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Mode

First day of last menstrual periodThe first day of bleeding, not the last.As-of dateDefaults to today.

Usual cycle lengthdays

Naegele's rule assumes a 28-day cycle. A 35-day cycle moves the estimated due date 7 days later, because ovulation happens later in the cycle.

Results

Gestational age

32w2d

In total days226 days

Estimated due date8 October 2025

Estimated conception15 January 2025

TrimesterTrimester 3

Days to due date54 days

Percent complete80.7%

Fetal age (from conception)30w2d

If born todayPreterm

Next milestoneGroup B strep swab window (35w0d) — in 19 days

Bookmark this gestational age calculator

Gestational age changes every single day. Save the page and reopen it whenever you need the current week and days, the next milestone, or the countdown to your due date.

Formula

Gestational age formulas

Gestational age = As-of date − LMP, reported as completed weeks + days. EDD = LMP + 280 + (cycle length − 28). LMP = EDD − 280. EDD = conception + 266, day-3 transfer + 263, day-5 blastocyst + 261. Fetal age = gestational age − 14 days.

Gestational age counts from the first day of the last menstrual period, not from conception, which is why you are already 2 weeks pregnant on the day you conceive. Report completed weeks plus leftover days (22w4d), never a rounded decimal.

Step by step

How the calculation works

1From LMP: enter the first day of your last period and your usual cycle length. Cycles longer than 28 days push the due date later by the difference.
2From Due Date: if a dating scan has already given you a due date, use this mode — the scan is a better input than a recalled period date.
3From Conception or IVF: for known conception dates, day-3 transfers, day-5 blastocyst transfers and egg retrievals, each of which uses a different day count.
4The as-of date defaults to today, but you can set it to any date to see where a pregnancy was, or will be, at that point.

Example

Worked example — due date 15 December 2025, as of 15 August 2025

LMP = 15 December minus 280 days = 10 March 2025. From 10 March to 15 August is 158 days, and 158 ÷ 7 = 22 remainder 4 — so 22w4d, second trimester, 122 days to go, 56.4% complete. Conception was around 24 March and fetal age is 20w4d.

Related guides

Learn more about this calculation

Guide

Pregnancy Due Date: How Due Dates Are Calculated and What to Expect

Learn how due dates are estimated from LMP, cycle length, conception, and ultrasound, plus what full term and overdue really mean.

Guide

Pregnancy Conception: Understanding Fertile Windows and Timing

Learn how conception timing works, why luteal phase assumptions matter, and how to estimate fertile days without overtrusting one date.

Due Date

Due Date Calculator

A simpler tool when you only need the estimated due date and not the full week-by-week breakdown.

Timeline

Pregnancy Calculator

Track the pregnancy timeline week by week, with trimester milestones and development stages.

Conception

Conception Calculator

Work backwards from a due date or period to the most likely conception window.

Fertility

Ovulation Calculator

Estimate ovulation day and the fertile window from your cycle length, before conception.

Hydration

Water Intake Calculator

Daily fluid targets including the pregnancy and breastfeeding allowances.

Dates

Age Calculator

General-purpose date arithmetic for exact ages in years, months and days.

health calculator

Gestational Age Calculator

Calculate gestational age in weeks and days from your last period, due date, or IVF transfer, with due date and trimester.

Formula Verified by Dr. Sarah Jenkins, PhD

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Mode

First day of last menstrual periodThe first day of bleeding, not the last.As-of dateDefaults to today.

Usual cycle lengthdays

Naegele's rule assumes a 28-day cycle. A 35-day cycle moves the estimated due date 7 days later, because ovulation happens later in the cycle.

Results

Gestational age

32w2d

In total days226 days

Estimated due date8 October 2025

Estimated conception15 January 2025

TrimesterTrimester 3

Days to due date54 days

Percent complete80.7%

Fetal age (from conception)30w2d

If born todayPreterm

Next milestoneGroup B strep swab window (35w0d) — in 19 days

Bookmark this gestational age calculator

Gestational age changes every single day. Save the page and reopen it whenever you need the current week and days, the next milestone, or the countdown to your due date.

Summary

Key output metrics

Gestational age

32w2d

226 days since your LMP. Fetal age is 30w2d.

Estimated conception

15 January 2025

LMP + 14 days, shifted by your 28-day cycle.

Estimated due date

8 October 2025

54 days from your as-of date.

Trimester

Trimester 3

80.7% complete. If birth happened today: preterm.

D3 visualization

How far along

80.7% of a 280-day pregnancy has elapsed at 32w2d.

Days elapsed226 days

Days remaining54 days

D3 visualization

Where you sit across the three trimesters

ACOG boundaries: the first trimester runs to 13w6d, the second from 14w0d to 27w6d, and the third from 28w0d onwards.

D3 visualization

Days until your next milestones

The upcoming appointments and thresholds measured against your current gestational age.

D3 visualization

What cycle length does to your due date

Naegele’s rule assumes a 28-day cycle. Longer cycles ovulate later, so the pregnancy length used for dating stretches to match.

Guide

How to understand and use this calculator

Quick Answer — What Is Gestational Age?

Gestational age (GA) is how far a pregnancy has progressed, counted from the first day of the last menstrual period (LMP) — not from conception. It is written as weeks *and* days, like 22w4d, because a single week of difference matters clinically.

The two formulas that do almost all the work:

Gestational age = Today − LMP, expressed as weeks + days
Estimated due date (EDD) = LMP + 280 days (40 weeks)

Working backwards from a known due date: LMP = EDD − 280 days, then apply the first formula.

Worked example. LMP was 1 January 2025 and today is 15 August 2025. That is 226 days, which is 32 weeks and 2 days — you would say 'thirty-two and two'. The due date is 1 January + 280 = 8 October 2025, which is 54 days away.

The two-week offset that confuses everyone: because the count starts at the LMP but conception happens roughly two weeks later, you are already considered '2 weeks pregnant' on the day you conceive. This is not an error; it is the clinical convention, because the LMP is a date women can usually recall and conception is not.

Key reference points: a full-term pregnancy is 40 weeks from LMP. Fewer than 5% of babies arrive on the exact due date. Fetal (embryonic) age = gestational age − 2 weeks.

How to Use This Calculator — Three Modes

The mode selector above the inputs switches between three genuinely different dating problems.

Mode 1 — From LMP. The standard route. Enter the first day of your last period, your usual cycle length, and the date you want the answer for. You get gestational age, EDD, trimester, conception date, fetal age and the milestone calendar.

Mode 2 — From Due Date. The reverse calculation, and the one most people actually need. If a scan has already given you a due date, this works backwards to your dating LMP and forwards again to today's gestational age. This is what people mean by a 'pregnancy week calculator by due date'.

Mode 3 — From Conception or IVF. For known conception dates, IVF embryo transfers and egg retrievals, where the fertilisation date is precisely known and the LMP is irrelevant or absent. Choose day-3 transfer, day-5 blastocyst transfer, egg retrieval, or a natural known conception date.

Example input (Mode 2): Estimated due date = 15 December 2025 • As-of date = 15 August 2025.

Output: LMP = 10 March 2025 • Gestational age = 22w4d (158 days) • Second trimester • 122 days to the due date • 56.4% complete • conception around 24 March 2025 • fetal age 20w4d.

Example input (Mode 3, IVF): Day-5 blastocyst transfer on 20 March 2025 • As-of date = 15 August 2025.

Output: EDD = 6 December 2025 • LMP-equivalent = 1 March 2025 • Gestational age = 23w6d. Note this is a *different* answer from a day-3 transfer on the same date, which gives 23w4d — the two-day difference is exactly the two extra days the embryo spent developing in the lab.

If you only want the due date and not the full week-by-week picture, the due date calculator is the simpler tool. For fertile-window planning before conception, use the ovulation calculator.

Gestational Age vs Fetal Age vs Conception Age

Three terms, two-week gaps between them, and endless confusion. Here is the whole thing.

Gestational age is measured from the first day of the LMP. It is what every clinician, scan report, and pregnancy app means by 'how many weeks'. Full term is 40 weeks.

Fetal age, also called embryonic age or conceptional age, is measured from fertilisation. It is gestational age minus 14 days. A pregnancy at 22w4d gestational is at 20w4d fetal age. Full term is 38 weeks on this scale.

Conception age is another name for fetal age. Some non-clinical sources and older textbooks use it, which is exactly why people end up two weeks apart when comparing notes.

Why the clinical world picked the more confusing one: the LMP is a date a woman can usually name. The moment of fertilisation is almost never known — even with a positive ovulation test, sperm can survive several days in the reproductive tract. Dating from a knowable event beats dating from an unknowable one, even at the cost of a two-week fiction at the start.

The practical consequence: at the moment of conception you are '2 weeks pregnant', and in weeks 1 and 2 of a pregnancy there is no pregnancy at all. This is normal and universal.

A worked illustration. LMP 1 January 2025, 28-day cycle. Conception around 15 January. On 15 August 2025 the gestational age is 32w2d, and the fetal age is 30w2d.

The Formulas — Naegele's Rule and Its Variants

Naegele's rule is the classical method, published in the early 1800s and still the basis of every due-date calculator: take the first day of the LMP, subtract 3 months, add 7 days, add 1 year.

The arithmetic version: EDD = LMP + 280 days. These two agree exactly most of the time, but they can differ by up to 3 days depending on which months are involved — LMP 10 May 2025 gives 14 February 2026 by the +280-day method and 17 February 2026 by the calendar shortcut. This calculator uses the +280-day method throughout, because day-counting is unambiguous and month arithmetic is not.

The cycle-length adjustment (Parikh's rule). Naegele's rule silently assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, ovulation is later, and the due date shifts:

EDD = LMP + 280 + (cycle length − 28)

Why this matters more than people think: a 35-day cycle pushes the due date 7 days later. LMP 15 January 2025 gives an EDD of 22 October 2025 on a 28-day cycle but 29 October 2025 on a 35-day cycle. A week is the difference between being scheduled for a post-dates induction and not.

Conception-based dating: EDD = Conception date + 266 days, because 280 − 14 = 266.

IVF dating, where fertilisation timing is known to the hour:

Egg retrieval date + 266 days
Day-3 embryo transfer + 263 days
Day-5 blastocyst transfer + 261 days

Trimester boundaries as defined by ACOG: the first trimester runs to 13w6d, the second from 14w0d to 27w6d, and the third from 28w0d to birth. Many popular sources use a simplified 1–12 / 13–27 / 28–40 split, which puts the first-trimester boundary about a week early.

Percentage complete = Gestational age in days ÷ 280 × 100.

Step-by-Step Worked Derivation

Take the 'from due date' example and do the whole thing by hand.

Known: EDD = 15 December 2025. Today = 15 August 2025.

Step 1 — Recover the dating LMP. LMP = EDD − 280 days. Counting back 280 days from 15 December 2025 gives 10 March 2025.

Step 2 — Count days elapsed. From 10 March to 15 August 2025 is 158 days.

Step 3 — Convert to weeks and days. 158 ÷ 7 = 22 remainder 4, so the gestational age is 22w4d. Note that you report the *completed* weeks plus leftover days — you do not round 22.57 up to 23. Getting this wrong is the single most common error in DIY pregnancy maths.

Step 4 — Trimester. 158 days is past 14w0d (98 days) and before 28w0d (196 days), so this is the second trimester.

Step 5 — Days remaining. 280 − 158 = 122 days to the due date, about 17 and a half weeks.

Step 6 — Percentage complete. 158 ÷ 280 = 56.4%.

Step 7 — Conception date. LMP + 14 days = 10 March + 14 = 24 March 2025.

Step 8 — Fetal age. 158 − 14 = 144 days = 20w4d.

Check your work in reverse: LMP 10 March + 280 = 15 December. It closes. If the reverse calculation does not return your starting date, you have made an off-by-one error somewhere — see the mistakes section below, which is largely a list of ways this happens.

Why Ultrasound Beats the Calendar — and When Doctors Redate

This calculator does arithmetic on the dates you give it. A first-trimester ultrasound does something better: it measures the embryo directly. If the two disagree beyond a defined threshold, the scan wins and the pregnancy is formally 'redated'.

The measurement used in the first trimester is crown-rump length (CRL), the straight-line distance from the top of the head to the bottom of the buttocks. Before about 14 weeks, embryos grow at a remarkably uniform rate regardless of genetics, so CRL predicts gestational age to within a few days. After that, growth becomes increasingly influenced by individual factors and the estimate loses precision — which is why a late scan cannot reliably redate a pregnancy.

The ACOG / AIUM / SMFM redating criteria (Committee Opinion 700) set out exactly how far off the scan must be before the dates are changed:

At 8w6d or earlier: redate if the ultrasound differs by more than 5 days
9w0d to 15w6d: redate if it differs by more than 7 days
16w0d to 21w6d: redate if it differs by more than 10 days
22w0d to 27w6d: redate if it differs by more than 14 days
28w0d and beyond: redate if it differs by more than 21 days

The window widens deliberately as pregnancy advances, because ultrasound gets *less* accurate over time while the LMP-based date does not change. By the third trimester a scan has to be nearly three weeks off before it overrules the original dating.

One due date, kept. Once dating is established — ideally at the first-trimester scan — that EDD is not revised again by later scans. A baby measuring small at 32 weeks is a growth question, not a dating question.

What this means for you: if you have had a dating scan, use its due date in Mode 2 rather than recalculating from your LMP. The scan is the better input.

Trimesters, Milestones and the Term Classifications

First trimester — LMP to 13w6d. Positive pregnancy test typically around 4–5 weeks. Cardiac activity visible on ultrasound from about 6 weeks. Dating scan usually between 8 and 13 weeks, and this is the window where gestational age can still be corrected.

Second trimester — 14w0d to 27w6d. The anatomy scan (also called the anomaly scan or 20-week scan) lands between 18 and 22 weeks. Fetal movement usually becomes noticeable between 16 and 22 weeks. The threshold of viability is generally placed around 22–24 weeks and varies by institution.

Third trimester — 28w0d onwards. Glucose tolerance screening around 24–28 weeks, Group B strep swab at 35–37 weeks, and increasingly frequent monitoring towards the end.

Term classifications matter more than most people realise, because 'term' is not one category. ACOG splits it four ways:

Preterm — before 37w0d
Early term — 37w0d to 38w6d
Full term — 39w0d to 40w6d
Late term — 41w0d to 41w6d
Post-term — 42w0d and beyond

Why the split exists: babies born at 37–38 weeks have measurably higher rates of respiratory difficulty and NICU admission than those born at 39–40 weeks. The categories were introduced specifically to discourage non-medically-indicated deliveries before 39 weeks, which had been drifting earlier for scheduling convenience.

A note on the due date itself: only about 4–5% of babies are born on it. Roughly half arrive within a week either side. Treating the EDD as a deadline rather than the midpoint of a distribution is a reliable source of unnecessary anxiety.

Calculating Gestational Age in Excel or Google Sheets

Date arithmetic is one of the things spreadsheets are genuinely good at, because dates are stored as serial numbers and subtract cleanly. Put the LMP in B2 and the as-of date in B3.

Estimated due date: =B2+280

Cycle-adjusted due date, with cycle length in B4: =B2+280+(B4-28)

Gestational age in total days: =B3-B2 — format the result cell as Number, not Date, or Excel will helpfully show you 1900-06-06.

Completed weeks: =INT((B3-B2)/7)

Leftover days: =MOD(B3-B2,7)

The whole thing as a label: =INT((B3-B2)/7)&"w"&MOD(B3-B2,7)&"d"

Days remaining to the due date: =B2+280-B3

Percentage complete: =(B3-B2)/280 formatted as a percentage

Trimester, using the ACOG boundaries: =IFS((B3-B2)<98,1,(B3-B2)<196,2,TRUE,3)

Term classification: =IFS((B3-B2)<259,"Preterm",(B3-B2)<273,"Early term",(B3-B2)<287,"Full term",(B3-B2)<294,"Late term",TRUE,"Post-term")

Conception date estimate: =B2+14+(B4-28)

Fetal age in weeks and days: =INT((B3-B2-14)/7)&"w"&MOD(B3-B2-14,7)&"d"

Live 'weeks pregnant today' cell: =INT((TODAY()-$B$2)/7)&"w"&MOD(TODAY()-$B$2,7)&"d" — this recalculates every time the file opens.

The alternative EDD formula using Naegele's calendar shortcut is =EDATE(B2,9)+7. It usually matches =B2+280 exactly, but as noted above it can differ by a few days across certain month boundaries. If you need the two to agree, use the day-count version.

Two traps worth naming. First, Google Sheets and Excel both silently reformat a subtraction of two dates as a date; always set the format explicitly. Second, if you import dates from a CSV, check whether they parsed as DD/MM or MM/DD before trusting a single result — a due date that is off by exactly the day-month swap is the classic symptom.

Realistic Scenarios

Scenario 1 — Standard 28-day cycle. LMP 1 January 2025, as of 15 August 2025. GA 32w2d, EDD 8 October 2025, third trimester, 54 days remaining, 80.7% complete, conception around 15 January.

Scenario 2 — Long cycle. LMP 15 January 2025, as of 15 August 2025, but a 35-day cycle. EDD moves from 22 October to 29 October 2025, and conception shifts from 29 January to 5 February. The gestational age reads 30w2d either way, because gestational age counts from the LMP regardless of cycle length — but the *percentage complete* drops from 75.7% to 73.9%, and the due date is a week later.

Scenario 3 — Known due date only. EDD 1 January 2026, as of 15 August 2025. Dating LMP 27 March 2025, GA 20w1d, second trimester, 139 days to go, 50.4% complete. This is the halfway point.

Scenario 4 — Nearly there. EDD 1 September 2025, as of 15 August 2025. GA 37w4d, 17 days remaining, 93.9% complete, and the birth classification if delivery happened today would be early term — full term does not begin until 39w0d.

Scenario 5 — IVF day-5 blastocyst. Transfer on 20 March 2025, as of 15 August 2025. EDD 6 December 2025, LMP-equivalent 1 March 2025, GA 23w6d. IVF pregnancies are the one case where the dating is genuinely exact rather than estimated, because the fertilisation moment is known.

Scenario 6 — Egg retrieval date. Retrieval on 15 March 2025, as of 15 August 2025. EDD 6 December 2025, GA 23w6d — identical to a day-5 transfer five days later, which is exactly the consistency check you would want.

Scenario 7 — Natural known conception. Conception 23 March 2025, as of 15 August 2025. EDD 14 December 2025, LMP-equivalent 9 March 2025, GA 22w5d.

Practice Problems

Problem 1. LMP is 15 January 2025 with a 28-day cycle. What is the estimated due date?

Problem 2. The due date is 1 November 2025. What was the dating LMP?

Problem 3. LMP is 1 February 2025 and today is 1 September 2025. How many weeks and days pregnant?

Problem 4. Conception was 1 April 2025. What is the due date, and what is the gestational age on 15 August 2025?

Problem 5. The due date is 31 December 2025 and today is 31 August 2025. How far along, and which trimester?

Problem 6. A day-3 embryo transfer took place on 20 March 2025. What is the due date, and how does it compare with a day-5 transfer on the same date?

Solutions:

1. 15 January + 280 days = 22 October 2025.

2. 1 November 2025 − 280 days = 25 January 2025.

3. 1 February to 1 September 2025 is 212 days. 212 ÷ 7 = 30 remainder 2, so 30w2d — third trimester, since 212 is past the 196-day boundary.

4. EDD = 1 April + 266 = 23 December 2025. The LMP-equivalent is 1 April − 14 = 18 March 2025, so on 15 August the gestational age is 150 days = 21w3d.

5. LMP = 31 December − 280 = 26 March 2025. From 26 March to 31 August is 158 days = 22w4d, which is the second trimester (still 38 days short of the 28w0d boundary).

6. Day-3 transfer: 20 March + 263 = 8 December 2025. Day-5 transfer: 20 March + 261 = 6 December 2025. The day-5 due date is 2 days earlier, because the embryo was already 2 days further along at transfer.

Common Gestational Age Mistakes

Counting from conception instead of the LMP. The single most common error, and it puts you exactly 2 weeks behind. If your number disagrees with your midwife's by 14 days, this is why.
Rounding weeks instead of reporting weeks + days. 158 days is 22w4d, not 23 weeks. 158 ÷ 7 = 22.57, and rounding that to 23 overstates the pregnancy by three days. Clinical dating always uses completed weeks plus the remainder.
Using 266 instead of 280 when reversing from a due date. LMP = EDD − 280. The 266-day figure applies to conception dates, not menstrual dates. Mixing them produces a consistent 14-day error in whichever direction you were not expecting.
Ignoring cycle length. Naegele's rule assumes a 28-day cycle. On a 35-day cycle the due date is a week later, and that week can determine whether you are offered a post-dates induction.
Trusting an LMP-based date after a first-trimester scan. If a dating scan has been done, it is the better input. Use Mode 2 with the scan's due date instead of recalculating from a half-remembered period date.
Applying the same redating threshold at every gestation. The allowable discrepancy widens from 5 days in early pregnancy to 21 days in the third trimester. A scan that is 10 days off at 8 weeks changes the dates; the same 10 days at 30 weeks does not.
Using the simplified trimester split. The 1–12 / 13–27 / 28–40 version puts the first-trimester boundary about a week early relative to the ACOG definition of 13w6d. Harmless in conversation, misleading if you are counting screening windows.
Assuming the due date is a deadline. Fewer than 5% of babies arrive on it. It is the midpoint of a distribution roughly two weeks wide in either direction.
Off-by-one errors on the day count itself. Always sanity-check by running the calculation backwards: your LMP plus 280 days should land exactly on your due date.

What This Calculator Can and Cannot Do

It can:

Convert between LMP, due date, conception date and IVF transfer dates in any direction
Report gestational age as completed weeks plus days, the way a clinician would
Adjust for cycle lengths other than 28 days
Handle day-3 transfers, day-5 blastocyst transfers and egg retrieval dates separately
Show trimester, fetal age, percentage complete, days remaining and the birth classification
Lay out the upcoming milestone calendar against your specific dates

It cannot:

Replace a dating ultrasound, which is more accurate than any calendar method in the first trimester
Confirm that a pregnancy is viable or ongoing
Detect complications, growth restriction, or multiple pregnancy — twins and higher-order multiples follow different growth and delivery timelines entirely
Predict the actual date of birth, which is a distribution and not a point
Account for irregular or absent cycles, where the LMP is not a reliable anchor at all
Substitute for antenatal care of any kind

The honest framing: this is arithmetic on dates you supply. It is exactly as good as its inputs, and for anyone with irregular cycles or an uncertain LMP, its inputs are the weak link. That is precisely what the dating scan exists to fix.

Final Summary

The formulas worth remembering:

Gestational age = Today − LMP, reported as completed weeks + days
EDD = LMP + 280 days, or LMP + 280 + (cycle − 28) for a non-28-day cycle
LMP = EDD − 280 days
EDD = Conception + 266 days • day-3 transfer + 263 • day-5 transfer + 261
Fetal age = Gestational age − 14 days

Trimesters: first to 13w6d, second 14w0d–27w6d, third 28w0d onwards.

Term: preterm before 37w0d, early term 37w0d–38w6d, full term 39w0d–40w6d, late term 41w0d–41w6d, post-term from 42w0d.

If you take one thing from this page: report weeks *and* days, not rounded weeks, and if you have had a first-trimester dating scan, use its due date as your input rather than recalculating from your LMP. The scan is the better measurement, and the arithmetic here is only as good as what you feed it.

Related tools: the due date calculator for a simpler EDD-only answer, the pregnancy calculator for the week-by-week timeline, the conception calculator for working backwards to a likely conception window, the ovulation calculator for fertile-window planning, and the water intake calculator for the pregnancy and breastfeeding fluid allowances.

Related concepts

Naegele's rule

The classical due-date method: last menstrual period minus 3 months, plus 7 days, plus 1 year — equivalent to adding 280 days. It assumes a 28-day cycle with ovulation on day 14.

Crown-rump length (CRL)

The head-to-buttocks measurement used to date a pregnancy by ultrasound. Before about 14 weeks, embryonic growth is uniform enough that CRL predicts gestational age to within a few days.

Estimated due date (EDD)

The date 280 days after the last menstrual period. It is the midpoint of a delivery distribution rather than a prediction — fewer than 5% of babies arrive on it.

Redating

Formally replacing an LMP-based due date with an ultrasound-based one when the two differ beyond a gestation-specific threshold, ranging from 5 days in early pregnancy to 21 days in the third trimester.

Frequently Asked Questions

What is a gestational age calculator?+

A tool that converts between the dates of a pregnancy — last menstrual period, estimated due date, conception date or IVF transfer date — and tells you how far along the pregnancy is in weeks and days. It also returns the trimester, fetal age, days remaining, and the milestone calendar for those specific dates.

How do I calculate gestational age?+

Subtract the first day of the last menstrual period from today's date to get the number of days, then divide by 7 and keep the remainder. From 10 March to 15 August 2025 is 158 days, and 158 divided by 7 is 22 remainder 4 — so the gestational age is 22 weeks and 4 days, written 22w4d. Report the completed weeks plus the leftover days rather than rounding.

How do I calculate my baby's due date?+

Add 280 days, which is 40 weeks, to the first day of your last menstrual period. An LMP of 1 January 2025 gives a due date of 8 October 2025. If your cycle is not 28 days, add the difference: on a 35-day cycle the due date shifts 7 days later.

How many weeks pregnant am I if I know my due date?+

Subtract 280 days from the due date to recover the dating LMP, then count forward from there to today. With a due date of 15 December 2025, the LMP is 10 March 2025, so on 15 August 2025 the gestational age is 158 days, or 22 weeks and 4 days.

What is the difference between gestational age and fetal age?+

Gestational age is counted from the last menstrual period and reaches 40 weeks at term. Fetal age, also called embryonic or conceptional age, is counted from fertilisation and is always exactly 14 days less — so 38 weeks at term. A pregnancy at 22w4d gestational is at 20w4d fetal age.

Why am I 2 weeks pregnant at conception?+

Because gestational age is counted from the first day of your last period, which is roughly two weeks before ovulation and conception. The convention exists because the LMP is a date most women can recall while the exact moment of fertilisation almost never is. It means weeks 1 and 2 of a pregnancy contain no pregnancy at all.

How do I use a conception date to calculate the due date?+

Add 266 days to the conception date, since 280 minus the 14-day offset is 266. A conception date of 1 April 2025 gives a due date of 23 December 2025. For IVF, use 266 days from egg retrieval, 263 from a day-3 transfer, or 261 from a day-5 blastocyst transfer.

How is an IVF due date calculated?+

From the known fertilisation timing rather than a menstrual date. Add 266 days to the egg retrieval date, 263 days to a day-3 embryo transfer, or 261 days to a day-5 blastocyst transfer. A day-5 transfer on 20 March 2025 gives 6 December 2025, while a day-3 transfer on the same date gives 8 December — the two-day gap is the extra time the embryo spent developing before transfer.

Does cycle length change the due date?+

Yes, and by more than people expect. Naegele's rule assumes a 28-day cycle with ovulation on day 14. Add the difference between your cycle length and 28 to the standard 280 days. An LMP of 15 January 2025 gives 22 October on a 28-day cycle but 29 October on a 35-day cycle — a full week.

When does a doctor change my due date?+

When an ultrasound differs from the LMP-based date by more than a defined threshold that widens with gestation: more than 5 days at 8w6d or earlier, 7 days from 9w0d to 15w6d, 10 days from 16w0d to 21w6d, 14 days from 22w0d to 27w6d, and 21 days from 28w0d. Once dating is established, later scans do not revise it again.

How accurate is a due date?+

Only about 4 to 5% of babies are born on their exact due date, and roughly half arrive within a week either side. The due date is the midpoint of a distribution about two weeks wide in each direction, not a deadline. First-trimester ultrasound dating is accurate to within a few days; dating from an uncertain LMP is considerably less so.

What are the trimester boundaries?+

ACOG places the first trimester from the LMP to 13w6d, the second from 14w0d to 27w6d, and the third from 28w0d to birth. Many popular sources use a simplified 1 to 12, 13 to 27, 28 to 40 week split, which sets the first-trimester boundary about a week early.

What counts as full term?+

Full term is 39w0d to 40w6d. Below that, 37w0d to 38w6d is early term and anything before 37w0d is preterm. Above it, 41w0d to 41w6d is late term and 42w0d onwards is post-term. The categories exist because babies born at 37 to 38 weeks have measurably higher rates of respiratory difficulty than those born at 39 to 40 weeks.

Is this gestational age calculator free?+

Yes — completely free, no registration, and nothing you enter leaves your browser. All three modes calculate locally as you type.

Formula

Gestational age formulas

Gestational age = As-of date − LMP, reported as completed weeks + days. EDD = LMP + 280 + (cycle length − 28). LMP = EDD − 280. EDD = conception + 266, day-3 transfer + 263, day-5 blastocyst + 261. Fetal age = gestational age − 14 days.

Step by step

How the calculation works

1From LMP: enter the first day of your last period and your usual cycle length. Cycles longer than 28 days push the due date later by the difference.
2From Due Date: if a dating scan has already given you a due date, use this mode — the scan is a better input than a recalled period date.
3From Conception or IVF: for known conception dates, day-3 transfers, day-5 blastocyst transfers and egg retrievals, each of which uses a different day count.
4The as-of date defaults to today, but you can set it to any date to see where a pregnancy was, or will be, at that point.

Example

Worked example — due date 15 December 2025, as of 15 August 2025

Related guides

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