Pregnancy Due Date & Gestational Age Calculator

Introduction to Pregnancy Dating, Due Dates and Gestational Age

Pregnancy dating answers two everyday questions at once: how far along is this pregnancy today, and roughly when is the birth expected? This calculator answers both. It converts a starting point — the first day of the last menstrual period, a known conception or embryo-transfer date, or the gestational age measured at a dating ultrasound — into an estimated due date, a gestational age in completed weeks plus days, a trimester, a fetal (conception) age, and the calendar dates of the milestone windows that prenatal care is organised around.

Every method on this page reduces to the same internal step. Whatever you enter is first converted into a single dating reference: the day that counts as day zero of gestation. The estimated due date is then 280 days after that reference, and gestational age on any chosen date is simply the number of days between the reference and that date, expressed as completed weeks and leftover days. Working this way keeps the due date and the gestational age arithmetically consistent, which is exactly how a clinical dating wheel behaves.

An estimated due date is a planning anchor, not a prediction of the birthday. It exists so that screening windows, growth assessments, vaccination timing and delivery decisions all sit on one shared timeline. Births cluster around the estimate rather than landing on it, and the clinical definitions of term birth deliberately span five weeks for that reason.

Key Pregnancy Dating Terms Used on This Page

• LMP: the first day of the last menstrual period — the traditional day zero for gestational age.
• Dating reference: the day this calculator treats as gestational day zero after any cycle-length or ultrasound adjustment has been applied.
• EDD (estimated due date): the calendar date 280 days, or 40 weeks 0 days, after the dating reference.
• Gestational age (GA): completed weeks plus days since the dating reference, written in the clinical style 20+6 for 20 weeks and 6 days.
• Fetal age: also called conception age or fertilisation age; approximately 14 days less than gestational age.
• Trimester: first trimester runs 0+0 to 13+6, second 14+0 to 27+6, third 28+0 onward.
• Term status: the ACOG labels early term, full term, late term and post-term, which apply from 37+0 onward.

How to Use This Due Date and Gestational Age Calculator

Choose a dating method, enter the matching date, and optionally choose the date you want the gestational age reported for. The calculator does the rest and shows its working, including the dating reference it derived.

1. From the last menstrual period: enter the first day of bleeding of your last period, not the day it ended. If your usual cycle is not 28 days, set the cycle length field; the calculator shifts the dating reference by the difference.
2. From a conception or transfer date: enter the date of conception, of a timed insemination, or of an embryo transfer if you have it. The calculator subtracts 14 days to obtain the equivalent dating reference so that gestational age remains on the conventional menstrual scale.
3. From a dating ultrasound: enter the date of the scan and the gestational age the sonographer reported on that day, in weeks and days. This is the most reliable option when a first-trimester scan is available, because it does not rely on the cycle at all.
4. Choose an evaluation date: leave the evaluation field empty to report gestational age today, or set a future or past date to see the gestational age at an appointment, a planned scan, or a travel date.
5. Read the result: the panel shows the estimated due date, gestational age, fetal age, trimester, term status, days remaining, and a table of milestone dates. The chart below the panel places the current gestational age on a 0-to-42-week band.

Choosing the Right Dating Method

If a first-trimester ultrasound has been performed, use it. ACOG, the Society for Maternal-Fetal Medicine and the American Institute of Ultrasound in Medicine agree that a crown-rump length measurement taken up to and including 13 weeks 6 days is the single most accurate dating tool available, and that the earliest accurate scan should be used to establish or confirm the due date. Menstrual dating is the right choice when no early scan exists and the cycle is reasonably regular. Conception dating is useful after assisted reproduction, where the fertilisation or transfer day is known precisely.

Formula Reference: Naegele's Rule and Gestational Age Arithmetic

Naegele's rule is the classical statement of the due date: add 280 days, or 40 weeks, to the first day of the last menstrual period. The familiar shortcut of subtracting three months, adding seven days and adding one year produces the same date in most months, and the calculator uses exact day counting rather than the shortcut so that month lengths, leap years and month-end rollovers are always handled correctly.

Gestational age on any evaluation date is the number of days since the dating reference, reported as completed weeks and the remaining days.

Here D is the whole number of days from the dating reference to the evaluation date. A gestational age of 20+6 therefore means 146 days have elapsed. Because the due date sits at 280 days, gestational age and days remaining always sum to 280.

Adjusting Naegele's Rule for a Cycle Longer or Shorter Than 28 Days

The 280-day interval assumes a 28-day cycle with ovulation on about day 14. When the usual cycle is longer, ovulation and therefore conception occur later, and the due date moves later by the same amount. A shorter cycle moves it earlier. The calculator applies the difference to the dating reference so that the due date and the gestational age shift together.

C is the usual cycle length in days. A 33-day cycle adds five days; a 24-day cycle subtracts four. This correction is an approximation, because the luteal phase is more consistent than the follicular phase but neither is fixed, and cycle length itself varies from month to month. It is a refinement of menstrual dating, not a replacement for an early scan.

Dating From a Conception or Embryo-Transfer Date

Conception occurs about two weeks after the first day of the last period, so a known conception date is 14 days ahead of the dating reference. The due date is 266 days after conception, which is the same as 280 days after the derived reference.

For a day-5 blastocyst transfer, subtract five days from the transfer date to recover the equivalent fertilisation date before entering it; for a day-3 transfer, subtract three days. This calculator does not perform that step for you.

Dating From a First-Trimester Ultrasound

An ultrasound reports the gestational age measured on the day of the scan. Subtracting that measured age from the scan date recovers the dating reference, and the due date follows in the usual way.

w and d are the weeks and days reported at the scan. This is the method that most closely matches how a due date is established in prenatal care, because the earliest accurate ultrasound is the reference standard.

Gestational Age Compared With Fetal or Conception Age

Gestational age is counted from the last menstrual period and therefore includes roughly two weeks before conception occurred. Fetal age is counted from conception. The two differ by about 14 days, and mixing them is one of the most common sources of confusion when comparing a pregnancy app, an embryology report and a clinic note. Everything reported by this calculator, apart from the explicitly labelled fetal age line, uses gestational age.

Worked Example: An LMP of 3 March 2025 With a 33-Day Cycle

Suppose the first day of the last menstrual period was 3 March 2025, the usual cycle is 33 days, and you want the gestational age on 1 August 2025.

1. Cycle adjustment: 33 − 28 = +5 days, so the dating reference moves from 3 March 2025 to 8 March 2025.
2. Estimated due date: 8 March 2025 + 280 days = Saturday, 13 December 2025.
3. Gestational age on 1 August 2025: 8 March to 1 August is 146 days, and 146 = 20 × 7 + 6, so the gestational age is 20 weeks 6 days (20+6), which is the second trimester.
4. Fetal age on the same day: 146 − 14 = 132 days = 18 weeks 6 days.
5. Days remaining on 1 August 2025: 280 − 146 = 134 days, or 19 weeks 1 day.
6. Milestone dates from the same reference: 37+0 falls on 22 November 2025, 39+0 on 6 December 2025, 41+0 on 20 December 2025 and 42+0 on 27 December 2025.

Entering 3 March 2025 with a 33-day cycle and an evaluation date of 1 August 2025 reproduces exactly these numbers. Without the cycle adjustment the due date would be 8 December 2025 and the gestational age on 1 August would be 21+4, which shows how much a five-day shift matters when a screening window is being booked.

ACOG Term Definitions and the Milestone Windows This Calculator Reports

Since 2013 ACOG and the Society for Maternal-Fetal Medicine have replaced the single label "term" with four named intervals, because outcomes differ measurably across them. The calculator labels the current gestational age against these definitions and prints the calendar date on which each boundary is reached.

When an Ultrasound Should Replace the Menstrual Due Date

ACOG Committee Opinion No. 700 gives explicit thresholds for the difference between menstrual dating and ultrasound dating. If the discrepancy exceeds the value for the gestational age at which the scan was performed, the due date should be changed to the ultrasound date; if it does not, the menstrual date is retained. The thresholds widen with gestational age because ultrasound biometry becomes less precise as the pregnancy advances.

Two practical consequences follow. First, a pregnancy with no ultrasound before 22+0 is described as suboptimally dated. Second, once the due date has been set from the earliest accurate scan it should rarely be revised; a smaller or larger fetus on a later scan is usually a growth finding, not a dating error, and is assessed as such.

Limitations, Assumptions and Medical Disclaimer

The assumptions built into this calculator are deliberately simple, and they are worth stating plainly:

• The 280-day interval is a population average. Individual pregnancy length varies by roughly two weeks either side even when dating is perfect.
• Menstrual dating assumes ovulation 14 days after the first day of bleeding. Late or early ovulation moves the true dating reference and no cycle-length correction can detect that.
• The cycle-length adjustment assumes a fixed luteal phase, which is a reasonable but not exact approximation.
• Conception dating assumes fertilisation on the entered date. Sperm survival means a single intercourse date is not a conception date.
• The tool assumes a single pregnancy. Multiple pregnancies are dated the same way but managed on a different timeline, with earlier expected delivery.
• It cannot account for a planned induction or caesarean, pregnancy complications, or a due date already established and documented by your care team.
• Dates are computed in whole calendar days from the components you enter, with no time-of-day, time-zone or daylight-saving component. Results therefore never shift by a day when the page is opened in a different time zone.
• The reported due date is not a redating decision. Comparing menstrual and ultrasound estimates against the ACOG thresholds is a clinical judgement made with your record in front of the clinician.
• Input checks are deliberately explicit. A future date, a cycle length outside 20 to 45 days, a scan gestational age outside 0+0 to 42+0, an unreadable date, or an evaluation date before gestational day zero are refused with an explanation. A dating reference more than 300 days before today is still calculated, but it is flagged because it is outside the range of a pregnancy still in progress, and gestational age is then reported at the estimated due date unless you supply an evaluation date.

Important: this calculator provides an estimate for general information and planning only. It does not diagnose, does not establish or change a clinical due date, and does not replace assessment and advice from a midwife, obstetrician, family doctor or other qualified clinician. If you are pregnant or think you might be, arrange care with a qualified health professional, and contact them promptly about bleeding, pain, reduced fetal movement, or any concern about your pregnancy.

Common Questions About Due Dates and Gestational Age

Sources Verified for This Pregnancy Dating Calculator

This page is educational. Estimates are produced from the dates you enter using Naegele's rule and the ACOG definitions cited above, and they do not replace the due date recorded in your prenatal notes or the judgement of your care team.