VitalMetrics

Pregnancy Due Date Calculator

Estimate when your baby is due from the first day of your last period, a known conception date, or an IVF transfer date.

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Enter a date to see your estimated due date.

Why your cycle length moves your due date

Pregnancy is counted from the first day of your last period, about two weeks before conception. Standard dating assumes a 28-day cycle with ovulation on day 14. The phase after ovulation is fairly steady at about 14 days, so a longer cycle means later ovulation, and a later due date. Drag the cycle length.

0481216202428323640weeks since last periodperiodovulation ≈ day 14due ≈ day 280

Only about 4% of babies arrive on their due date; most come between 37 and 42 weeks. Early ultrasound dating usually takes over if it differs from period dating by more than a set margin (ACOG Committee Opinion 700).

Three ways to date a pregnancy

  1. Last menstrual period (Naegele’s rule): first day of your last period + 280 days, or “add 7 days, subtract 3 months, add a year”. We add or subtract the difference if your cycle isn’t 28 days.
  2. Conception date: + 266 days (38 weeks), since conception usually happens about two weeks after the period starts.
  3. IVF transfer: + 261 days for a day-5 embryo or + 263 days for a day-3 embryo, following ACOG guidance.

Whichever you use, your midwife or obstetrician may adjust the date after your first ultrasound, which is the most accurate method in early pregnancy.

What “full term” means

ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks, and post-term as 42 weeks and beyond. Your due date is simply the middle of the full-term window.

Contact your maternity unit or doctor straight away if you have bleeding, severe pain, fluid leaking, reduced baby movements later in pregnancy, or you feel something is wrong. Don’t wait on a calculator.

Questions people ask

How is a due date calculated?

By convention the estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, which assumes a 28-day cycle. This calculator shifts the date for longer or shorter cycles, or counts 266 days from a known conception date.

How do I calculate my due date after IVF?

ACOG advises dating IVF pregnancies from the transfer: add 261 days for a day-5 (blastocyst) embryo, or 263 days for a day-3 embryo. Choose “IVF transfer” above.

How accurate is a due date calculator?

It gives an estimate. Only around 4% of babies are born on their due date, and most arrive within two weeks either side. A first-trimester ultrasound is the most accurate way to date a pregnancy.

What if I don’t know my last period date?

Use the conception date if you know it, or wait for a dating ultrasound. Your midwife or doctor will set the official due date from the scan.

Can my due date change?

Yes, usually after the first ultrasound. ACOG recommends changing the period-based date if the early scan differs by more than a set amount (for example, by more than 7 days when the scan is done between 9 and 14 weeks). Later in pregnancy, dates are usually not changed.

Sources

  1. ACOG Committee Opinion No. 700 (2017). Methods for estimating the due date: EDD = LMP + 280 days; IVF day-5 transfer + 261 days, day-3 + 263 days; ultrasound redating thresholds.
  2. Mittendorf R et al. (1990). The length of uncomplicated human gestation. Obstet Gynecol 75:929–932.
  3. Mayo Clinic. Pregnancy week by week: counting 40 weeks from the start of the last period.

Every formula on VitalMetrics is listed with its original reference on Sources & methods.

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