Pregnancy Due Date Calculator

How a due date is estimated

Pregnancy is counted in weeks from the first day of the last menstrual period (LMP), not from conception. In a typical 28-day cycle, ovulation and conception happen about two weeks after the period starts, so by the time a pregnancy test turns positive you are already counted as about four weeks pregnant. The estimated due date (EDD) is 40 weeks, or 280 days, after the LMP. Only a small share of babies arrive on that exact day; most come in the weeks around it, which is why doctors talk about a due "window" rather than a deadline.

How to use the calculator

1. Choose what you know: the first day of your last period, the conception date, or the date of an IVF embryo transfer.
2. Enter the date. For the period method, also enter your usual cycle length, counted from the first day of one period to the first day of the next.
3. Read your due date, how many weeks and days along you are today, your trimester and the key dates in the table.

The formulas used

• From the last period (Naegele's rule): LMP + 280 days, adjusted by your cycle length minus 28. A 32-day cycle adds 4 days because ovulation probably happened later; a 25-day cycle takes 3 days off.
• From conception: conception date + 266 days (38 weeks).
• IVF, day-5 transfer: transfer date + 261 days, because the embryo was already 5 days old.
• IVF, day-3 transfer: transfer date + 263 days.

How far along you are is then counted back from the due date, so all four methods give the same week for the same pregnancy.

A worked example

Say your last period started on 10 March 2026 and your cycle is 28 days. Adding 280 days gives an estimated due date of 15 December 2026. On 1 July 2026 you would be 16 weeks and 1 day pregnant, in the second trimester, with 167 days to go. The first trimester ends on 15 June (13 weeks 6 days), the second on 21 September (27 weeks 6 days), early term begins on 24 November (37 weeks) and full term on 8 December (39 weeks). If your cycle were 32 days instead, every date would move 4 days later, to a due date of 19 December 2026. A conception date of 24 March, a day-5 transfer on 29 March or a day-3 transfer on 27 March all lead to the same 15 December date.

What "term" means

The American College of Obstetricians and Gynecologists describes 37 weeks 0 days to 38 weeks 6 days as early term, 39 weeks 0 days to 40 weeks 6 days as full term, 41 weeks as late term and 42 weeks onwards as post-term. The table uses those boundaries.

Limits, and why your scan date may differ

• The period method assumes ovulation about 14 days before your next period. Irregular cycles, recent hormonal contraception or breastfeeding make it less reliable.
• An early ultrasound measures the baby directly and is generally used to confirm or correct the date. If your scan gives a different due date, go by the one your doctor gives you.
• This page is an estimate for general information and is not medical advice. It does not replace antenatal care, so please consult your doctor or midwife about your pregnancy, and contact them straight away about bleeding, pain or anything that worries you.