How a pregnancy due date is worked out: periods, cycles and IVF

Guide · Updated 2026-11-10

Where the 280-day rule comes from, why pregnancy weeks start before conception, and how the due date calculator handles long cycles, conception dates and IVF transfers.

Open the Due date Free, in your browser, nothing uploaded.

The due date calculator estimates when a baby is likely to arrive, and how far along a pregnancy is on any day you choose. It can start from the first day of the last period, a known conception date, or the date of an IVF embryo transfer, and it lists the dates when the main stages begin. It is for expecting parents who want a rough calendar, and students learning how pregnancy is dated. It is general education, not medical advice: your midwife or doctor, and above all an early scan, set the date that counts. Before a pregnancy, the ovulation calendar estimates fertile days; during one, the water intake calculator and sleep calculator can help with daily routines, with your care team’s advice coming first.

What it is

A pregnancy is usually counted as 40 weeks, or 280 days, but not from conception. It is counted from the first day of the last menstrual period (LMP), because that date is easy to know and conception usually is not. In a 28-day cycle, ovulation, and so conception, comes about two weeks after the period starts. So by the time an egg is fertilised, the pregnancy is already “2 weeks” by this count.

This gives a family of simple rules:

• From the LMP (Naegele’s rule): due date = LMP + 280 days + (C − 28) days, where C is the usual cycle length in days. A longer cycle means ovulation came later, so the due date moves later by the same number of days.
• From conception: due date = conception + 266 days (280 minus the 14 days before ovulation).
• From an IVF transfer: due date = transfer + 266 days − embryo age. A day-3 embryo gives transfer + 263 days; a day-5 embryo gives transfer + 261 days.

The gestational age on any day is the number of days since that counting start, written as weeks plus days, for example 18+4. Trimesters split it into three: the first runs to the end of week 13, the second from week 14, and the third from week 28.

How to use it

1. In Work it out from, pick what you know: first day of last period, conception date, or IVF transfer of a day-3 or day-5 embryo.
2. Enter that date in the date box below it. Its label changes to match your choice.
3. If you chose the period date, type your Usual cycle length in days. The box hides for the other methods, where cycle length does not matter.
4. Today’s date fills in from your device. Change it to see how far along the pregnancy is on another day, such as a planned trip.
5. Read the estimated due date, the line with weeks, days, trimester and days to go, and the sentence that shows the rule used.
6. Check the table of key weeks: the usual first scan window, the start of each trimester, the mid-pregnancy scan, early term, the due date and post-term. Reset restores the example.

A worked example

Suppose the last period started on 20 March 2026 and cycles are usually 32 days, and we look at the week count on 1 August 2026.

The cycle is 4 days longer than 28, so the due date is 20 March 2026 + 280 + 4 = 284 days later: Tuesday 29 December 2026. The counting start moves 4 days too, to 24 March 2026. On 1 August 2026 that is 130 days in, or 18 weeks 4 days (18+4), second trimester, with 150 days to go.

With the cycle left at 28 days, the tool would say Friday 25 December 2026 and 19+1 instead.

Now check it another way. In a 32-day cycle ovulation comes about 18 days after the period starts, so around 7 April 2026. Choose Conception date, enter 7 April 2026, and the tool gives 7 April + 266 days = 29 December 2026 again. The two methods agree, which is a good sign the inputs make sense.

From the key weeks table for this pregnancy: week 20 starts on 11 August 2026, week 28 on 6 October 2026, and week 37 (early term) on 8 December 2026.

Where it is used

• Planning leave: parents use the due date and the early-term date to talk to employers about maternity, paternity or parental leave. Rules vary by country, so check the official source.
• Booking appointments: the scan windows in the table help when a clinic asks which week you are in.
• Fertility care: after IVF, the transfer date gives a firmer starting point than a period date.

Common mistakes

• Forgetting the cycle length. A regular 35-day cycle moves the date a week later than the 28-day default. Type your own number.
• Counting weeks from conception. A scan report that says 12 weeks means 12 weeks from the LMP-style start, which is about 10 weeks after conception.
• Treating the date as a deadline. It is the middle of a spread. Plan for a window of a few weeks either side, and let your midwife or doctor guide what happens near and after it.
• Keeping the period date after a scan. If an early scan gives a different date, go by the scan.

Questions people ask

Why does my cycle length change the date but not for conception or IVF? Cycle length only tells the tool when ovulation probably happened. If you already know conception or the transfer date, that step is not needed.

What if my cycles are irregular? Then the period date is a weak guide, and the tool falls back to 28 days for lengths outside 20 to 45. An early ultrasound is the better way to date the pregnancy; ask your care team.

Is week 40 when labour starts? Not usually exactly. It marks the estimated date, and many healthy births happen in the weeks around it. Talk to your midwife or doctor about what to expect for you.