Due date calculator: your due date and how far along you are

All four methods start from the same rule: a pregnancy lasts on average 280 days (40 weeks) from the first day of your last period, which is 266 days (38 weeks) from conception.

From your last period

First day of your last period + 280 days; this is Naegele's rule, "add 9 months and 7 days". If your cycle isn't 28 days, the date moves by the difference: with a 32-day cycle, 4 days later, because ovulation comes about 14 days before the next period. Example: last period on 1 January 2026, 28-day cycle, due date 8 October 2026.

From your conception date

Conception date + 266 days. Useful if you know the day you ovulated, for example from ovulation tests. The fertile window is six days long and ends on the day of ovulation, though, so the day you had sex isn't necessarily the day you conceived.

From an IVF embryo transfer

Transfer date + 266 days − the embryo's age: 261 days for a 5-day embryo (blastocyst), 263 for a 3-day embryo, 260 for a 6-day embryo. With IVF, doctors date the pregnancy exactly this way, because the day of fertilisation is known.

From an ultrasound

Scan date + (280 days − how far along you were at the scan). After a scan at 12 weeks and 3 days, 193 days remain. A first-trimester scan, up to 13 weeks and 6 days, that measures the crown-rump length (CRL) is the most accurate way to date a pregnancy.

It's an estimate, not an appointment. A study that tracked the day of ovulation in 125 women found that pregnancy length naturally differs from one woman to another by as much as 37 days, even when the day of ovulation is known.

That's why ACOG guidance says the date from your last period should be corrected when a first-trimester scan disagrees with it by too much: by more than 5 days before 9 weeks, by more than 7 days between 9 weeks and 13 weeks 6 days. Once set, the date isn't changed by later scans except in rare cases.

This calculator gives you a good idea. The date your doctor sets comes first.

Pregnancy weeks are counted from the first day of your last period, not from conception. Scan reports write them as 12w 3d or 12+3: 12 full weeks and 3 days. That means you're in your 13th week.

The first trimester runs to 13+6, the second from 14+0 to 27+6, and the third from 28+0 until birth. A pregnancy is full term between 37+0 and 41+6; from 42+0 it's called post-term.

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