Pregnancy Due Date Calculator

due date = first day of the last period + 280 days, adjusted for cycle length. That is the Naegele rule, and 280 days is 40 weeks. It assumes a 28 day cycle with ovulation on day 14, so a 32 day cycle pushes ovulation to day 18 and the due date four days later. The two weeks that confuse everybody: pregnancy is dated from the last period, which is about two weeks before conception. So at "8 weeks pregnant" the embryo is about 6 weeks old. Every scan, chart and appointment uses the same convention, so it is consistent even though it is strange.

How accurate is a due date?

It is a midpoint, not an appointment. About 4 in 100 babies arrive on the exact date, and most arrive within two weeks either side of it. A dating scan in the first trimester is more accurate than any calculation from a period, and if the two disagree by more than about a week, the scan usually wins.

Why does it count from my period and not from conception?

Because the date of a period is something you know and the date of conception almost never is. Sperm can survive several days, so even a known day of intercourse does not fix the day of fertilization. Medicine picked the date that can actually be recorded.

My cycle is not 28 days. Does that matter?

Yes, and this calculator adjusts for it. A longer cycle means you ovulate later, so conception happened later and the due date moves back by the same number of days. A 35 day cycle pushes the due date a week later than the standard rule would give.

What does "full term" mean?

From 37 weeks. Before that is preterm, 39 to 41 weeks is called full term proper, and past 42 is post-term, which is where induction is usually discussed. The window is wide because normal is wide.

Is this medical advice?

No. It is the same arithmetic your doctor uses, but they also have a scan, your history and an examination, none of which a web page has. Treat this as a way of understanding the dates you have been given, not as a replacement for them.