Due date by conception date
If you know the date of conception, enter it here. That is common after fertility treatment or when intercourse happened on a single day. The page adds 266 days to find the due date.
How it works
Add 266 days to the date of conception. That is 38 weeks. The pregnancy length shown adds 14 days, to match the way doctors count from the last period.
Worked example
Say conception was on 3 October. Add 266 days to reach 26 June. Day 100 after conception is 11 January. Add 14 days to count from the last period. That makes 114 days, which is 16 weeks and 2 days. This example uses a 365-day year.
Common questions
How do I know the date of conception?
It is rarely known exactly. Ovulation tests, fertility treatment dates or a single intercourse date give the best guess. If conception was on 3 October, the due date is 26 June.
Why 266 days?
A pregnancy lasts about 38 weeks from conception, which is 40 weeks from the last period less the two weeks before ovulation. From 3 October, 266 days ends on 26 June and 280 days would end on 10 July.
Can sperm survive for days?
Yes. Sperm can live for several days. The date of intercourse can be earlier than the date of conception. A conception date 3 days after 3 October would move the due date to 29 June.
Does IVF change the calculation?
For IVF the transfer date and embryo age are used, and clinics give their own due dates. Use their date over this estimate. Here 3 October gives 26 June, which a clinic may adjust.