Quick answer: With a due date of 8 May 2027, conception most likely happened on 15 August 2026 (266 days earlier), and intercourse from 10 to 15 August 2026 could have led to the pregnancy.
How it's calculated
A pregnancy lasts about 266 days (38 weeks) from conception, so the likely conception date is the due date minus 266 days. From the last period, conception is taken as the day of ovulation: the first day of the last period plus the cycle length minus 14, since the second half of the cycle (the luteal phase) is usually about 14 days.
Conception can follow intercourse on the day of ovulation or in the five days before it, because sperm can survive that long; the egg lives for about a day. That six-day span, found by Wilcox and colleagues in 1995, is shown as the fertile window.
The result is an estimate. Ovulation can come early or late, and a due date set by ultrasound can itself be a few days out. A dating scan cannot pin conception to one day, which is why medical dating counts from the last period instead. Some calculators show a range of about two days either side of this date rather than a single day.
Worked examples
- Due date 8 May 2027: − 266 days = 15 August 2026; window 10–15 August
- Last period 1 August 2026, 28-day cycle: + 28 − 14 = 15 August 2026
- Last period 1 August 2026, 32-day cycle: + 32 − 14 = 19 August 2026
Frequently asked questions
Can I find the exact day I got pregnant?
No. Dates and scans narrow it to a few days at best: ovulation varies, and sperm can live for up to five days, so intercourse on several different days could have led to the same pregnancy.
Why is my pregnancy two weeks further along than my conception date?
Pregnancy is counted from the first day of the last period, which is about two weeks before conception. So a pregnancy is already about 2 weeks along on the day of conception.
Is conception the same as ovulation?
Very nearly. Fertilisation happens within about a day of ovulation, so the two are treated as the same day for dating.
Can an ultrasound show when I conceived?
It estimates the age of the pregnancy. A scan in the first trimester is the most accurate, usually to within about 5 to 7 days, so it still gives a range rather than an exact day.
Does the calculator work for IVF?
With IVF the conception timing is known from the procedure, so you don't need an estimate. Use the transfer date in the due date calculator instead.
How soon after conception does a test show positive?
Usually about two weeks after conception, around the time your period is due. The hormone that tests detect needs time to build up after implantation.