Free Pregnancy Tool
Work out when conception most likely happened, from the last period or the due date. Private - nothing leaves this page.
28 days is average. Longer cycles mean later ovulation.
A due date from a dating scan gives the most accurate result.
Don't know the due date? Use our Due Date Calculator to find out.
A conception calculator estimates when a pregnancy began - the day an egg was most likely fertilised - and the few days around it when sex could have led to this pregnancy. You can start from the first day of the last period or from a due date you've already been given.
Conception happens around ovulation, which is usually about 14 days before the next period would have started. From the last period, the calculator adds the cycle length and counts back 14 days. From a due date, it counts back 266 days - the average time from conception to birth. Because sperm can survive up to 5 days, the likely window runs from 5 days before that date to the day after.
Pregnancy weeks are counted from the last period, so at "6 weeks" conception was only about 4 weeks ago. Knowing the actual conception date helps the numbers make sense - and it's the date most people are really asking about when they say "when did it happen?"
FAQ
How conception dates are estimated, and why the answer is a window rather than a single day.
From the last period: add the cycle length to the first day of the last period, then subtract 14 days. For a 28-day cycle that's day 14. From a due date: subtract 266 days (38 weeks). Both give the most likely day of ovulation and fertilisation.
Not usually. Ovulation timing varies, and sperm can survive up to 5 days, so sex several days before ovulation can still lead to pregnancy. That's why the calculator shows a window, not just a single day. IVF pregnancies are the exception - the dates are known precisely.
Pregnancy is dated from the first day of the last period, because that date is known and ovulation usually isn't. Conception typically happens around 2 weeks later, so a pregnancy is already "2 weeks" along on the day of conception.
If the due date came from a dating scan, use that. A first-trimester scan is accurate to within about 5-7 days, which is more reliable than a remembered period date, especially with irregular cycles.
No. Because of the multi-day window and normal variation in ovulation and pregnancy length, a calculator can't reliably separate dates a few days apart. Only a paternity test can do that.
Enter the first day of the last period and cycle length to see the fertile window, ovulation day, and next period for the coming months.
Use tool →Find your due date from the first day of the last period, or from how many weeks pregnant you are today.
Use tool →Most partners assume pregnancy lasts nine calendar months. Here is why doctors use weeks instead, and why 'five months pregnant' is harder to interpret than it sounds.
Read article →Learn why couples often wait 12 weeks, who to tell earlier, and how to decide when to announce your pregnancy.
Read article →Built for dads & partners
Stop asking. Stop guessing.
Open the app and you're caught up - week by week, until your baby is here.
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