When did I conceive? Working out your conception date

Whether you’re curious, planning, or trying to piece together dates, it’s natural to want to know when you conceived. You can usually work it out to within a few days, but rarely to an exact day. Here’s how, and how accurate each method is.

Why it’s hard to know the exact day

Conception is when a sperm fertilises an egg, and it happens around ovulation. But sperm can survive inside you for up to 7 days, so you can conceive several days after you had sex. Ovulation itself also moves from cycle to cycle. So even if you know exactly when you had sex, the day you conceived may have been a few days later.

Method 1: from your last period

Pregnancy is counted from the first day of your last period, not from conception. If you have regular 28-day cycles, you probably ovulated, and conceived, around 2 weeks after your period started.

If your cycle is longer or shorter, adjust by the difference. Ovulation usually happens 10 to 16 days before your next period would have been due, so with a 35-day cycle it’s more likely to be about 3 weeks after your period started.

How accurate? It’s a rough guide. It depends on remembering the date of your last period and on your cycle being regular.

Method 2: from your due date

A due date is worked out as 40 weeks (280 days) from the first day of your last period. In a 28-day cycle, conception is about 2 weeks after that, so roughly 38 weeks (266 days) before your due date. Count back 38 weeks from your due date to get an estimate. Our due date calculator can do this the other way round.

How accurate? Only as accurate as the due date. If your due date came from your period dates, it has the same limits as method 1. If it came from a scan, it’s more reliable.

Method 3: from your dating scan

You’ll be offered a dating scan at around 10 to 14 weeks. The sonographer measures your baby from the top of the head to the bottom (the crown–rump length) and uses it to work out how many weeks pregnant you are. If you’re more than about 14 weeks, they measure the head instead. The scan gives a more reliable due date than your period dates, so if they’re different, your due date is usually based on the scan.

How accurate? It’s the most accurate method for most people. In a large international study, scan estimates were usually within 3 to 4 days of the date worked out from reliable period dates, and about 19 in 20 were within 6 to 8 days. Your conception date is then roughly 2 weeks after the scan’s ‘first day of your last period’.

If you had IVF or IUI

With IVF, the date of fertilisation is known, so your clinic will work out your due date from your embryo transfer date, and it won’t usually change because of the scan. With IUI, conception will have been within a few days of the insemination.

Putting it together

If your dating scan and your own dates agree, you can be fairly confident you conceived within a few days of the estimate. If they disagree, the scan is the better guide, and in the UK your due date is normally set from it. It’s common to ovulate earlier or later than you think.

If you need to know for a sensitive reason

Sometimes people want to know when they conceived because it matters who the biological father is. Dates can narrow it down, but if sex with different partners was only days apart, dates alone can’t tell you. Your midwife, GP or a sexual health clinic can talk this through confidentially.

Questions people ask

How many weeks pregnant am I if I conceived 2 weeks ago?

About 4 weeks pregnant, because pregnancy is counted from your last period, roughly 2 weeks before conception.

Can a scan tell you exactly when you conceived?

No. An early scan usually dates a pregnancy to within about a week either way, which narrows it down a lot, but it can’t give you an exact day.

Why did my due date change after my scan?

Because the scan measures your baby directly, which is more reliable than working it out from your period, especially if your cycles aren’t 28 days or you’re not sure of the date.

Can I work out conception from a positive test?

Most pregnancies show on a test by the day your period is due, about 2 weeks after conception, and some show a few days earlier on an early test. So the day you first got a positive only gives a rough idea. See how early can I take a pregnancy test?