How the conception date is worked out
Midwives and doctors date a pregnancy from the first day of the last period, and a full pregnancy lasts 280 days (40 weeks) from that day to the due date. Conception does not happen on that first day, though. It happens around ovulation, which in a typical 28-day cycle comes about 14 days later.
So the calculator counts back 266 days (38 weeks) from the due date to find the most likely day of conception. This is the same rule the due date calculator uses forwards: conception date plus 266 days gives the due date.
If you enter a date of birth instead, the calculator needs to know how long the pregnancy lasted. It starts from the birth date, goes back by the length of the pregnancy to find the first day of the last period, and then adds 14 days. If you leave the length at 40 weeks, a birth date works exactly like a due date.
A worked example
Say your due date is 8 October 2026. Counting back 266 days lands on 15 January 2026: that is the most likely conception date. The window around it, two days either side, runs from 13 to 17 January.
Now say your baby was born on 17 September 2026, at 37+0, three weeks before that due date. A pregnancy of 37 weeks is 259 days, so the last period started on 1 January 2026, and conception most likely happened 14 days later, again on 15 January. The early birth does not change when the pregnancy began; it only means it was shorter.
What the result means
The calculator gives three things.
- The most likely conception date. This is the day of ovulation, when the egg and sperm meet. The egg lives for only about a day after it is released, so fertilisation happens on or very close to that day.
- A likely window of five days, two days either side of the estimate. Ovulation does not come on exactly day 14 for everyone: the time from ovulation to the next period is fairly stable, but the first half of the cycle varies from person to person and month to month.
- The fertile window leading up to it. Sperm can survive for several days, usually up to five, so sex in the five days before ovulation can still lead to a pregnancy. The sex that led to conception may therefore have happened a few days before the conception date itself.
This is why the date of conception and the date of the sex that started the pregnancy are not always the same day, and why nobody can pin either down to a single day from a due date alone.
How accurate it is
The answer can only be as accurate as the date you put in.
- A due date from an early scan is the best starting point. Early in pregnancy babies grow at very similar rates, so a scan between about 8 and 14 weeks dates the pregnancy to within about 5 to 7 days.
- A due date based only on the last period assumes a regular 28-day cycle. With longer or irregular cycles, ovulation may have been days or even weeks later than the calculator assumes.
- A date of birth is precise, but the length of the pregnancy at birth matters. Your birth records or the baby's health book often note it as weeks plus days, for example 38+2. If you do not know it, 40 weeks is the usual assumption, but most babies are born anywhere between 37 and 42 weeks, so the estimate can then be out by a couple of weeks.
After IVF the timing is known exactly, so you do not need an estimate: conception is, on paper, the day of egg collection and fertilisation, and your clinic will have the dates.
Limits of this calculator
This tool gives the textbook estimate, the same one a midwife would make with a pregnancy wheel. It cannot know your cycle length, when you actually ovulated, or how the pregnancy was dated. It is not precise enough to tell apart two dates a few days apart, so it should never be used to decide questions of paternity or to settle a disagreement. If you need the dates of your own pregnancy confirmed, your midwife or doctor can go through your scan results with you.