Doctors date a pregnancy from the first day of the last period, about two weeks before conception actually happens. That’s why people often ask when they really conceived.
Start from a due date and count back, or start from your last period and count forward.
How to use the Conception Calculator#
- Choose what you know. Your due date, or the first day of your last period.
- Enter the date. If you use the period, add your cycle length too.
- Read the estimate. You get the likely conception date and a range around it.
How the estimate is worked out#
If your last period began on 1 January and your cycle is 28 days, ovulation, and so conception, was around 15 January. Your due date would be 280 days from 1 January, which is 8 October.
Why the date is a range#
Sperm can live for several days, and ovulation doesn’t always happen on day 14. A conception date is really a window of a few days, so the calculator shows a range of three days either side. Only a small share of babies arrive on their due date, and most are born within a couple of weeks of it.
Using it carefully#
An early ultrasound is more accurate for dating than your last period if your cycles vary, and your doctor may adjust the due date. This tool is for general information and shouldn’t be used to settle questions that need a medical or legal answer. See also the ovulation calculator.
How the dates line up#
Pregnancy is dated from the first day of the last period, not from conception, which is why a pregnancy is said to last 40 weeks when the baby has only developed for about 38. The calculator works from either end. From a due date, it counts back 266 days for conception and 280 days for the last period. From a last period with a 28-day cycle, ovulation falls on day 14 and the due date is day 280. For a cycle of 32 days, ovulation moves to day 18, and the due date moves 4 days later.
| Cycle length | Ovulation on day | Conception around | Due date after the last period |
|---|---|---|---|
| 24 days | 10 | Day 10 | 276 days |
| 28 days | 14 | Day 14 | 280 days |
| 32 days | 18 | Day 18 | 284 days |
| 35 days | 21 | Day 21 | 287 days |
Why the date is a window, not a day#
Sperm can survive in the body for up to five days, while an egg lives for around a day after ovulation. That makes the fertile window about six days, ending on the day of ovulation or the day after. Conception can therefore happen on any of those days, and a single act of intercourse may not coincide with the moment of fertilisation. The calculator shows the most likely three days either side of its estimate and the wider fertile window. Embryo implantation happens about a week later, and a pregnancy test is usually positive from about the day of a missed period.
Using the result with your doctor#
Doctors usually confirm the due date with an early ultrasound, which measures the baby and can correct a date based on a period if the two differ by more than a few days. Only about 4 in 100 babies arrive on the due date itself, and most are born in the two weeks either side. If you are trying to conceive and have not succeeded after a year, or after six months if you are over 35, speak to a doctor. The ovulation calculator helps you find the fertile days in a cycle, and the dog, cat and other animal calculators on the biology page do the same job for pets and livestock. This page is information only and does not replace medical advice.
Frequently asked questions#
Count back about 266 days from your due date, or add your cycle length minus 14 days to the start of your last period.
No. Pregnancy weeks are counted from the last period, about two weeks before conception.
Within a few days at best. An early ultrasound is more accurate.
About 40 weeks from the last period, or 38 weeks from conception.
It is a good estimate when the cycle is regular and you know the date of the last period well. If your cycles are longer, shorter or irregular, the date can be off by a week or more. An early ultrasound gives a more accurate measure, and your midwife or doctor will usually use it to confirm or adjust the date.