You can estimate your conception date by counting 280 days back from your due date to the first day of your last period (ACOG). Then count forward to ovulation, which usually comes about 14 days before your next period (NHS). It is an estimate: fertilisation takes place in a fallopian tube, so the day cannot be seen (ACOG).
How is a conception date worked out?
ACOG (the American College of Obstetricians and Gynecologists) gives the average length of a pregnancy as 280 days, or 40 weeks. It counts from the first day of the last period. So 280 days before your due date is the first day of your last period. If you enter your last period instead, the calculator starts there. Conception, or fertilisation, is the union of an egg and a sperm into a single cell (ACOG).
The NHS (the UK’s National Health Service) says you are more likely to get pregnant around the time you ovulate (release an egg). This is usually about 14 days before the start of your next period (NHS). For most women the gap is 10 to 16 days (NHS). ACOG says the days between ovulation and the next period are the most consistent part of a cycle (ACOG), so the calculator counts back from the next period. To reach ovulation it counts forward your cycle length minus 14 days from the first day of the last period. An egg survives about 24 hours after ovulation (ACOG), so the calculator names the ovulation day as the estimated conception day.
The formula
First day of the last period = due date − 280 days
Ovulation ≈ first day of the last period + cycle length − 14 days
Estimated conception ≈ the ovulation day
Fertile window = ovulation − 5 days to ovulation + 1 day (starting no earlier than the day after a 5-day period)
A worked example
Your due date is and your cycle is 28 days long.
- First day of the last period
- ( − 280 days)
- Ovulation
- around ( + 28 days − 14 days)
- Estimated conception
- around
- Fertile window
- – August 2026
- In the Hijri calendar
- last period on , ovulation around , due date
Change the cycle to 35 days and the same due date gives ovulation around : seven days later, because the cycle is seven days longer.
Why is conception a window, not one day?
Several things keep the date uncertain. The NHS says it is difficult to pinpoint exactly when ovulation happens (NHS). Sperm can survive for several days: about 3 days, sometimes up to 5, according to ACOG (ACOG), and up to 7 days according to the NHS (NHS). So the day you had sex is not always the day you conceived, and conception can happen some days after sex. ACOG says you can become pregnant if you have sex anywhere from 5 days before ovulation until 1 day after (ACOG). That is why the calculator shows a window around its estimated day.
The calculator cannot know your own ovulation day. It uses the cycle length you type, and it works best when your cycles are regular. If they vary, take the date as a rough guide only.
Is a scan more accurate than counting back?
A date worked out from the calendar is only as good as the dates it starts from. The NHS says its 12-week scan can estimate more accurately how many weeks pregnant you are (NHS). In England the dating scan is offered at around 10 to 14 weeks (NHS). When the date of the last period is uncertain, ACOG says a first-trimester scan measures the embryo or fetus. A formula then estimates the gestational age from that measurement (ACOG). With a due date from a scan, the cycle length you type still moves the estimate, even though a scan does not use it.
After IVF (in vitro fertilisation), ACOG says the due date is set by the age of the embryo and the date it was transferred to the uterus (ACOG). So use your clinic’s dates, not this calculator’s.
When to see a doctor
If you think you might be pregnant, take a test (NHS); if it is positive, tell a doctor or midwife. A pregnancy test looks for the hormone hCG, which starts to be produced around 6 days after fertilisation (NHS). The NHS says tests are most reliable from the first day of a missed period (NHS). The pregnancy test date calculator gives the day your next period is due. If you are not sure of your dates, speak to a midwife or GP (family doctor) (NHS).
Trying to get pregnant? The ovulation calculator shows your fertile days for the next cycles. Read about the signs of ovulation too. Already expecting? The pregnancy week calculator counts your weeks and days, and the due date calculator turns a last period into a due date.
Estimates only, not medical advice: no calculator can tell you the real day you conceived.
Questions about your conception date
Can I find the exact day I conceived?
Is the conception date the day I had sex?
Why does my cycle length change the date?
My scan gave me a new due date. What happens to the estimate?
The dating scan works out how many weeks pregnant you are and your due date (NHS). Enter the new due date and the conception estimate moves with it.
Can I use this after IVF (in vitro fertilisation)?
No. After IVF, ACOG says the due date is set by the age of the embryo and the date it was transferred to the uterus (ACOG). Your clinic knows these dates; this calculator counts from a last period.
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under Sources. They have not reviewed it, and neither has a doctor. The calculator runs the formula above in your browser. Its ovulation and fertile-window rules are the ones the Period Calc app uses; counting back from a due date is this page’s own. The calculator works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see when to see a doctor.
Sources
- Periods and fertility in the menstrual cycle (NHS)
- Trying to get pregnant (NHS)
- Fertility Awareness-Based Methods of Family Planning (ACOG)
- How Your Fetus Grows During Pregnancy (ACOG)
- When Pregnancy Goes Past Your Due Date (ACOG)
- Pregnancy due date calculator (NHS)
- 12-week scan (NHS)
- Doing a pregnancy test (NHS)