How your conception date is estimated
Conception usually happens on or within a day of ovulation — about 266 days (38 weeks) before your due date, or about 14 days after the first day of your last period in a 28-day cycle.
- From a due date: conception ≈ due date − 266 days.
- From your last period: conception ≈ LMP + (cycle length − 14) days. For a 28-day cycle that’s day 14; for a 32-day cycle, day 18.
- From a birth date: conception ≈ birthday − 266 days, assuming the baby was born on their due date.
Want to go the other way — from conception to due date? Use the due date calculator and pick “Conception date”.
The conception window: why it’s a range
Pregnancy is only possible on about six days of each cycle: the five days before ovulation and ovulation day itself. Sperm can live in the reproductive tract for up to five days, while the egg survives only 12–24 hours after it’s released. In a landmark study of couples trying to conceive, every pregnancy came from intercourse in that six-day window (Wilcox et al., 1995).
That’s why the calculator shows two things:
- Conception (fertilization) date — the estimated day of ovulation, when sperm met egg.
- Likely intercourse window — the 5 days before through 1 day after that date.
About 6–12 days after fertilization, the embryo implants in the uterus (most often on days 8–10). After implantation the body starts making hCG, the hormone home pregnancy tests detect — see when to take a pregnancy test.
Reverse due date chart
Here’s the estimated conception date for a due date on the 1st of each month:
| Due date | Estimated conception date (previous year for most months) |
|---|---|
| January 1 | April 10 |
| February 1 | May 11 |
| March 1 | June 8 |
| April 1 | July 9 |
| May 1 | August 8 |
| June 1 | September 8 |
| July 1 | October 8 |
| August 1 | November 8 |
| September 1 | December 9 |
| October 1 | January 8 |
| November 1 | February 8 |
| December 1 | March 10 |
How accurate is it — and can it prove paternity?
The estimate is only as good as the date you start from. A due date from a first-trimester ultrasound is accurate to about a week; one based on your last period can be off further if your cycles are irregular. Ovulation itself can shift by several days from cycle to cycle, even in people with regular periods.
A conception calculator cannot prove paternity. When two possible dates are close together, the conception window often covers both. Only a DNA test — which can be done safely during pregnancy — can confirm who the father is.
Trying to conceive right now? Our ovulation calculator predicts your fertile days for the next six cycles.
Frequently asked questions
How do I calculate my conception date from my due date?
Subtract 266 days (38 weeks) from your due date. If your baby is due on October 8, conception most likely happened around January 15. This is the reverse of the standard due-date formula.
Can a conception calculator tell me the exact day I got pregnant?
No. It estimates the day you ovulated, which is when fertilization usually happens. Sperm can survive up to 5 days, so the intercourse that led to pregnancy could have happened up to 5 days before that date, or on the day itself.
Can a conception date prove paternity?
No. Because sperm can survive for days and ovulation timing varies, a calculated conception date usually leaves a window of a week or more. Only a DNA test can establish paternity.
How accurate is conception dating by ultrasound?
A first-trimester ultrasound dates a pregnancy to within about 5–7 days. If your due date came from an early scan, the conception estimate is about that accurate; if it came from your last period, it depends on how regular your cycles are.
When was I conceived if I was born on my due date?
Count back 266 days from your birthday. If you were born early or late, the true date shifts by the same number of days, so this is only a rough guide unless you know your original due date.
Is a reverse due date calculator the same as a conception calculator?
Yes. A reverse due date calculator starts from a due date and works backwards to the conception date and last period — exactly what this tool does.
Sources
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21
- Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 1999;340:1796–9
- MedlinePlus — Fetal development