Conception Calculator - When Did I Conceive?

How conception date is estimated

Pregnancy is dated from the first day of the last menstrual period (LMP), about two weeks before conception actually happens. So a conception calculator works backward from the same anchor your provider uses.

From LMP: conception ≈ LMP + (cycle length − 14) days. In a 28-day cycle that is day 14. In a 32-day cycle, day 18. The 14 is the typical luteal phase, the fairly stable stretch between ovulation and the next period. The due date is then conception + 266 days (38 weeks), which is the same date Naegele's rule gives (LMP + 280 days) for a 28-day cycle.

From a due date: conception ≈ due date − 266 days. If your due date came from an early ultrasound, use this mode. First-trimester ultrasound measurements are generally more accurate than LMP dating, especially when cycles are irregular or the LMP date is uncertain.

Why it gives a range, not a day

Three sources of uncertainty stack up. Ovulation can shift by several days even in regular cycles. Sperm can survive up to 5 days in the reproductive tract, so sex several days before ovulation can lead to pregnancy. And the egg lives only about 12-24 hours after release, so conception happens around ovulation day, not at the time of sex.

The landmark study here is Wilcox, Weinberg and Baird (New England Journal of Medicine, 1995). Tracking 221 women with daily hormone tests, they found conception only followed intercourse during a six-day window ending on the day of ovulation. The chance per act ranged from about 10% five days before ovulation to about 33% on ovulation day. That six-day window is what the calculator shows as the fertile window, and the ±3-day range reflects normal variation in when ovulation happened.

Worked example

LMP: January 1, 2026. Cycle: 30 days. Conception ≈ January 1 + (30 − 14) = January 17. Likely range: January 14 to January 20. Fertile window for intercourse: January 12 through January 17. Due date: January 17 + 266 days = October 10, 2026, which matches Naegele's rule adjusted for a 30-day cycle (January 1 + 280 + 2 days).

Due-date mode: an ultrasound due date of October 15, 2026 gives conception ≈ October 15 − 266 days = January 22, 2026, and an implied LMP of January 8.

It cannot settle paternity

Many people search this because they need to know who the father is. A calculator cannot answer that. If two partners were within about a week of each other around the estimated conception date, the dates alone cannot separate them, and even an early ultrasound has a margin of several days. The only reliable answer is a DNA paternity test. Non-invasive prenatal paternity tests can be done from a blood draw from about 7-8 weeks of pregnancy; otherwise testing can be done after birth.

Common mistakes

Using the last day of the period instead of the first.

Assuming ovulation is always day 14. It is only day 14 in a 28-day cycle with a 14-day luteal phase.

Mixing gestational age and fetal age. A pregnancy "8 weeks" along is about 6 weeks after conception, because dating starts at LMP.

Ignoring the ultrasound date. If your provider changed your due date after a scan, use the new date in due-date mode.

Using this for IVF. With IVF, conception timing is known from the transfer date and embryo age. Your clinic's dates are exact; a calendar estimate is not needed.

When to talk to your provider

Contact your OB or midwife if your cycles are irregular and you are unsure of your LMP, if you had bleeding in early pregnancy that might have been mistaken for a period, or if the date matters for medical decisions like screening windows. Seek urgent care for one-sided pelvic pain, heavy bleeding, or shoulder-tip pain in early pregnancy, which can signal an ectopic pregnancy. For how far along you are and trimester dates, use the due date calculator.