Conception calculator

How the conception date is estimated

A due date is set 280 days after the first day of the last period, on the assumption that ovulation came 14 days after it. Working backward, the estimated day of ovulation, and of conception, is 266 days before the due date. From a last period, it’s 14 days before the next period would have started, so a longer cycle moves it later.

Conception itself happens within about a day of ovulation, but the sex that led to it may have been several days earlier. ACOG notes that sperm can survive up to 5 days and an egg about 24 hours, so sex from 5 days before ovulation until 1 day after can lead to pregnancy. The calculator shows that seven-day window. The days aren’t equally likely: in Wilcox and colleagues’ 1995 study, the chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% for sex on the day of ovulation.

Worked examples

• From a due date: due February 14, 2027. Ovulation was most likely around May 24, 2026, and conception most likely happened between May 19 and May 25. Implantation usually followed between Jun 1 and Jun 3.
• From a last period, 28-day cycle: a period on March 1, 2026 puts ovulation around Mar 15 and the window at Mar 10–16.
• Same period, 35-day cycle: ovulation moves a week later, to about Mar 22, and the window to Mar 17–23.

Conception windows for some due dates

Each row counts back 266 days from the due date to ovulation, about 38 weeks, so a due date in early autumn goes back to the December before.

Implantation usually follows 8 to 10 days after ovulation: for the Jun 30 due date, around Oct 15–17, a few days before the next period would have been due.

Why it’s a window, not a day

Three things blur the date. First, sperm survival: the sex that led to a pregnancy can be up to five days before ovulation. Second, ovulation moves: the time from ovulation to the next period averaged 14.1 days with a standard deviation of 1.4 in a study of 327 cycles, and a 2000 study found the fertile window highly unpredictable even in regular cycles. Third, the due date is itself an estimate: ACOG Committee Opinion 700 rates an early scan at about ±5 to 7 days, and later scans less precisely. Treat the window as the most likely week, with a few days of give either side.

Working back from a birth date is looser still. In a study of 125 pregnancies where ovulation was measured with daily hormone tests, the time from ovulation to birth had a median of 268 days and varied by 37 days, even leaving out preterm births.

Dates and paternity

Many people look up a conception date because they want to know who the father is. It’s an understandable question, and the honest answer is that dates can rarely settle it. If sex with two different partners happened within about a week, both dates can fit the same window, because of how long sperm survive and how much ovulation varies. Scans don’t narrow it enough either. Only a DNA paternity test can establish paternity. A doctor, midwife or a confidential clinic can explain the options and support you, whatever the situation.

The early weeks

After fertilization, the embryo takes several days to reach the uterus. Wilcox and colleagues found that implantation happened 6 to 12 days after ovulation, most often on day 8 to 10. The pregnancy hormone hCG appears only from then, which is why a test can’t pick up a pregnancy straight after conception. Because pregnancy weeks are counted from the last period, implantation 8 to 10 days after ovulation falls at about 3 weeks 1 day to 3 weeks 3 days of pregnancy. By the time a period would be due, about 14 days after ovulation, you’re counted as 4 weeks pregnant, two weeks after conception.

For your due date, trimesters and term dates, use the due date calculator, which also handles IVF transfers and dating scans. If you’re planning a pregnancy, the ovulation calculator estimates your next fertile windows, and the sleep calculator can help with rest along the way.

Sources

• American College of Obstetricians and Gynecologists. Methods for estimating the due date. Committee Opinion 700, Obstetrics & Gynecology, 2017 (reaffirmed 2025).
• American College of Obstetricians and Gynecologists. Fertility awareness-based methods of family planning. Patient FAQ, 2019.
• Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995.
• Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. New England Journal of Medicine, 1999.
• Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle. BMJ, 2000.
• Lenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle. British Journal of Obstetrics and Gynaecology, 1984.
• Jukic AM, Baird DD, Weinberg CR et al. Length of human pregnancy and contributors to its natural variation. Human Reproduction, 2013.

An estimate from averages, not medical advice. A doctor or midwife can help you make sense of your dates and scans, and it’s always fine to ask.