Health · Pregnancy
Conception date calculator.
Enter a due date, a birth date or the first day of the last period and see the estimated conception date and the days intercourse most likely happened.
Inputs
Estimated conception
Choose a date and press Calculate.
How this is calculated
Pregnancy is counted from the first day of the last menstrual period (LMP), and the due date is 40 weeks, or 280 days, after it (NICHD; ACOG Committee Opinion 700, Methods for Estimating the Due Date). Conception is taken as ovulation, about 14 days after the LMP in a 28-day cycle, so conception = due date − 266 days. The same 266-day figure is the “assumed ovulation to delivery” length behind Naegele’s rule (Mittendorf et al. 1990). If your cycle is not 28 days, ovulation is placed at LMP + (cycle length − 14).
If you enter a birth date, the calculator subtracts the gestational age at birth (40 weeks 0 days unless you change it) to get the LMP equivalent, then applies the same rule. If you enter an LMP, it adds your ovulation offset directly.
Likely intercourse window: in 625 cycles, conception occurred only when intercourse took place in the six days ending on the day of ovulation (Wilcox, Weinberg and Baird, NEJM 1995), so the window is the estimated conception date and the 5 days before it. In 125 natural pregnancies with measured ovulation, pregnancy length varied over 37 days (Jukic et al., Human Reproduction 2013), so the estimated conception day is not a medically bounded interval. Ultrasound sets the official due date; this tool is not medical advice. Sources checked 2026-10-02.
Frequently asked questions
How do you calculate the conception date from a due date?
Subtract 266 days from the due date. A due date is 280 days (40 weeks) after the first day of the last period, and ovulation usually falls about 14 days after that day, so conception is about 280 minus 14 = 266 days before the due date. Enter your due date above and the calculator does the subtraction for you.
Can I find the conception date from a baby's birth date?
Yes. Choose “Birth date”, enter the date and the gestational age at birth (40 weeks 0 days if you do not know it). The calculator works back to the last period equivalent and then to ovulation. A baby born early or late shifts the answer, so use the real gestational age if your records show it.
How accurate is an estimated conception date?
It is an estimate, not a day that rules other dates in or out. It assumes ovulation 14 days after the last period, but cycles and pregnancy length vary. Use an obstetric estimate from your clinician for medical decisions.
What dates could intercourse have happened?
In a study of 625 cycles (Wilcox et al. 1995), conception happened only when intercourse fell in the six days ending on the day of ovulation. So the likely intercourse window runs from 5 days before the estimated conception date to the estimated conception date itself. If the due date came from an early ultrasound, the window is still only an estimate.
Why is my conception date different from my ultrasound date?
Ultrasound dating measures the fetus and sets the official due date; it does not record when conception happened. ACOG recommends using the best obstetric estimate, from the last period and early ultrasound, for the due date. A conception date worked back from that due date is an estimate, and ultrasound can be a few days either way.
What percent of babies are born on their due date?
Only about 4 percent, a figure commonly quoted by obstetric sources. Most babies arrive in the weeks either side of the due date, which is one reason a conception date worked back from the due date is a range and not a fact.