Conception Calculator (conception date from due date or birthday)

Understanding the conception calculator — concept and principle

This calculator estimates when conception probably happened from a due date or, if the baby is already born, from the birth date and the weeks of pregnancy at birth. It shows the estimated conception date, the likely window for the sex that led to pregnancy, the corresponding first day of the last period, the due date and the weeks from conception. Parents curious about when their pregnancy began use it.

Count back from the due date — with the default inputs: Fri 17 Jul 2026.

How it is calculated

From a due date: conception ≈ due date − 266 days
From a birth date: conception ≈ birth date − (gestational age at birth − 14 days)
Likely window for sex: the 6 days ending on the conception date
First day of last period ≈ conception − 14 days; due date = conception + 266 days

The principle

Pregnancy is dated from the first day of the last menstrual period, and a standard pregnancy lasts 280 days, 40 weeks, from that date. In a 28-day cycle, ovulation and conception happen about 14 days after the period starts, so conception is about 266 days before the due date. Sperm can survive up to about 5 days while the egg lives about a day, so the sex that leads to pregnancy usually happens in the 6 days ending on ovulation day, as Wilcox and colleagues showed in 1995.

Worked example

Defaults: due date 150 days from today (shown for 10 November 2026: Fri, Apr 9, 2027).

Conception = due date − 266 days = Fri, Jul 17, 2026.
Likely window: Sun, Jul 12, 2026 to Fri, Jul 17, 2026.
First day of last period = Fri, Jul 3, 2026; due date = Fri, Apr 9, 2027.
Weeks from conception to the due date = 266 ÷ 7 = 38.

How to read the result

The date is an estimate. A cycle longer or shorter than 28 days, or a due date set by an early ultrasound, shifts the real date by several days. Even with known intercourse dates, conception can happen up to about five days later. For medical dating, the due date from your care team is what counts.

Common mistakes

Assuming conception happened on the day of intercourse.
Using a 28-day rule with a much longer or shorter cycle.
Forgetting to enter the weeks at birth when using a birth date.
Using the estimate to settle questions of paternity; it is not precise enough.

Keep in mind: This is an estimate for general interest, not medical or legal evidence. Ask your midwife or doctor about dating your pregnancy.

Frequently asked questions

How is conception date calculated from a due date?

Subtract 266 days (38 weeks), assuming a 28-day cycle.

Why is the window six days long?

Sperm can survive up to about five days and the egg about one day.

How accurate is the estimate?

Usually within several days; cycle length and dating method can shift it.

Sources: ACOG Committee Opinion No. 700, "Methods for Estimating the Due Date" (2017) · Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", N Engl J Med 1995;333:1517–1521

Checked 9 Nov 2026. ACOG Committee Opinion No. 700, “Methods for Estimating the Due Date” (2017; 280 days from the LMP, conception about 2 weeks after the LMP); Wilcox AJ, Weinberg CR, Baird DD, “Timing of sexual intercourse in relation to ovulation”, N Engl J Med 1995;333:1517–1521 (six-day fertile window ending on ovulation day).

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