Conception Calculator

Conception = due date − 266 days; possible intercourse = conception − 5 days to conception + 1 day

Due date

The estimated date of delivery. Everything on this page is counted back from it, so it already contains your cycle length and does not need to be entered again.

266 days

The distance from ovulation to the due date. It is not the 280 days used when counting from your last period: the 14-day difference is the gap between the first day of that period and ovulation. (days)

Conception date

The estimated day of ovulation, which is also the day conception is most likely to have occurred. It is the middle of the three dates, not the earliest of them.

Earliest intercourse

Five days before conception. Sperm can survive in the body for about 3 days and sometimes up to 5, so days this early can still lead to the pregnancy. (days)

Latest intercourse

One day after conception. An egg survives for about 24 hours after ovulation, so the window closes a day later rather than opening symmetrically. (days)

38 weeks

The distance from conception to the due date, in weeks. It is 266 days divided by seven, and it is the number that makes the two-week offset visible. (weeks)

Use it when a due date is what you have: a scan report, a note from an appointment, or the result of the due date calculator. If you know the first day of your last period instead, the due date calculator and the pregnancy calculator both work forwards from it, and the ovulation calculator covers the fertile window in the current cycle.

1. A due date of 1 September 2026

   1. Subtract 266 days from 2026-09-01: the estimated conception date is 2025-12-09.
   2. The earliest day that could have led to it is 5 days before, 2025-12-04, which is how long sperm can survive inside the body.
   3. The latest is 1 day after, 2025-12-10, which is how long the egg survives after ovulation.
   4. From conception to the due date is 266 ÷ 7 = 38 weeks.

   The gap between the due date and the conception date is more than eight months, which surprises people who expect 40 weeks to be measured from conception. It is measured from the last period, and the two weeks before ovulation are part of it.

2. The due date the due date calculator produces for 1 January 2026

   1. Enter the due date that the due date calculator gives for a last period of 2026-01-01 and a 28-day cycle: 2026-10-08.
   2. Subtract 266 days: 2026-01-15.
   3. That date is 14 days after 2026-01-01, which is exactly where the ovulation calculator places ovulation for the same cycle.
   4. The window runs from 2026-01-10 to 2026-01-16.

   This is the check that keeps the four pages consistent with each other: a date produced forwards by one of them, fed backwards into this one, lands on the date a third one reports. If any page's offset drifted, this example would stop matching.

3. The earliest due date the field accepts, 1 January 1900

   1. 1900-01-01 minus 266 days falls in the previous year: 1899-04-10.
   2. The intercourse window is 1899-04-05 to 1899-04-11.
   3. The distance to the due date is still 38 weeks, because it does not depend on the date entered.

   The subtraction can cross a year boundary, and near the lower limit it crosses into the previous century. The field starts at 1900 for that reason: below it, subtracting 266 days reaches a year the date library will not represent.

This is a reverse calculation from a date that is itself an estimate. A due date is a midpoint, and ACOG notes that the 280-day rule assumes a regular 28-day cycle and an accurate memory of the last period, so a date that was revised after an ultrasound will move the conception date with it. A single day is also more precision than the method supports: the conception date is the most likely day rather than a day that can be identified. The window around it is the honest part of the answer, and it is still a range based on average survival times rather than on anything measured in your body.

Why subtract 266 days instead of 280?

Because 280 days is measured from the first day of your last period, and this page does not start there. Ovulation happens about 14 days after that first day, so the distance from ovulation to the due date is 280 minus 14, which is 266 days. Subtracting 280 from a due date would place conception about two weeks earlier than it was.

Can I find out the exact day I conceived?

No, and no arithmetic can. The conception date here is the estimated day of ovulation, which is the most likely day rather than a day that can be identified. Sperm survival is given as about 3 days and sometimes up to 5, which is why the answer that holds up is the window around the date rather than the date on its own.

Why is the answer only 38 weeks before my due date?

Because pregnancy is dated from the last period, not from conception, so 38 weeks from conception and 40 weeks of pregnancy describe the same due date. The two weeks in between are the days before ovulation, which are counted as part of pregnancy even though conception has not happened yet.

Does my cycle length change the answer?

Not if your due date is right. A longer cycle shifts ovulation later, which shifts the due date later by the same amount, so the 266 days between them stays fixed. There is no cycle length field on this page for that reason: entering it here would apply the same adjustment a second time.

What if my due date was changed after an ultrasound?

Then use the revised date. ACOG advises changing the estimated due date when a scan before 14 weeks differs from the last period by more than 7 days, and the revised date is the one your care is planned around. Whatever you enter here, the conception date moves with it, because this page is a consequence of the due date rather than an independent estimate.

Do I need to use this if I know my last period?

No. If the first day of your last period is what you have, the due date calculator works forwards from it and needs no reverse step. This page exists for the other situation: when a due date is the only figure you have been given, and you want to know what it implies about when you conceived.

• Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, United States, with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine (May 2017). Source of the 280-day convention and, through the in vitro fertilization dating rule it gives, of the 266 days between ovulation and the estimated due date that this page subtracts.
• Fertility Awareness-Based Methods of Family Planning (patient FAQ) — American College of Obstetricians and Gynecologists, United States. Source of the survival times behind the window: sperm for about 3 days and sometimes up to 5, an egg for about 24 hours, and the statement that pregnancy is possible from 5 days before ovulation until 1 day after it.
• Fetal development (Medical Encyclopedia) — MedlinePlus, United States National Library of Medicine. Explains that gestational age is measured from the first day of the last menstrual cycle rather than from conception, and that in the 40th week of pregnancy it has been 38 weeks since conception.