Conception Calculator

When was I conceived, or when did I get pregnant?

Estimate when conception occurred based on your due date or baby's birth date.

Quick answer: conception usually happens about 266 days (38 weeks) before the due date, or around 2 weeks after the first day of the last period. From a birthday, count back about 38 weeks. Enter a date below to see the exact estimate and the fertile window.
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How is the Conception Date Calculated?

The conception date calculator works backward from a known date — either your due date or your baby's birth date. Since pregnancy is typically 266 days (38 weeks) from conception, or 280 days (40 weeks) from the last menstrual period (LMP), the calculator subtracts these values to estimate when conception most likely occurred.

Keep in mind that the "conception date" is an estimate. Sperm can survive up to 5 days in the reproductive tract, so intercourse that led to conception may have occurred several days before the actual fertilization of the egg.

Conception Date vs. LMP Date

The last menstrual period (LMP) date is approximately 2 weeks before conception. Doctors use the LMP to calculate gestational age because most women know when their last period started, but few know the exact date of conception. The LMP-based calculation adds 280 days, while conception-based calculation adds 266 days — both arrive at the same due date.

Why Would I Want to Know My Conception Date?

Knowing your approximate conception date can help satisfy curiosity about when your baby was conceived, verify pregnancy dating, or help with paternity considerations. It can also help you understand the timeline of early pregnancy development.

Frequently Asked Questions

How do I calculate my conception date?
Subtract 266 days (38 weeks) from your due date. If you know the first day of your last period, add about 14 days instead: that is roughly when ovulation, and conception, happened in a 28-day cycle. The calculator above does the math and shows the likely fertile window around it.
When was I conceived based on my birthday?
Count back about 266 days (38 weeks) from your birthday if you were born around your due date. Babies born a week or two early or late shift the estimate by the same amount, so treat the result as a window of a few weeks, not an exact day.
Is the conception date the same as the date I had intercourse?
Not necessarily. Sperm can survive up to 5 days in the reproductive tract, so intercourse several days before ovulation can still lead to conception on the day of ovulation.
How accurate is a conception date calculator?
It gives an estimate within a window of about 5 days. Since sperm can live up to 5 days and the egg 12–24 hours, conception could have happened on any day of the fertile window shown. An early ultrasound dates a pregnancy more precisely than any calculator.
Why is the conception date about 2 weeks after the last period?
Pregnancy is dated from the first day of the last menstrual period (LMP), but ovulation usually happens about 14 days later. That's why doctors count 280 days from the LMP and 266 days from conception: both land on the same due date.

Further reading

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