One of the most common questions among new mothers is: when did conception occur? Knowing the approximate date has emotional value and helps you understand your pregnancy timeline in relation to medical tests.
Enter your due date or last period date to find out when conception likely occurred.
Use the Free CalculatorThere are two main methods, both based on statistical averages:
If you know your estimated due date, subtract 266 days (38 weeks) to get the probable conception date. This works because pregnancy lasts an average of 266 days from fertilization.
If you remember the first day of your last period, add 14 days — this is the average ovulation day in a 28-day cycle. Conception typically occurs within 24 hours of ovulation.
Conception doesn't happen on a single exact day — it occurs within a fertile window of about 6 days: the 5 days before ovulation plus the day of ovulation itself. This is why the calculator gives a date range rather than a single day.
The standard calculation assumes a regular 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, ovulation shifts accordingly and the estimated conception date may be off by several days.
In such cases, a first-trimester ultrasound (performed between weeks 10–13) measures the crown-rump length of the embryo and can determine gestational age with ±5–7 days accuracy.
No. Even an ultrasound provides an estimate (±5–7 days), not a certainty. Mathematical calculation is a good starting point but less precise.
Not necessarily. Sperm can survive for days waiting for ovulation. Conception may occur hours or days after intercourse.
Because the LMP date is easier to remember and verify. In reality, no embryo exists in the first two weeks of "pregnancy" — conception occurs around week two.
This article is for informational purposes only. Always consult your doctor for personalized medical advice.