Conception Calculator
Free conception calculator: estimate when your baby was conceived from the due date, the birth date or the last period, with the fertile window and due date.
This is an estimate. Sperm can survive up to five days and ovulation does not always happen on the same day of the cycle, so the real date can be several days earlier or later. An ultrasound early in the pregnancy dates it more precisely than any calculation.
Also: Pregnancy Due Date Calculator Β· Due Date Calculator Β· Ovulation Calculator
What it does
Doctors count a pregnancy from the first day of the last period, and conception usually happens about two weeks later, at ovulation. A due date is 280 days (40 weeks) after that period, so conception is about 266 days before the due date. This calculator runs that count backwards from the due date, from the birth date or from the last period.
How to use it
- Choose what you know: the due date, the birth date or the first day of the last period.
- Enter the date; for a birth date, add the weeks of pregnancy at birth if the baby came early or late, and for the last period, your usual cycle length.
- Read the estimated conception date and the fertile window around it.
The arithmetic runs in your browser. Nothing you type is sent anywhere and nothing is stored, so you can use these with real numbers.
Frequently asked questions
How accurate is a conception date?
It is accurate to within a few days at best. Cycles vary, ovulation can come early or late and sperm can live for days, so the calculator shows a fertile window of six days rather than a single day.
Why are there two weeks between the last period and conception?
In a 28-day cycle the egg is released around day 14, and that is when conception can happen. With a longer or shorter cycle ovulation moves by the same number of days, which is why the calculator asks for your cycle length.