Conception Date Calculator
Estimate when conception happened from a due date, last period or birth date.
About this tool
Choose what you know: the due date, the first day of your last period (with your usual cycle length) or the baby's birth date, then enter the date. The calculator shows the most likely conception date, the likely window around it, and the matching due date or last-period date, so you can see how the dates fit together.
How the conception date is estimated
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception. In a 28-day cycle, ovulation and conception happen about 14 days after the LMP, so the due date, LMP + 280 days, is about 266 days, or 38 weeks, after conception. Working back from a due date of 1 March 2027 gives a most likely conception date of 8 June 2026. With a longer cycle, ovulation happens later: a 32-day cycle moves conception to about day 18. Sperm can survive up to about five days and the egg about a day, so conception may have happened a few days either side of the estimate. Working back from a birth date is less precise, because only about 1 in 20 babies arrives on the due date and most are born within two weeks either side. Honest limits: an early dating ultrasound is the most accurate guide to when conception happened, and no date calculation can establish paternity. Not medical advice. The due date calculator works forward from the last period and the ovulation calculator finds fertile days.
Frequently asked questions
How accurate is a conception date calculator?
It gives a likely window of a few days. Cycle length varies and sperm can survive for days, so an early ultrasound is more accurate.
How many days before the due date is conception?
About 266 days, or 38 weeks, assuming a 28-day cycle with ovulation on day 14.
Can a conception date prove who the father is?
No. Dating can't establish paternity; only a DNA test can.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.