Counting back from a birth
Pregnancy lasts about 280 days from the last period, or 266 days from conception. Given only a birth date, the best single guess is 266 days earlier. The catch is that "full term" is a five-week range, so unless the gestation at birth is known the answer is a window, not a day.
If you know the pregnancy lasted, say, 38 weeks and 2 days, enter that. The calculator then subtracts that gestation minus 14 days and narrows the window to a few days of dating error.
What the answer can and cannot tell you
It can tell you roughly when a pregnancy started, which is what most people want for curiosity or for a timeline. It cannot settle paternity: sperm survive up to five days, dating carries error, and two candidates can easily both fall inside a two-week window. If that is the question, a DNA test is the only reliable route. For the forward direction, from conception to a due date, use the due date by conception calculator.
Frequently asked questions
How do you calculate a conception date from a birthday?
A full-term pregnancy is about 38 weeks from conception, so the calculator subtracts 266 days from the birth date. If the pregnancy is known to have lasted a different number of weeks, for example a baby born at 37 weeks, it subtracts that gestation minus the two weeks before ovulation instead.
How accurate is a conception date worked back from a birth date?
Without knowing the gestation at birth it is only a window. Babies are born anywhere from 37 to 42 weeks, so the true conception date could be about ten days either side of the estimate. With a known gestation, the estimate narrows to a few days, limited by how precisely the pregnancy itself was dated.
Can this calculator tell me who the father is?
No. The window is usually one to three weeks wide, sperm can survive up to five days before ovulation, and pregnancy dating carries its own error. Date maths can only say who is and is not plausible; it cannot decide between two people whose dates both fall in the window. A DNA test is the only reliable answer.
Why is the conception date two weeks after the last period?
Ovulation and conception happen around the middle of the cycle, roughly 14 days after a period starts in a 28-day cycle. Pregnancy is counted from that period, which is why a 40-week pregnancy contains only about 38 weeks of actual fetal development.
I am pregnant now and want my conception date, not a baby's
Use the conception date calculator, which works from your current due date or last period. This page is for looking back from a birth that has already happened.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.