Reverse Due Date Calculator
Work back from a due date, or from the day the baby actually arrived. Other tools only take the due date, which is the number that turned out to be wrong.
Which day was most likely
Odds that sex on each day led to this pregnancy, from Wilcox, Weinberg & Baird (NEJM, 1995).
Those days on a calendar
Sex could have caused it
Conception
Why the weeks at birth matter
Between 4 and 5 percent of babies are born on their due date. Everyone else arrives early or late, and a reverse calculation that assumes 40 weeks inherits that whole error.
A baby born at 37 weeks was conceived three weeks later than a naive count from the birth date suggests. One born at 41 weeks was conceived a week earlier. Your discharge notes or red book will have the gestational age at birth written on them.
| Working back from | Subtract | Gives you |
|---|
| Due date | 266 days | Conception |
| Due date | 280 days | First day of last period |
| Birth date | gestational age at birth, minus 2 weeks | Conception |
One date is never the honest answer
Conception happens at ovulation, but sperm survive up to five days. Sex on any of six days can produce a conception on the last of them, which is why the answer above gives a span as well as a single most likely date.
Stack the errors before you conclude anything. A six-day fertile window, plus five to seven days of uncertainty in a scan-based due date, plus more if the date came from a remembered period. A DNA test is the only thing that settles paternity.
Questions people actually ask
Why does the conception date land after the due date was set?
It should not. If it does, check whether you entered a birth date in the due date field. Pregnancy is counted from the last period, roughly two weeks before conception, so conception always sits about 266 days before the due date.
Can this tell me who the father is?
No. The window is six days wide and the date it starts from carries days of error on top. Where two possibilities both fall inside that span, only a DNA test answers it.
I want a baby born in a particular month. When do I need to conceive?
Set the tool to a due date in the middle of your target month. The conception date it returns is the ovulation you would need to hit, and the span is the week to aim at.
What if the pregnancy was IVF?
Then the date is exact and there is no window. Fertilisation happened in the lab on a recorded day. Use the IVF calculator instead.