Conception Calculator
Work back from a due date, a last period, or a scan. Sperm live up to five days, so you get the six days that could be responsible.
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
Conception and sex are not the same day
Conception is the moment a sperm fertilises the egg, which happens within about a day of ovulation. Sperm can wait. In fertile cervical mucus they survive up to five days, so sex on the Monday can cause a conception on the Friday.
A calculator that returns a single date is really returning the ovulation date. The span above is the honest version of the same answer.
How much error is in the answer
The output inherits the error of whatever you fed it.
| Starting point | Typical error | Why |
|---|
| First-trimester scan | ±5 to 7 days | Crown-rump length varies little this early |
| Due date from a clinic | ±5 to 7 days | Usually set by that same scan |
| Last period, regular cycles | ±5 days | Ovulation still moves a little |
| Last period, irregular cycles | ±2 weeks | Ovulation day is the unknown |
| Scan after 22 weeks | ±2 weeks | Babies grow at different rates by then |
Add the six-day window to the error above before drawing any conclusion from this. A DNA test is the only thing that settles paternity, and it can be done from a blood sample at about 8 weeks without touching the pregnancy.
Questions people actually ask
Is the conception date the same as the day we had sex?
Usually not. Fertilisation happens at ovulation. If sex was several days earlier, the sperm were waiting. Only sex on the day of ovulation itself makes the two dates line up.
Why does my due date suggest a conception date before I was pregnant?
Pregnancy is counted from the first day of your last period, which is about two weeks before you conceived. At the moment of conception you are already counted as two weeks pregnant. It confuses everyone and every clinic uses it.
Can a conception date settle who the father is?
No. The window is six days wide and the date it is built on carries its own error of five days or more. If two possibilities fall inside that span, only a DNA test answers it.
What if I conceived on birth control or right after stopping it?
The dates still work the same way, but your first cycle off hormonal contraception can be unusually long or short, which widens the error on any calculation from a last period. A scan is worth more than arithmetic here.
Does IVF change this?
Yes, and it makes it exact. Conception happened in the lab on the day of fertilisation, and your clinic recorded it. For a day-5 transfer, fertilisation was five days before the transfer date.