Due Date Calculator
The standard formula assumes you ovulate on day 14. If your cycle is not 28 days, your due date moves. This one adjusts.
When babies actually arrive
When labour started on its own in low-risk first-time mothers. Inductions and planned caesareans are excluded, so real birth dates skew earlier than this.
Why the standard formula is wrong for most people
Naegele's rule adds 280 days to the first day of your last period. It was published in the early 1800s and it bakes in one assumption: that you ovulated on day 14.
Ovulation happens about 14 days before your next period, not 14 days after your last one. On a 35-day cycle you ovulate around day 21, so conception happened a week later than the formula assumes. Your pregnancy is a week younger than the standard date says, and your due date belongs a week later.
This calculator adds the difference between your cycle length and 28 days. A 32-day cycle pushes the date four days later. A 24-day cycle pulls it four days earlier.
Your clinic will probably still use the unadjusted date until a scan overrides it. Bring your cycle length to the first appointment, because a due date that is a week out changes when people start talking about induction.
A due date is a middle, not a deadline
Somewhere between 4 and 5 percent of babies are born on their due date. Looking at when labour starts by itself in low-risk first-time mothers, the median was 40.1 weeks, the single most likely week was the one after the estimated due date, and about one in five had not started by 41 weeks.
First babies tend to come later than later ones. If this is your first, the far side of the range is more likely than the near side.
Questions people actually ask
Which date wins, my dates or the ultrasound?
The ultrasound, when it was done in the first trimester. Measuring crown-rump length before 14 weeks dates a pregnancy to within about five to seven days, which beats a remembered period date. After about 22 weeks, scan measurements spread out and the earlier estimate is normally kept.
What if I do not know my last period?
Ask for an early scan. If you know roughly when you conceived, add 266 days to that date and use the setting above. Both are better than guessing at a period you are unsure of.
Does IVF change the calculation?
Yes, and it makes it more exact, because the date of transfer is known. For a day-5 blastocyst transfer the due date is the transfer date plus 261 days. For a day-3 transfer it is the transfer date plus 263 days. Your clinic will have given you a date already, and theirs is the one to use.
How many weeks pregnant am I?
Counted from the first day of your last period, which means you are counted as two weeks pregnant at the moment you conceive. That convention confuses everyone and it is what every clinic uses. The answer above shows your week and day in the standard format.