Pregnancy & Due Date Calculator

How pregnancy is calculated

Pregnancy lasts about 280 days (40 weeks), counted from the first day of your last menstrual period. That first day is treated as the start of pregnancy when counting the weeks, even though conception usually happens about two weeks later — fetal development typically lags the menstrual date by around two weeks.

Calculating a due date is not an exact science: only a small share of women give birth on the estimated day, but it gives a useful approximate window.

How the due date is calculated

For a regular 28-day cycle there are two common methods:

Naegele's rule

A simple formula: add 7 days to the first day of the last period, then subtract 3 months, to get the estimated due date. For example, if the last period began on 1 November, adding 7 days gives 8 November, and subtracting 3 months gives an estimated due date around 8 August of the following year.

The pregnancy wheel

Most doctors use a pregnancy wheel: you line the pointer up with the first day of the last period, and the wheel shows the expected delivery date. It's the same idea as the calculator above, just in physical form.

If you don't remember your last period

This is more common than you might think, and there are still ways to estimate the date:

• If you know roughly which week your period fell in, a doctor can approximate the due date.
• If you can't recall it at all, an ultrasound scan can determine gestational age and due date.

Irregular or longer cycles

Some women consistently have longer cycles. The second half of the cycle is always about 14 days (the time between ovulation and the next period), so ovulation shifts later in a longer cycle — for a 35-day cycle it's around day 21. Knowing roughly when ovulation occurs lets a doctor adjust the last-period date and recalculate the due date accordingly.

Why a doctor might change the due date

A doctor may adjust the expected date if the fetus measures noticeably smaller or larger than average for its gestational age. An ultrasound is used to determine gestational age when ovulation is irregular, the last period is uncertain, or pregnancy occurs despite oral contraceptives.