How to calculate a due date

Open the due date calculator with the date of the last period at hand. Use the cycle length from a period app if you have one. An estimate, not medical advice; an early ultrasound overrides it. Nothing is uploaded.

1. Know Naegele's rule

   Pregnancy is counted from the first day of the last period, not from conception. Due date = that day + 280 days (40 weeks) for a 28-day cycle. A longer cycle ovulates later, so each extra day moves the due date a day later.

   

2. Open Due date calculator on cv.cm

   Last period is selected with a sample date and a 28-day cycle. The due date and today's week sit on the right. Nothing is uploaded.

   Open Due date calculator

3. Enter the last period and cycle length

   Type the first day of the last period, for example 1 January. Leave 28 days or change it: 32 days moves the due date from 8 October to 12 October.

4. Read today's week and trimester

   On 12 March that pregnancy is 10 weeks 0 days, first trimester, 210 days to go. The second trimester starts at 13 weeks (2 April), the third at 27 weeks (9 July).

   

5. Or start from conception or a known due date

   Switch I know my to Conception date (due date is 266 days later) or to Due date from a scan. The weeks, trimesters and other dates follow from it.

6. Treat it as a window, then copy

   Only about 4 in 100 babies arrive on the due date; most come within two weeks either side. Copy puts the dates on the clipboard for the first appointment.