Calculation of birth date / when you are due

When you've found out that you're pregnant, it is exciting to find out when the birth can be expected to take place, i.e. when you are due.
If you have a regular menstrual cycle of 28 days, your due date can be calculated on the basis of the first day of your last period. The due date is therefore 40 full weeks later.

That is to say, that you (purely in terms of calculation) are already in the third week of pregnancy at the time conception takes place - you thus get approx. 2 weeks "free".

The due date can be calculated more accurately as follows:
Add one week to the first day of your last period. Then deduct three months from that and you've worked out the due date.

An example:
The first day of your last period was 1/1/2004. Plus one week gives the date of 8/1. Minus three months gives 8/10. In this example, a woman is thus due on 8/10/2004.

If your periods are irregular, the due date can be accurately determined using an ultrasound scan of the foetus. The result will be most accurate if the scan is done during the early stages of your pregnancy. The due date is calculated on the basis of the size of the foetus' head.

If you have a menstrual cycle longer than 28 days, fertilisation will be a little later than 14 days after the first day of your last period. This will mean that the due date us a little later than for the usual calculation.
If you have a shorter menstrual cycle, the due date will correspondingly be slightly earlier.

If you've also stopped taking the contraceptive pill, and haven't had a period since, the due date should under any circumstances be established by a scan.

It is important to establish a reasonably definite due date. This will make it easier to determine whether the baby is growing correctly, whether anything is wrong, and to decide whether the birth should be induced if you've gone too far past the due date.

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