Understanding weeks of amenorrhoea and weeks of pregnancy

"Congratulations! How far along are you?" "18 WA, oh no, 16 WP…" But how many months is that exactly? Let us try to understand weeks of amenorrhoea and weeks of pregnancy, because it is very easy to get confused!

And as usual, your favourite blog Maman et Bébé Nature is here to help!

Calculating the length of pregnancy: how does it work?

First of all, it is important to know that a pregnancy lasts approximately 9 months, or between 270 and 280 days. To get a fairly accurate idea of your due date, you will need to know the date your last period began.

Concretely, when you take a pregnancy test and it comes back positive, you will need to go to a laboratory for a blood test. The HCG level will confirm that you are indeed pregnant.

To find out more, our article on "The key appointments during pregnancy" should shed some light!

Understanding weeks of amenorrhoea and weeks of pregnancy

Your gynaecologist speaks in terms of weeks of amenorrhoea (WA), but you may also hear the term weeks of pregnancy (WP). What does each one mean exactly?

WP is calculated from the presumed date of fertilisation — in other words, the first day of the foetus's life. Following this calculation, a pregnancy lasts 41 WA.

However, the date of ovulation is not a very reliable reference point, as every woman ovulates at a different time. That is why medical professionals prefer to use weeks of amenorrhoea, as it is more precise.

An example to make things clearer

Examples are always a good way of illustrating a point and making things that bit easier to understand.

Let us say that the first day of Mrs A.'s last period was 18 August 2021. Her ovulation period therefore took place approximately 2 weeks after that date.

Mrs A.'s EDD (Estimated Due Date) is therefore 25 May 2022.

In weeks of amenorrhoea (WA), the pregnancy is calculated from 18 August. And in weeks of pregnancy (WP), you simply add 2 weeks to that date.