When someone asks what week you are in, the answer can be two different things, because it depends on how you calculate. Are you in week 18 when you are in week 17+2, or when you are in week 18+2? In this article, we clear up all the question marks about how to calculate the due date and why you count weeks of pregnancy.
We rarely talk about which month of pregnancy you are in during a pregnancy, instead we talk about weeks. How you, as an expectant parent, calculate which week you are in is often different from how healthcare professionals calculate.
What week am I in? Don't you count months of pregnancy?
Midwives and other healthcare professionals always count the full week that you have completed. That is, if you are in week 26+2, health professionals say you are in week 26. Expectant parents often count the week you are "in", that is, if you are in week 26+2, they say you are in week 27.
The pregnancy is considered full term when 37 full weeks have passed, i.e. week 37+0 (when you "enter" week 38). If a baby is born before week 37+0, it is considered premature. If the pregnancy goes to week 42+0, the baby is considered overdue.
Most pregnancy platforms count based on the week you are currently in. So if you are in week 24+2, you are in week 25!
How do you actually calculate BF (estimated date of birth)?
The first step to figuring out how far along your pregnancy is, will be to identify the first day of your last period. Then you calculate a month of 28 days (four weeks), which is also the average length of a menstrual cycle. This will tell you how far along your pregnancy is.
This means that the first day of your last period is also the first day of your pregnancy, even though you haven't actually conceived yet because conception happens at ovulation, around week 2+0.
Based on which week you are in; you will also get a provisional estimated due date but the correct estimated date of delivery is often given later in the pregnancy when you have an ultrasound. This happens either around week 13, and/or around week 19 when routine ultrasound is usually performed. The midwife/doctor then measures the growth of the fetus and calculates how many weeks you have been pregnant. This may therefore move your due date forward or back a few days. But it may also be the same estimated date you were given in the first place.
If you have an extra short or long menstrual cycle, this date can be moved forward or back many, many days.
What is pregnancy month vs calendar month in a pregnancy?
Sometimes midwives/doctors can talk about calendar month and month of pregnancy (as if it wasn't complicated enough anyway!). A calendar month counts as 30 days and is calculated from the date you actually became pregnant, i.e. the day you ovulated. According to the calculation method, you are then in week 2+0.
A month of pregnancy, also known as a lunar month, is counted from the first day of your last period and lasts 28 days. This is the time when a pregnancy test does not yet show that you are pregnant. You will usually find out around week 5+0.
This means that the pregnancy (40 weeks) actually lasts for 10 months from the first day of your period. The 9-month period that is usually talked about is therefore based on the period from about week 4-5 when you can see or find out that you are pregnant, up to the date of delivery.
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