When someone asks you what week you are in, the answer can be two different ones, because it depends on how you count. Are you in week 18 when you are in week 17+2, or when you are in week 18+2? In this article, we'll clear up all the questions about how to calculate BF and why we count weeks of pregnancy.

People rarely talk about what month of pregnancy you are in during a pregnancy, instead they talk about weeks. The way you, as an expectant parent, calculate which week you are in often differs from how health professionals count.

What week am I in? Don't you count the month of pregnancy?

Midwives and other health 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 at 37 completed weeks, i.e. week 37+0 (when you 'enter' week 38). If a baby is born before week 37+0, the baby is considered premature. If the pregnancy continues until week 42+0, the baby is considered overweight.

Most pregnancy platforms, including Baby Journey, calculate based on the week you are currently in. So if you are in week 24+2, you are in week 25 with us!

How should the BF (estimated date of delivery) be calculated?

The first step in calculating how far along the pregnancy is, is to figure out the last menstrual period first day. Then you count a month of 28 days (four weeks), which is also the average length of a menstrual cycle. This tells you how far along the pregnancy is.

This means that the first day of the last period is also the first day of pregnancy, although actually have not yet become pregnant because fertilization occurs at ovulation, around week 2+0.

Based on the week you are in, you will also receive a provisional estimated date of delivery. This is called the BF, but the correct date of estimated delivery is often given later in the pregnancy when you do an ultrasound. This takes place either around week 13, when you usually also have a CUB, and/or around week 19, when you usually have a RUL (routine ultrasound). The midwife will then measure the growth of the fetus and calculate how many weeks you have been pregnant. This is can This means that your BF is moved forward or back a few days. However, it may also be the same estimated date you were originally given.

If you have an extra short or long menstrual cycle, this date can be moved back or forth by many days.

Baby Journey can calculate your BF automatically based on the first day of your last menstrual period, so once you've added a stick, we can be the first to give you an approximate date to work from, giving you the answer to when your baby will finally arrive.

With Baby Journey's Pregnancy Calculator you can easily calculate your BF!

What is month of pregnancy vs calendar month in a pregnancy? 

Sometimes midwives may talk about calendar month and month of pregnancy (as if that wasn't complicated enough!). A calendar month counts as 30 days and is calculated from the date on which you actually became pregnant, the day you ovulated. According to the calculation method, you are then in week 2+0.

A pregnancy month, also known as a lunar month, is counted from the first day of your last menstrual 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 10 months from the first day of menstruation. The length of 9 months that is usually talked about is therefore based on approximately week 4-5 when you can see or find out that you are pregnant, and up to BF.

Here you can read about bleeding during pregnancy.