When someone asks what week you are in, there are two different answers because it depends on how you count it. 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 questions about how to calculate pregnancy weeks and due date!

You rarely talk about which month of pregnancy you are in during a pregnancy, instead you talk about weeks. As an expectant parent, the way you count which week you are in often differs from how health professionals count them.

Which week am I in?
Midwives and other healthcare professionals always count the full, completed week you have reached. That is, if you are in week 26+2, the healthcare staff say you are in week 26. Parents-to-be often count the week you are “in”, i.e. if you are in week 26+2, you are said to be in week 27.

The pregnancy is considered at full-term when 37 complete weeks have passed, i.e. week 37+0 (when you “enter” week 38). If a child is born before week 37+0, the child is considered premature. If the pregnancy goes to week 42+0, the child is considered overdue.

Most pregnancy platforms, Baby Journey included, calculate based on the week you are currently in. If you are in week 24+2, that means you are in week 25 with us!

How to actually calculate your estimated due date?
The first step to figuring out how far along the pregnancy is, is to figure out the first day of your last period. Then you count a month of 28 days (four weeks), which is also the average length of a menstrual cycle. This will then tell you how far along the 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 really had time to get pregnant yet because fertilisation takes place at ovulation, around week 2+0.

Based on which week you are in, you will also receive a preliminary estimated due date. This is just an estimate, but you are often given the correct due date later on when you go for an ultrasound. This happens either around week 13, which is when you usually also do a CUB, and/or around week 19 when you usually do an RUL (routine ultrasound). Then the midwife measures the growth of the foetus and then calculates how many weeks you have been pregnant. So, this may push your due date forward or backward by a couple of days. However, it may also stay the same as what you were told from the beginning.

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

Baby Journey automatically calculates your estimated due date based on the first day of your last period, so when you’ve received that plus sign on your stick, we can be the first to give you an approximate date to start from and tell you when your baby will finally arrive.

What is a pregnancy month vs calendar month in a pregnancy?
Sometimes midwives may talk about calendar month and pregnancy month (as if things weren’t complicated enough already!). A calendar month is counted 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 pregnancy month, also called a lunar month, is counted from the first day of the last menstrual period and lasts 28 days. This is the time when a pregnancy test does not yet show that you are pregnant. You usually find this out around week 5+0.

This means that pregnancy (40 weeks) actually lasts 10 months from the first day of menstruation. The length of 9 months that is usually talked about is thus based on approximately week 4-5 when you can see or find out that you are pregnant, up until your estimated due date.