How long does a pregnancy last?

A pregnancy lasts an average of 40 weeks. When a woman becomes pregnant, her due date at the first doctor's visit. The due date is the day when the woman has been pregnant for 40 weeks.

It is important to know that there is also a term period. It lasts from gestational week 37+0 to gestational week 42+0. Giving birth within this time period is called birth to term. Here your baby is full-term. Only about 2% of babies are born on their due date. It is therefore not so important for pregnant women to focus on the due date, but rather on the due date.

How is your due date calculated?

Midwives and doctors calculate your due date based on the first day of your last period. Add 280 days to this, which is 40 weeks, to get your due date. You can calculate your due date yourself by taking the first day of your last period + 7 days + 9 months. This will give you your due date.

Calculating your due date in this way, however, requires that your periods have been regular and have a cycle of approximately 28 days.

If you have had fertility treatment, you know with great certainty when you ovulated. That date can also be used to calculate your due date. An egg can be fertilized within a relatively small window of 24-48 hours after ovulation. This allows for a reasonably accurate due date calculation by taking the date of your ovulation + 266 days (equivalent to 38 weeks).

If you accept the offer of a 12-week scan at the hospital (which can actually be between weeks 11+3 – 13+6 of pregnancy), the due date here will be determined based on your baby's head circumference. The due date found at the scan is the one used for the rest of the pregnancy.

What should the due date be used for?

• We professionals use the due date to relate to your pregnancy progress, e.g. when the individual examinations should be performed.
• We professionals also use the due date to assess your child's growth.
• The due date helps determine the recommendations for you.
• The due date is used to determine the date when you can go on maternity leave.
• The due date helps determine when a possible initiation will be recommended.

The due date itself is therefore primarily a working tool for professionals and the labor market. For those of you who are expecting a child, the due date is the one I would recommend that you relate to.

Wishing you a good pregnancy

Most affectionately

Siff Stephenson, HeartMidwife