Most people have heard "9 months", but in practice, pregnancy is typically measured in weeks in healthcare. The most important thing for you to know is how the weeks are counted, how the due date is set, and what is normal if the birth happens a little before or after.

A pregnancy typically lasts about 40 weeks, counted from the first day of your last menstrual period. This is often called LMP (last menstrual period) and is a standard way of calculating because many people don't know the exact date of conception.

If you count from the fertilization itself, the pregnancy is typically about 38 weeks. The difference is due to the fact that ovulation and fertilization most often occur about 2 weeks after the first day of menstruation in a "classic" cycle, but cycle lengths vary from person to person.

The due date is an educated estimate, not a guarantee. It can be calculated in two common ways: based on the last menstrual period and based on ultrasound.

Here, the starting point is the first day of your last menstrual period, and approximately 40 weeks are added. The method is simple and often used initially, but it can be inaccurate if you have irregular cycles or don't remember the date precisely.

During an early scan, the size of the fetus can be used to estimate the length of the pregnancy and thus adjust the due date. Ultrasound is often used to make the due date calculation more precise, precisely because it does not depend on when you last had your period.

Many are surprised by how much variation there can be without it necessarily being abnormal. Healthcare professionals often refer to the baby being full-term around the due period, but it is common for births to happen both before and after the date you have been given.

If you go overdue, you will typically be followed more closely with check-ups, and a plan can be made with the maternity ward regarding when it makes sense to wait and when induction might be recommended. What is relevant for you depends, among other things, on your course, your baby, and your measurements.

Several factors can influence when labor begins - and much of it is beyond your control.

It can feel confusing if your due date shifts after a scan. Most often, this is because ultrasound provides a better estimate of pregnancy length than the date of your last menstrual period.

Multiple pregnancies, previous births, and individual differences in body and pregnancy course can affect timing. If you are in doubt about your symptoms, or if you experience bleeding, waters breaking, significantly reduced fetal movements, or persistent pain, it is always advisable to contact a midwife, doctor, or maternity ward.

Knowing which week you are in makes it easier to plan your daily life and preparations step by step.

The body changes gradually, and many experience that the need for support can particularly increase with abdominal growth and weight. If you need physical relief, a support belt can be a practical help during periods when your lower back and pelvis feel strained. See e.g., Maternity Support Belt.

It can bring peace of mind to have the most basic things ready for the days after birth, when the focus is often on recovery and rest. A complete package of necessary care products can be a simple way to prepare. See e.g., New Mum Care Pack.

Danish reference points: Sundhed.dk (Patienthåndbogen) and information on ultrasound scanning in pregnancy on Sundhed.dk.