Many people are surprised when they are first introduced to the week system in pregnancy. The logic behind the way pregnancy weeks are counted does not immediately make sense: technically, you have entered the first week of pregnancy even before the egg has been fertilized. But that is how the clinical week system works.
Let's take a closer look at how the weeks are counted, why the system makes sense physiologically, and how you can best use it if you are trying to get pregnant – or already are.
Why weeks and not months?
You may have heard people ask, “How many months along are you?” Many people answer politely, but roll their eyes internally, because that's not how healthcare professionals and doctors count. Everything about pregnancy is about weeks.
There is a good reason for this. Pregnancy develops in a continuous process, where every single week makes a huge difference – and this applies to both the baby and the mother. Whether you have just discovered your pregnancy or are only toying with the idea, you will quickly understand why everyone uses weeks as a basis.
But when does week 1 of pregnancy actually start?
It all starts on the first day of your last period – even if you are not yet pregnant at that point. The first two weeks or so are all about your body getting ready: the egg matures and the lining of the uterus prepares for possible fertilization. But for the healthcare system, it is still the weekend when the blood appears that marks the starting point.
It may seem strange to count the first two weeks, even though you are not yet fertilized. This is done because it can be quite difficult to determine the exact time of ovulation and fertilization, whereas in most cases it is fairly easy to remember the date of the first day of menstruation – at least more tangible for healthcare professionals.
Terminology: What does week 4+0 mean?
When we talk about, for example, “week 14+5,” it means that 14 full weeks and five days have passed since the first day of your period. Week 4+0 is therefore four full weeks after your period began. It also means that your pregnancy is actually counted from week 0+0, which is the first day of your last period.
The advantages of the precise week system
It may feel unnecessary and bureaucratic, but the week system actually provides both pregnant women and doctors with an incredibly valuable common language.
- Less room for misunderstandings
- Possibility of precise monitoring of fetal development
- Easier access to important services (e.g., scans) that are scheduled for specific weeks
All of this makes it easier to ensure the best for both mother and child throughout the pregnancy.
Counting: From day 0 to 40+0
Pregnancy lasts, theoretically, 40 weeks – that is, 280 days. And it is these 40 weeks that you and your healthcare professionals will be keeping an eye on. Some women give birth earlier, some later, but the vast majority of children are born in weeks 37-42.
In practice, the weeks are divided up as follows:
- First trimester (0-12 weeks): The early stages of development, the risk of miscarriage is highest, all organs are formed.
- Second trimester (13-27 weeks): The belly begins to show, many experience fewer discomforts, the fetus develops rapidly.
- Third trimester (28-40 weeks): The fetus's growth accelerates, you feel kicks, and your body prepares for childbirth.
So it's much more detailed than “nine months.”
Week 0 – the start of menstruation
You may not even be aware that you could potentially be on your way to becoming pregnant. But the system is already counting from this point, because this is where the cycle starts.
Week 2 – ovulation is approaching
Towards the end of this week, most women ovulate. Fertilization usually takes place now or at the beginning of week 3. However, it takes exactly 6-7 days for the fertilized egg to attach itself to the lining of the uterus.
Week 4 – “you are now pregnant”
If everything goes as it should, your period will now be late. Many women take a pregnancy test during this week.
Before fertilization: Folic acid and good habits
When you start trying to get pregnant, there are some important things to consider. One of the most important is to meet your folic acid requirements.
- Folic acid (400 micrograms daily): Reduces the risk of neural tube defects in the baby.
- Check your vitamins: Most common multivitamins already contain the necessary amount.
- Cut out alcohol and cigarettes: They have a negative impact on both your fertility and your baby's health.
This also applies to your partner, of course, whether it's general lifestyle or vitamin supplements.
Once pregnancy is confirmed: How to use the week system
Knowing your exact week is very beneficial, as it is important for:
- When you can book the recommended scans
- Which tests and consultations you will be offered
- What you can expect from your body and when
For example, everyone is offered a nuchal scan in weeks 11-13 and a malformation scan in weeks 18-20.
You can quickly calculate which week you are in by counting the number of weeks and days since the first day of your last menstrual period. If you want to make it super easy, there are tons of online pregnancy calendars you can look up or download as an app.
Important milestones
Along with the weekly offers and exams, there are also personal and physiological milestones along the way. Here are some that many people notice:
- Missed period (often week 4-5)
- First scan (week 8-13)
- First noticeable kicks (week 16-22)
- Regular midwife visits and health checks (from week 24 onwards)
Understanding removes uncertainty
Once you understand the logic behind the week system, much of the confusion disappears. It helps both you and your partner keep track of developments – not least because it is actually in weeks that you can compare your own experiences, expectations, and concerns with those of others.
The week system quickly becomes your guide: not only in your medical records, but also in conversations with other expectant parents, colleagues, and friends.
Removing years and months from the reporting is not just a matter of professional precision. It enables you to understand the rhythm of pregnancy, look forward to the milestones, and celebrate both the small and large steps that are spread out over almost ten months. And who knows – maybe you'll already be smiling the next time someone asks, “How far along are you?”
