Skip to content

Calculate the week of pregnancy: Based on these data you can determine the SSW & and the month

You are holding your positive pregnancy test in your hand right now and can hardly believe your luck? The greatest miracle of nature awaits you in a few months and from now on you will want to follow every stage of development of your embryo/fetus full of excitement and tension. Pregnancy is the beginning of a wonderful time that every future mom should fully enjoy. And because you also want to do everything right, you are naturally particularly interested in which week you are always in. After all, this is how you find out what exactly is developing in the unborn child and also which nutrients are now particularly important. The only question that remains is how to calculate the week of pregnancy in the first place. We explain it to you!

9 months of pregnancy vs. 40 weeks of pregnancy – Yes, which one?!

Pregnancy week calculate on the basis of the cycle

How are the weeks of pregnancy calculated? Pregnancy is officially counted from the last period, which results in 40 weeks (280 days). So the term “weeks of pregnancy” (SSW) is actually not quite right. So if you have already wondered why people talk about 9 months of pregnancy but end up counting 40 weeks – this is why. But why do gynecologists count this way?

Calculating the week of pregnancy with the help of the time of ovulation

So, the 9 months refers to the period from fertilization to birth, but the 40 weeks are counted from the first day of the last period and the reason is simply that the latter variant is much more precise. You don’t know exactly on which of the fertile days you were actually fertilized, but the first menstrual day is certain (provided you have written it down or remember it). The gynecologist also uses this day to determine the expected date of delivery (ET; Entbindungstermin).

Overview of how to calculate the week of pregnancy: What are the options?

Pregnancy week calculate on the basis of the fertilization

But how many weeks are you in right now? W hat is the correct way to calculate the weeks of pregnancy? So, in principle, you can come up with the solution in three different ways. But in all three cases, you need to know at least one specific date: the cycle, ovulation, or delivery date. There are numerous calculators online where you enter one of these dates to have the week calculated for you.

Calculation based on ovulation

Follow the development of the fetus with the help of the right week


If you want to calculate the weeks of pregnancy based on ovulation, you should of course know when it occurred. In the case of artificial insemination, this is a good option because you now know exactly on which day it was performed. You can therefore calculate the week of pregnancy based on the fertilization. It is more difficult if you have become pregnant naturally, because ovulation takes place about 14 days after your period and the exact time varies from woman to woman.

Calculate pregnancy week - overview with weeks and months

In addition, actual fertilization can occur at least 12 and up to 24 hours after ovulation. So on average six fertile days are possible in each cycle, sometimes even more, which makes it difficult to set an exact date. If you still want to calculate the week of pregnancy in this way, be prepared that certain deviations are possible.

Pregnancy weeks and days calculate by means of the period

40 weeks lasts the pregnancy of a woman


Since women are always advised to track and record their cycle, you will surely know the exact day when your last period started. From this day, you can count off the weeks. However, online calculators also include the length of the cycle (the period from the first day of your period to the last day without a period), as this also determines the fertile days and thus fertilization. This can vary between 25 and 35 days, which makes an average of 28 days.

Count the weeks and look forward to the birth

Since the length of your cycle may also vary a little each month, you can simply average the last few months to calculate it and use it. If you use an app to track your cycle each month, it will calculate it quite automatically.

Calculate the week of pregnancy – the date of birth as a starting point.

Pregnancy week calculate by counting backwards from birth date

The first two variants are also suitable for determining the ET. However, if your gynecologist has already calculated it by giving you the day of fertilization (in the case of artificial insemination) or the first day of your last period, but you got confused and don’t know which week you are in, you can also calculate it using the due date. To do this, simply count down the weeks from the ET until you get to the current calendar week. Then subtract the number you get from 40 (the 40th week) and the result is your current week.

Which week falls in which month of pregnancy?

Is the pregnancy counted in weeks or months

You can now use the calculated SSW to determine your month of pregnancy. If this seems to be too complicated for you, you can use the following list, in which we have clearly assigned the weeks to the respective month.

  • 1st month: 1st to 4th week
  • 2nd month: 5th to 8th week
  • 3rd month: 9th to 2nd week
  • 4th month: 13th to 16th week
  • 5th month: 17th to 20th week
  • 6th month: 21st to 24th week
  • 7th month: 25th to 28th week
  • 8th month: 29th to 32nd week
  • 9th month: 33rd to 36th week
  • 10th month: 37th to 40th week

Of these, the first 12 weeks (3 months) are the first trimester, weeks 13 to 28 are the second, and the last from the 29th to 40th are the third.

Calculate the ET using the first day of the last menstrual period