Calculate Due Date: Calculator and Weeks of Pregnancy

When is your baby due? Use the calculator above to figure out your due date and see which week of pregnancy and which trimester you’re in. Here, you’ll learn how the calculation works, how to correctly interpret the weeks, and why your due date is really more of a range.

Calculating Your Due Date: How the Naegele Rule Works

On average, a pregnancy lasts 40 weeks or 280 days, counting from the first day of your last period. That’s how the University Hospital of Zurich and the German website familienplanung.de explain it. Experts refer to this as the Naegele rule, named after a gynecologist from Heidelberg.

To calculate it by hand, here’s the rule: Subtract 3 months from the first day of your last period, then add 7 days and 1 year. For example: Your last period began on January 10, 2026. Subtract 3 months to get October 10, add 7 days to get October 17, and add 1 year to get October 17, 2026.

Cycle Adjustment for Longer or Shorter Cycles

The Naegele rule is based on a 28-day cycle. If your cycle is longer or shorter, according to the Professional Association of Gynecologists, you should add or subtract the difference. So, for a 32-day cycle, add 4 days: October 17 becomes October 21. The calculator above does this automatically for cycles ranging from 21 to 45 days.

If your cycle is irregular or you don’t know the exact date of your last period, your doctor will determine the gestational age using an early ultrasound, according to the University Hospital of Zurich.

Calculating from conception or embryo transfer

If you know the date of conception, according to the Professional Association of Obstetricians and Gynecologists, you add 266 days—that’s 38 weeks. In the example above, conception would have been around January 24, and the due date remains October 17. After in vitro fertilization (IVF), the Swiss Society for Ultrasound in Medicine calculates it as follows: the date of embryo transfer plus 266 days, minus the age of the embryo in days.

Important: Even if you start counting from conception, the weeks of pregnancy are still counted from the start of your last period. According to familienplanung.de, conception occurs about two weeks after your period, so on that day, you’re already at about 2+0.

Understanding Pregnancy Weeks Correctly: What Does 12+3 Gestational Weeks Mean?

Pregnancy weeks (SSW) are counted from the first day of your last period. So for the first two weeks or so, you’re not actually pregnant yet; according to familienplanung.de, conception doesn’t occur until about two weeks later.

Experts often write the weeks with a plus sign; for example, the ultrasound guidelines state “11+0 to 13+6.” 12+3 means 12 full weeks and 3 days. So you’re right in the middle of your 13th week of pregnancy. This is exactly where misunderstandings often arise:

• “13+0” is the first day of the 14th week.
• “In the 13th week of pregnancy” means: somewhere between 12+0 and 12+6.
• The estimated due date is 40+0: that’s when exactly 40 weeks have elapsed.

Specialized agencies use both forms of expression; for example, the IQWiG’s German health portal states “from the 42nd week of pregnancy (from 41+0).” If you’re unsure which week is being referred to, just ask.

Trimesters: The Three Stages of Pregnancy

Pregnancy is divided into three trimesters, which are periods of about three months each. We count the same way as the calculator above: The first trimester extends to 13+6, the second from 14+0 to 27+6, and the third begins at 28+0. For the first trimester, we follow the ultrasound guidelines of the German, Austrian, and Swiss professional associations, which define the first-trimester timeframe as extending up to 13+6. The 28+0 cutoff is used, for example, by the U.S. ICD-10-CM classification. Other organizations define the boundaries slightly differently: familienplanung.de counts the first trimester from the 1st to the 13th week, the second from the 14th to the 26th, and the third starting at the 27th week. This classification is for general guidance; medically, the individual weeks are what matter. When you hear “ten months,” this refers to four-week periods: 10 times 4 equals 40 weeks.

If you’re feeling so nauseous during the first trimester that you can barely keep anything down, read our guide on morning sickness and contact your doctor or midwife.

How an Ultrasound Can Adjust Your Due Date

The gestational age can be determined most accurately during the first trimester. The Swiss Society for Ultrasound in Medicine (SGUM) recommends measuring the crown-rump length between 11 and 14 weeks. Its recommendations have been approved by a commission of the Swiss Society of Gynecology and Obstetrics (SGGG).

Here’s how the adjustment works:

1. Measurements: During the ultrasound, your baby is measured from crown to rump.
2. Comparison: The doctor compares the measured gestational age with the gestational age based on your last menstrual period.
3. Correction: If there is a discrepancy of more than 5 days between the two, the gestational age is corrected according to SGUM guidelines, and you’ll be given a new appointment.
4. Recording: According to the SGUM, a later correction is no longer permitted.

According to the Federal Office of Public Health (FOPH), basic health insurance covers two ultrasound exams during a normal pregnancy: one between the 12th and 14th weeks and one between the 20th and 23rd weeks of pregnancy. The SGUM specifies the exact time frames as 11+0 to 14+0 and 20+0 to 23+0. If your ultrasound results in a corrected due date, use that as your guide, including when counting the weeks.

Important Weeks of Pregnancy

These time points are important in Switzerland; differences for Germany and Austria are noted.

When will the baby actually arrive?

The due date is just a rough estimate. According to the University Hospital of Zurich, it’s accurate for only about 5 percent of births. According to familienplanung.de, about 90 percent of babies are born within two weeks before or after the due date.

A pregnancy is considered normal if it lasts from 37+0 to 41+6. According to the University Hospital of Zurich, if your baby is born before 37+0, it is considered a preterm birth; according to familienplanung.de, a pregnancy lasting 42+0 or longer is considered post-term. The British National Health Service (NHS) also lists 37 to 42 weeks as the normal duration.

The guide “Contractions and Your Water Breaking: When to Go to the Hospital?” explains when to go to the hospital once your contractions start. To help you avoid having to search for things later, check out the “Packing Your Hospital Bag” checklist. You’ll find everything your baby will need afterward in the “Essential Items Checklist.”

Past Your Due Date: What Now?

Once you’ve passed your due date, you’ll receive more frequent monitoring. The IQWiG recommends about two checkups per week, including fetal heart rate monitoring (CTG) and ultrasound. According to the IQWiG, starting at 41+0, your doctor or midwife will usually offer to induce labor. Starting at 42+0, the obstetric team strongly recommends not waiting any longer. The joint guideline from the German, Austrian, and Swiss professional associations states something similar: Induction can be offered starting at 41+0, should be recommended starting at 41+3, and is strongly recommended starting at 42+0. You can read about how this works in the guide on labor induction.

Maternity Leave: Why the Due Date Also Matters for Work

In Switzerland, maternity leave does not begin until after the birth. According to the AHV/IV information sheet, it lasts 14 weeks or 98 days. You will receive 80 percent of your previous earned income, up to a maximum amount. There is no paid maternity leave before the birth. However, starting eight weeks before the birth, you are not allowed to work between 8 p.m. and 6 a.m., according to the Canton of Zurich. After the birth, there is an eight-week ban on working. As of October 2026.

In Germany, on the other hand, maternity leave begins six weeks before the estimated due date; in Austria, it begins eight weeks before.

The guide “Maternity and Paternity Leave in Switzerland” explains everything you need to know about eligibility, duration, and paternity leave.

Frequently Asked Questions

Sources

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7. KVG Amendment: Cost-Containment Measures – Package 2, Federal Office of Public Health (BAG)
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16. Trimester, familienplanung.de, BIÖG
17. When the baby is late, familienplanung.de, BIÖG
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20. Guideline on Induction of Labor, AWMF Register 015-088 (S2k, as of 12/2020, valid until 12/2025, revision pending, PDF), DGGG, OEGGG, and SGGG
21. Maternity Leave, Federal Family Portal, Germany
22. Maternity Leave Regulations, Austrian Chamber of Labor
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24. ICD-10-CM, Chapter O00–O9A: Pregnancy, Childbirth, and the Puerperium (Chapter Note on Trimesters), Reproduction of the U.S. ICD-10-CM classification on icd10data.com
25. Preterm birth (Fact sheet), World Health Organization (WHO)
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