Due Date Calculator
Enter the start of your last period – get your calculated due date instantly
- Updated
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Default: 28 days (21–45 days)
Estimated due date
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–Please enter a cycle length between 21 and 45 days.
How is the due date calculated?
The Naegele rule and its medical significance
The Naegele rule is the most widely used method worldwide for calculating the expected date of birth. It was described in 1812 by the German obstetrician Franz Karl Naegele and remains the standard procedure in obstetrics to this day. The formula adds 280 days (40 weeks of pregnancy) to the first day of the last menstrual period (LMP).
Our calculator extends the classic Naegele rule with a cycle correction: since the original formula assumes a standard 28-day cycle, the calculated date shifts accordingly for a deviating cycle length. For a 30-day cycle, the calculator computes EDD = LMP + 282 days (+2 days), and for a 26-day cycle EDD = LMP + 278 days (−2 days). This correction significantly improves the accuracy of the estimate.
It is important to understand that the calculated due date is a statistical estimate. In fact, only about 5 percent of all babies are born exactly on the due date. Most births occur in the period from one week before to two weeks after the calculated date. An ultrasound examination in the first trimester (usually between weeks 9 and 12 of pregnancy) allows for a significantly more precise determination of the date by measuring the embryo's crown-rump length.
In Germany, the maternity guidelines (Mutterschaftsrichtlinien) require at least three ultrasound examinations during pregnancy. The first screening serves, among other things, to confirm or correct the calculated due date. In the event of a significant discrepancy between the calculated EDD (Naegele) and the sonographic EDD, doctors correct the date based on the ultrasound measurement, as this is more precise in early pregnancy.
Key milestones in pregnancy
The calculator counts trimesters as defined by familienplanung.de (German Federal Institute of Public Health, glossary entry “Trimenon / Trimester”): 1st trimester weeks 1–13 (up to 12+6), 2nd trimester weeks 14–26 (13+0 to 25+6), 3rd trimester from week 27 (from 26+0). Some sources count differently, e.g. the 3rd trimester only from week 28.
Trimesters at a glance
- 1st trimester (weeks 1–13, up to 12+6)
- Implantation, embryonic development, organ formation. First prenatal check-up and first-trimester screening. Highest risk of miscarriage.
- 2nd trimester (weeks 14–26, 13+0 to 25+6)
- Visible weight gain, first fetal movements (from around week 18–22). Second ultrasound examination (anomaly scan) around week 20.
- 3rd trimester (from week 27, from 26+0)
- Rapid growth, lung maturation, preparations for birth. Third ultrasound around week 30. From week 37, the pregnancy is considered full-term.
- Post-term (from week 42)
- From 42+0 weeks, births are considered post-term. Closer monitoring via CTG and ultrasound; labor induction if necessary.
Calculation examples
- Last period start
- 1 January 2026
- Cycle length
- 28 days
- Cycle correction
- 0 days
- EDD = LMP + 280 days
- 8 October 2026
- Last period start
- 1 January 2026
- Cycle length
- 30 days
- Cycle correction
- +2 days
- EDD = LMP + 282 days
- 10 October 2026
Frequently asked questions about the due date calculator
Naegele rule, accuracy and medical background
How is the due date calculated using the Naegele rule?
The Naegele rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). For a cycle other than 28 days, the difference from the standard length is added or subtracted: EDD = LMP + 280 + (cycle length − 28) days. A 30-day cycle shifts the date forward by +2 days.
How accurate is the calculated due date?
The calculated due date (EDD) is a statistical estimate. Only about 5% of all babies are born exactly on the due date. Most births fall within a window of ±2 weeks around the due date. An ultrasound examination in the first trimester (weeks 9–12) allows for a significantly more accurate estimate based on the embryo's crown-rump length.
What is the difference between EDD and due date?
EDD stands for "estimated date of delivery" and refers to the calculated date of the expected birth. In medicine, the terms EDD and calculated due date are used interchangeably. It serves as a guide for prenatal check-ups and decisions in obstetric care.
Does cycle length affect the due date?
Yes. The Naegele rule assumes a 28-day cycle by default. For shorter cycles (e.g. 24 days), ovulation occurs earlier, moving the due date forward by −4 days. For longer cycles (e.g. 35 days), it shifts back accordingly. Our calculator automatically accounts for this correction.
From when is a pregnancy considered post-term?
In Germany, a pregnancy is considered post-term from 42+0 weeks of pregnancy (two weeks after the due date). From this point, the German Society of Gynecology and Obstetrics (DGGG) recommends closer monitoring, as the risk of complications rises slightly.