Rechnerpilot Calculator
Pregnancy Calculator
Calculate the estimated due date with Naegele's rule from the first day of the last menstrual period and determine the current week of gestation (SSW).
Explanation
How Pregnancy Calculator works
The pregnancy calculator determines two values from the first day of the last menstrual period: the estimated due date and the current week of gestation. Both figures are based on the standard medical formula.
Due date and gestational week
Due dateDue date = 1st day of last period + 280 daysNaegele's rule adds 40 weeks to the first day of the last menstrual period.
Gestational weekGestational week = completed weeks + days since the last periodThe time elapsed since the last menstrual period is split into full weeks and remaining days.
Practice
Practical examples
For illustration only – your own calculation is what counts.
Due date from the last menstrual period
The first day of the last menstrual period was 01.01.2026; the due date and the current gestational week on 01.03.2026 are sought.
Result: The estimated due date is 08.10.2026; on 01.03.2026 the pregnancy is at gestational week 8+3.
Current week of gestation
With the last menstrual period on 10.11.2025, the current week of gestation is determined for 14.02.2026.
Result: The due date is 17.08.2026; on 14.02.2026 the pregnancy is at gestational week 13+5.
Due date at the start of pregnancy
If the last menstrual period is dated 15.04.2026, the due date arises right at the start (gestational week 0+0).
Result: The estimated due date is 20.01.2027; the pregnancy begins mathematically at gestational week 0+0.
- Entering ovulation instead of the last menstrual period
The calculator expects the first day of the last menstrual period, not the day of ovulation.
Enter the first day of the last menstrual period.
- Counting the gestational week incorrectly
12+3 means twelve completed weeks and three days – not the twelfth running week.
Note the separate figures for completed weeks and the current gestational week.
FAQ
Frequently asked questions
What does the gestational week (SSW) mean?
SSW stands for gestational week. It is counted from the first day of the last menstrual period and given as “completed weeks + days”, for example 12+3 for twelve full weeks and three days.
How is the due date calculated?
The calculator uses Naegele's rule: 280 days (40 weeks) are added to the first day of the last menstrual period. This gives the estimated due date.
How accurate is the calculated date?
The due date is an estimate. Only a small share of births takes place exactly on the calculated day; most fall in the weeks before or after. Medical examinations are decisive.
Why is the calculation based on the last menstrual period?
The exact moment of conception is usually unknown, whereas the first day of the last menstrual period can be determined reliably. That is why it is the standard starting point for the calculation.
How is the current gestational week shown?
In addition to the figure “completed weeks + days”, the calculator states the current week of gestation. At 12+3, twelve weeks are completed, so the 13th week is running.
Does the calculator account for the cycle length?
No. The calculator uses the standard assumption of 280 days from the last menstrual period. With a strongly deviating cycle length, the medically determined date may differ.
What about the ultrasound date?
By ultrasound the due date is determined from the size of the baby and can differ from the calculated date. The date established by a doctor takes precedence.
Does the calculator replace medical advice?
No. The calculator only provides a non-binding estimate. Examinations and the assessment by a doctor or midwife are decisive.
- The calculator provides a non-binding estimate and no medical diagnosis.
- A standard duration of 280 days is assumed; individual cycle lengths are not taken into account.
Sources
Sources and references
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