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).

Available

Inputs

Enter values

Live

Angaben zur Schwangerschaft

Erster Tag der letzten Periode und optionaler Stichtag.

First day of the last menstrual periodi

Der erste Tag der letzten Menstruation (Basis der Naegele-Regel).

Reference datei

Optional. Ohne Stichtag verwendet der Rechner das heutige Datum für die SSW.

Load example

For illustration only – your own calculation is what counts.

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 dateGeburtstermin = 1. Tag der letzten Periode + 280 Tage

Naegele's rule adds 40 weeks to the first day of the last menstrual period.

Gestational weekSSW = vollendete Wochen + Tage seit der letzten Periode

The 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.

Notes

Common mistakes

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.

Limits

Limitations

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

Naegele-Regel zur Bestimmung des GeburtsterminsDeutsche Gesellschaft für Gynäkologie und Geburtshilfe
Professional society for gynaecology and obstetrics; Naegele's rule (last menstrual period + 280 days) is the established standard estimate.
View sourceRetrieved: 08/24/2026 · Verified on: 08/24/2026
Schwangerschaftsdauer und Zählung der SchwangerschaftswochenRechnerpilot Redaktion
Editorial derivation of the gestational-week count from the first day of the last menstrual period.
View sourceRetrieved: 08/24/2026 · Verified on: 08/24/2026

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