🤰Pregnancy Due Date Calculator

Pregnancy Calculator online — due date and pregnancy week

Our free pregnancy calculator estimates your due date and current week of pregnancy based on the first day of your last menstrual period (LMP). Simply enter that date and the calculator applies Naegele's rule — an established medical method that adds 280 days (exactly 40 weeks) to the LMP. You can also enter your cycle length if it differs from the standard 28 days — the calculator will adjust the date accordingly. The result includes the estimated due date, current pregnancy week and day, trimester, and a progress bar.

Naegele's rule — how is the due date calculated?

Naegele's rule was developed by Franz Karl Naegele in the 19th century and is still used as the starting point for estimating the due date. The calculation is simple: add 280 days (9 months and 7 days) to the LMP. The calculator also applies a cycle correction — for example, a 35-day cycle shifts the date forward by 7 days. Bear in mind, however, that the actual due date is determined by a doctor using an ultrasound scan and may differ from the calculator result by up to several weeks. This result is indicative only and does not replace medical advice.

Frequently asked questions

How does the pregnancy calculator work?

The calculator applies Naegele's rule: it adds 280 days (40 weeks) to the first day of the last menstrual period (LMP), optionally adjusting for cycle length differences from the standard 28 days. From this it calculates the estimated due date, current pregnancy week, and trimester.

Is the pregnancy calculator result accurate?

No. Naegele's rule gives an indicative result — the confirmed due date is established by a doctor based on a first-trimester ultrasound. Varying cycle lengths, off-schedule ovulation, and other factors can shift the actual date by days or weeks.

What is a trimester?

Pregnancy is traditionally divided into three trimesters: the first (weeks 1–13), the second (weeks 14–27), and the third (weeks 28–40). Each trimester represents a different stage of fetal development and comes with different medical recommendations.

Why can I enter my cycle length?

Standard Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, the calculator automatically adjusts the due date by the difference (e.g. a 35-day cycle shifts the date by +7 days), improving the accuracy of the estimate.