Pregnancy Week and Due Date Calculator

How is the pregnancy week calculated?

Pregnancy weeks are counted from the first day of the last menstrual period. To estimate the due date, 280 days (40 weeks) are added to that date. This is Naegele's rule and assumes a regular 28-day cycle. For a longer or shorter cycle the difference in days is added or subtracted.

How to use it

1. Choose the first day of your last period.
2. Optionally enter your cycle length (28 is assumed if blank).
3. Press Run to see your week, trimester, estimated conception date and due date.

Example table (28-day cycle)

Trimesters

Good to know

• The due date is a reference; most babies are born a few weeks before or after it. Weeks 37-42 are generally considered normal term.
• If your periods are irregular or you do not remember the date, the result can mislead. Your doctor may correct the date from an ultrasound.
• The conception date is a rough estimate and depends on the day of ovulation.
• This tool is not medical advice. For prenatal care, tests and health questions, consult your doctor or midwife.

To plan leave around the birth, try the Maternity Leave Calculator on this site.

Frequently asked questions

How is the due date calculated?

With Naegele's rule: add 280 days (40 weeks) to the first day of your last period. If your cycle differs from 28 days, the difference is added or subtracted. The tool does this automatically.

How are pregnancy weeks counted?

Weeks are counted from the first day of the last period, not from conception, so conception usually falls near the end of week 2.

Will I give birth on the calculated date?

The due date is a reference. Most babies are born before or after it; weeks 37 to 42 are generally considered normal term. Talk to your doctor about follow-up.

Why might the ultrasound date differ?

An ultrasound dates the pregnancy from the baby's measurements and is most reliable in the first trimester. If periods are irregular, your doctor may adjust the date to the ultrasound.