How are the trimesters defined?

The first trimester runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. These are measured in gestational weeks from your last menstrual period.

How is the due date calculated?

It uses Naegele's rule: 280 days (40 weeks) added to the first day of your last menstrual period. Ultrasound dating can differ, so always confirm with your clinician.

Is any information uploaded?

No. The calculation happens entirely in your browser and nothing is stored or sent anywhere.

Pregnancy Trimester Calculator

Pregnancy trimester calculator

Enter the first day of your last menstrual period (LMP) to see your estimated due date, your gestational age in weeks and days, and the exact dates each trimester begins and ends.

Dating follows Naegele’s rule — 40 weeks from your LMP — with trimester boundaries at weeks 13 and 27. Ultrasound dating may shift these dates, so use this as a guide and confirm details with your healthcare provider. Everything is computed privately in your browser.