Pregnancy week calculator

What week am I?

Enter the first day of your last period to find your pregnancy week, trimester, estimated due date and key milestones.

The third trimester starts at week 27 and lasts until birth.

Pregnancy is counted in weeks from the first day of your last menstrual period (LMP), not conception. The first trimester runs weeks 1 to 12, the second covers 13 to 26, and the third runs from week 27 until birth, usually around week 40. Enter your LMP below to see exactly which week and trimester you're in today.

The calculator uses Naegele's Rule, the standard clinical method that adds 280 days to your LMP for an estimated due date. It also shows how far along you are as a percentage and which developmental milestones you've passed.

Medical disclaimer

This calculator provides general information only and isn't a substitute for professional medical advice, diagnosis or treatment. Pregnancy dating should always be confirmed by your healthcare provider, ideally with an early ultrasound. Always consult a qualified obstetrician or midwife for personal guidance.

First trimester
Weeks 1–12
Organ formation
Second trimester
Weeks 13–26
Growth & movement
Third trimester
Weeks 27–40
Safe delivery: wk 37+

Pregnancy week calculator

Enter the first day of your last menstrual period. Results update instantly.

The first day of your most recent menstrual period.

About this calculator

Calculations use Naegele's Rule, the standard clinical method endorsed by ACOG, NHS and WHO, which adds 280 days (40 weeks) to the LMP date. Trimester boundaries follow ACOG definitions.

Last reviewed: May 2026 · Sources: ACOG, NHS, WHO

Pregnancy week: questions and answers

What week does the third trimester start?

The third trimester starts at week 27 of pregnancy and lasts until birth, typically at week 40. It begins approximately six months into pregnancy.

Which week of pregnancy is safe for delivery?

A pregnancy is considered full-term at 37 weeks. The optimal delivery window is weeks 39–41. Babies born at 34–36 weeks are called late preterm and may need additional support.

What is the third trimester?

The third trimester covers weeks 27 through 40 (or birth). During this phase, the baby undergoes rapid weight gain and organ maturation, particularly the lungs and brain.

How is the estimated due date calculated?

The standard method (Naegele's rule) adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This is an estimate — only about 5% of babies are born on their exact due date.

What is the LMP date?

LMP stands for Last Menstrual Period. It is the first day of your most recent period before becoming pregnant. Pregnancy is conventionally counted from this date, even though conception occurs around week 2.

Medical references & sources