Pregnancy week calculator

Enter the date of your last menstrual period to see your current week, trimester, and estimated due date based on Naegele's rule.

⚠️ Please note: this tool provides estimates only and is in no way a substitute for advice from your OB-GYN or midwife. Reliable pregnancy dating is done with a first-trimester ultrasound.

First day of your last menstrual period

–Weeks + days

–Trimester

–Estimated due date

–Weeks to go

🔒 Everything is calculated in your browser: no dates are saved or sent anywhere.

How Naegele's rule works

Naegele's rule estimates the due date by adding 280 days (40 weeks) to the first day of your last menstrual period. It's the convention obstetricians have used for over two centuries, and it assumes regular 28-day cycles with ovulation on day 14. That's why gestational age includes roughly two weeks during which conception hadn't happened yet. If your cycles are longer, shorter, or irregular, the estimate can shift by several days — only a first-trimester ultrasound can date a pregnancy accurately.

Counting weeks and trimesters

Pregnancy weeks are written as "weeks + days": 12+3 means 12 completed weeks and 3 days, which puts you in week 13. The first trimester runs through 13+6, the second covers 14+0 to 27+6, and the third goes from 28+0 until delivery. Only about 4–5% of babies arrive exactly on their due date: a birth between 37+0 and 41+6 is considered full term. Remember: this tool provides estimates only and is not medical advice — for any question, symptom, or decision, always talk to your doctor or midwife.