How do I calculate pregnancy in weeks?

Gestational age is counted from the first day of your last period: the number of days since then, converted into weeks and days. The calculator does this automatically and shows your trimester.

How is the due date estimated?

By Naegele’s rule: first day of the last period + 280 days, adjusted for cycle length. The calculator computes it instantly. Only about 4% of babies arrive on the due date — it’s a guide, not an exact day.

How does gestational age differ from age since conception?

Gestational age is counted from the last period, while conception happens about 2 weeks later. So gestational age is about 2 weeks more than embryonic (conception) age. Doctors and ultrasound use gestational age.

What about an irregular cycle?

With an irregular or long cycle, the period-based estimate is less accurate. Enter your real cycle length and the calculator adjusts the due date. A first-trimester ultrasound gives the most accurate date.

How accurate is the due date?

It’s an estimate. Birth from week 37 to 42 is normal, so the actual date can differ from the due date by a couple of weeks either way. Accuracy is higher with an early ultrasound.

Pregnancy due date and weeks calculator

Enter the first day of your last period and your cycle length — the calculator works out gestational age in weeks and days, your trimester and the estimated due date by Naegele’s rule. Know your conception date? Add it for a more accurate result.

Calculate gestational age and due date

Enter the date of your last period — the result appears instantly.

Pregnancy trimesters

Gestational age is counted from the first day of the last period, so it’s about 2 weeks more than “age since conception”.

Trimester	Weeks
1st trimester	weeks 1–13
2nd trimester	weeks 14–27
3rd trimester	weeks 28–40

How gestational age is calculated

In obstetrics, age is counted not from conception but from the first day of the last menstrual period (LMP) — it’s more practical, since the exact conception date is usually unknown. So gestational age is about 2 weeks more than the real “age since conception”, and pregnancy lasts on average 40 weeks from the LMP.

If your cycle is longer or shorter than the standard 28 days, ovulation shifts — the calculator accounts for this and adjusts the due date.

Due date by Naegele’s rule

The estimated due date is worked out by Naegele’s rule: first day of the last period + 280 days (40 weeks), adjusted for cycle length. It’s a guide: only about 4% of babies are born exactly on the due date, and birth from week 37 to 42 is considered normal.

The most accurate date is set by a doctor from a first-trimester ultrasound (crown-rump length), which is more reliable than the period-based estimate, especially with irregular cycles.

What the calculator shows

You’ll see your current age in weeks and days, the trimester, the estimated due date and how many weeks remain. It helps you keep track of screening and test timing but doesn’t replace care from an obstetrician.

Frequently asked questions

Gestational age is counted from the first day of your last period: the number of days since then, converted into weeks and days. The calculator does this automatically and shows your trimester.
By Naegele’s rule: first day of the last period + 280 days, adjusted for cycle length. The calculator computes it instantly. Only about 4% of babies arrive on the due date — it’s a guide, not an exact day.
Gestational age is counted from the last period, while conception happens about 2 weeks later. So gestational age is about 2 weeks more than embryonic (conception) age. Doctors and ultrasound use gestational age.
With an irregular or long cycle, the period-based estimate is less accurate. Enter your real cycle length and the calculator adjusts the due date. A first-trimester ultrasound gives the most accurate date.
It’s an estimate. Birth from week 37 to 42 is normal, so the actual date can differ from the due date by a couple of weeks either way. Accuracy is higher with an early ultrasound.

Make sense of your tests and screenings by week

Upload your lab and ultrasound results — AI explains the values in the context of your gestational week and flags what to watch.

Decode my tests

This calculator is for reference and information only. Gestational age and the due date are confirmed by a doctor via ultrasound and examination; the due date is a guide, not an exact date.