Gestational age calculator

Find how far along a pregnancy is — your current gestational age in weeks and days — from the last period or a known due date. This gestation calculator works everything out in your browser; nothing is saved or sent.

How to use it

How is gestational age calculated?

Gestational age is simply the time elapsed since the first day of your last menstrual period, expressed in completed weeks and days — for example, 12 weeks 3 days (often written 12+3). From a known due date, the calculator works backwards (due date minus 280 days gives the equivalent last period) and counts forward to your chosen date. The concept is explained in gestational age vs. fetal age.

How many weeks pregnant am I?

“How many weeks pregnant” and “gestational age” are the same thing — both count your gestational weeks and days from the last period. If you’re 12 weeks 3 days along, you’re in your 13th week of pregnancy — the week number is one ahead of the completed weeks, which is why the calculator shows both. For more on reading the number, see how far along am I.

Gestational age vs. fetal age

Gestational age counts from the last period; fetal (or embryonic) age counts from conception, about two weeks later. Clinicians use gestational age because it’s the standard for screening windows, growth charts and term definitions. Your result above uses gestational age throughout.

Why pregnancy is counted from the last period

The exact day of conception is rarely known, but the last period usually is — so it provides a consistent starting line. It also means “week 1” begins before conception, and the due date lands at 40 weeks even though a pregnancy runs about 38 weeks from conception. The dating rule behind this is covered in Naegele’s rule.

Accuracy and limitations

Last-period dating is only as reliable as the date and cycle length you enter, and it assumes fairly regular cycles. A first-trimester ultrasound is the most accurate method and may adjust the gestational age — see how accurate is your due date. For dating from an ultrasound, IVF or a conception date, use the full OB wheel.

Frequently asked questions

How many weeks pregnant am I from my LMP?

Count the days from the first day of your last period to today, then convert to weeks and days. This calculator does it for you — enter your last period and read the weeks-and-days figure.

Why is gestational age counted from the last period?

It gives a consistent, knowable start point. Since the exact day of conception is rarely known, dating from the last menstrual period (about two weeks before conception) keeps every pregnancy on the same clock.

Is gestational age the same as fetal age?

No. Gestational age is counted from the last period; fetal age is counted from conception, roughly two weeks later. So a pregnancy at 12 weeks gestational age has a fetal age of about 10 weeks.

What does “week 1” of pregnancy mean?

By convention, week 1 begins on the first day of your last period — before conception has even happened. That’s why the due date is 40 weeks out even though a pregnancy lasts about 38 weeks from conception.

How accurate is gestational age from the last period?

It depends on knowing your last period and cycle length. A first-trimester ultrasound is more accurate and may adjust the dating. If your cycles are irregular, lean on the scan.

Last meaningfully updated: 2026-08-25.