What Is Gestational Age?

Definition of Gestational Age

Gestational age (GA) is the age of a pregnancy measured in weeks and days from the first day of the last menstrual period (LMP). This is the universal standard used by obstetricians, midwives, and ultrasound technologists worldwide. Under this system, you are considered "2 weeks pregnant" at the moment of conception, because the period typically ended about 2 weeks before ovulation.

A full-term pregnancy is approximately 40 weeks gestational age (280 days from LMP). This is why due dates are calculated by adding 280 days to LMP.

Gestational Age vs Fetal Age

How Gestational Age Is Measured

There are two main ways gestational age is determined in clinical practice:

1. By Last Menstrual Period (LMP)

The simplest method: count from the first day of the LMP. This assumes ovulation at day 14 of a 28-day cycle. Our calculators use this method as the baseline.

2. By Ultrasound

Ultrasound measures fetal size to estimate GA. In the first trimester, Crown-Rump Length (CRL) in millimeters is used. A CRL of 10mm corresponds to approximately 7 weeks GA. In the second trimester, multiple biometric measurements are combined.

Gestational Age in IVF

In IVF pregnancies, gestational age is calculated slightly differently. Since the embryo’s age at transfer is known precisely, IVF clinics back-calculate a "virtual LMP." For a Day 5 blastocyst transfer, the virtual LMP is the transfer date minus 19 days (14 days standard LMP-to-ovulation + 5 days embryo age). This virtual LMP is then used for all subsequent gestational age calculations.