What is gestational age?
Gestational age (GA) is the age of a pregnancy counted from the first day of the last menstrual period (LMP). It is written in completed weeks and days — for example 24w3d — and it is the standard used for prenatal care, ultrasound reports and birth records. Because it starts at the LMP, gestational age is about two weeks more than the embryo’s or fetus’s actual age since conception (sometimes called conceptional or fetal age).
How gestational age is calculated
| You know | Gestational age on a date |
|---|---|
| LMP | date − LMP (in days), written as weeks + days |
| EDD (due date) | 280 − (EDD − date) days |
| Ultrasound | GA on the scan + days since the scan |
Use the optional Calculate for this date field to find the gestational age on any day — the date of a past appointment, a planned scan, or a future milestone — not just today. To work the other way and calculate the LMP from a gestational age, see the LMP Calculator.
Gestational age by LMP vs. by ultrasound
Gestational age by LMP is the starting point, but it assumes ovulation around day 14. A first-trimester ultrasound measurement is generally the most accurate way to estimate gestational age in a naturally conceived pregnancy, and the due date may be adjusted to match it. The Accurate Due Date Calculator explains when redating usually happens.
Gestational age after birth
For babies born early, gestational age at birth is used to work out corrected age. The Corrected Gestational Age Calculator does this for premature babies.