Gestational age (or menstrual age) is the time elapsed between the first day of the last normal menstrual period and the day of delivery. The first day of the last menstrual period occurs approximately 2 weeks before ovulation and approximately 3 weeks before implantation of the blastocyst. Because most women know when their last period began but not when ovulation occurred, this definition traditionally has been used when estimating the expected date of delivery. In contrast, chronological age (or postnatal age) is the time elapsed after birth.

Gestational age is conventionally expressed as completed weeks. Therefore, a 25-week, 5-day fetus is considered a 25-week fetus. Gestational age is often reported as W+D. For instance, 33 weeks and 2 days could be reported as 33+2.

Data model

Definition of the GestationalAge element
Field Type Multiplicity Description
weeks int32 1..1 Completed weeks of gestation according to the above definition. REQUIRED.
days int32 0..1 RECOMMENDED, If available

The following shows how the element can be used to report the gestational age of 33 weeks and 2 days.

    weeks: 33
    days: 2

The gestational age element is intended for use in phenopackets that describe prenatal clinical data.