Gestational age is the duration of gestation (being with child) measured from the first day of the last normal menstrual period. Gestational age is expressed in completed days or completed weeks (e.g. events occurring 280 to 286 completed days after the onset of the last normal menstrual period are considered to have occurred at 40 weeks of gestation).
Gestational age is frequently a source of confusion, when calculations are based on menstrual dates. For the purposes of calculation of gestational age from the date of the first day of the last normal menstrual period and the date of delivery, it should be borne in mind that the first day is day zero and not day one; days 0-6 therefore correspond to “completed week zero”; days 7-13 to “completed week one”; and the 40th week of actual gestation is synonymous with “completed week 39”.
Where the date of the last normal menstrual period is not available, gestational age should be based on the best clinical estimate. In order to avoid misunderstanding, tabulations should indicate both weeks and days.
Different gestational age groups can be distinguished, compared to the normal time period before giving birth:
- pre-term: less than 37 completed weeks (less than 259 days) of gestation;
- term: From 37 completed weeks to less than 42 completed weeks (259 to 293 days) of gestation;
- post-term: 42 completed weeks or more (294 days or more) of gestation.