Gestational age is the best clinical estimate of the duration of pregnancy at a specific point in time, based on the first day of the last menstrual period, ultrasound or physical examination of the baby. Gestational age is conventionally expressed in completed weeks. When gestational age is calculated using the first day of the last normal menstrual period, the first day is counted as day zero and not day one. Therefore, a 25 week, 5 day fetus is considered a 25 week fetus (25+0, 25+1, 25+2, 25+3, 25+4, 25+5, 25+6). When ultrasound is used to date a pregnancy, the earliest ultrasound examination should be used and should preferably be between 6 and 10 weeks gestation. Scans performed beyond 24 weeks gestation are unlikely to be reliable in estimating gestational age and should not be used for this purpose. The duration of gestation can be determined from the first day of the last normal menstrual period, from ultrasound or clinical assessment. For the purpose of the national collection, gestational age is expressed in completed weeks. The World Health Organization identifies the following categories: - 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; and
- post-term: 42 completed weeks or more (294 days or more) of gestation.
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