The Initiation and Course of Labour
In man, as with other mammals, the gestation period is remarkably constant, and has a duration of 40 ± 2 weeks (i.e. 10 lunar months) from the last menstrual period, with the majority of pregnancies lying between these limits. However, in extremely rare cases, normal babies have been born after as little as 32 or as much as 52 weeks of gestation. Gestational age is most often calculated from the first day of the last menstrual period which occurs 13–16 days before ovulation. This is termed menstrual age, and it is a useful measure since it is timed from an obvious and visible event which the mother is often able to date precisely. Fetal age is also commonly used to refer to the progress of gestation, and is defined as menstrual age minus two weeks. Because some women are unsure of the date of their last menstrual period, other methods may have to be used to determine how far pregnancy has advanced. These were discussed in chapter 10.
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