Engagement of the fetal head in primigravidae and its relationship to duration of gestation and time of onset of labour.

In a retrospective survey on the time of engagement of the fetal head in 462 unselected consecutive primigravidae it was seen that by the end of the 37th week of gestation (259 days), engagement had occurred in only 23 per cent of them. The highest rate of engagement was from 39 to 40 weeks of gestation (273 to 280 days) and in 50 per cent of patients, engagement occurred between 38 and 42 weeks (266 to 294 days). The mean interval between engagement and delivery was 1.39 weeks (9.7 days), the median was 7 days, and the mode was less than 7 days. In 80 per cent of patients the engagement-delivery interval was less than 14 days. These findings strongly suggest that in the majority of primigravid patients the fetal head does not engage between 36 and 38 weeks of gestation (252 to 266 days) as is commonly believed.