Doctors love conventions. They have them in exotic places and they frequently use them to defend what would otherwise be inexplicable calculations. Thus 38 weeks of true gestation is, of course, 40 weeks of amenorrhoea, which may also be viewed as 38–40 weeks of pregnancy if timing is based on a “conventional” menstrual cycle. Even if this leaves the outside world to wonder, the clinicians understand. Many of the proponents of such ambiguity have moved away from obstetrics into reproductive medicine but have carried with …