It was well known that a certain percentage of foetuses are always malformed, and, as a result, in some cases a pregnancy may abort spontaneously, leading to a miscarriage. But in others the pregnancy may continue and the mother prepare for birth with no knowledge that she is carrying a malformed foetus. Foetal malformation increases the risk of a labour obstructed by the bones of the mother's pelvis, especially if the foetus suffers from hydrocephaly as in the cases described by Jacques Bury or Louise Bourgeois. A surgeon's intervention may then become inevitable. To the Early Modern mind, a malformed foetus may be close to a monster, a subject which both terrorised and fascinated the wider public. Ambroise Paré published a famous study of monsters, while Jacques Duval devoted the last third of his obstetrical treatise to the nature and conception of hermaphrodites. Many other authors were also interested in hydatiform moles, often assumed by the lay public to be monstrous conceptions, even though they contained no foetal matter.