Observations related to the aetiology of placenta praevia, with special reference to the influence of age and parity.

It is generally accepted that the frequency of placenta praevia is higher among multigravidae than among primigravidae. This observation has prompted the suggestion that low implantation of the ovum is a result of functional or structural changes in the uterus resulting from repeated pregnancies (Solomons and Canter, 1932; Davis and Campbell, 1946; Shaw, 1947; Findley, 1951). It is possible, however, that the association of placenta praevia with parity is secondary to its association with age, a result which would have different implications in respect of aetiology. It is surprising that this possibility has received little attention. Penrose (1939) analysed a small series (72 cases) using a modification of the Greenwood-Yule method. He concluded that increasing maternal age is significant in central placenta praevia, and suggested that in non-central cases multiparity may be more important. These conclusions cannot be accepted unreservedly since a number of mothers had not reached the end of the reproductive period. Use of the Greenwood-Yule method in these circumstances is likely to give a false impression of the importance of increasing parity or maternal age. (This' subject has received fuller consideration elsewhere-McKeown and Record, 1956). Kalmus (1946) compared a larger series (162 cases) with a random sample of hospital births and came to conclusions similar to those of Penrose. The control group was not entirely satisfactory, however, since hospital deliveries as a whole contain an abnormally high proportion of primiparae. No selection is applied to admission of cases of placenta praevia, which may therefore show *an apparent association with multiparity. It appears that a more reliable assessment of the relative importance of age and parity in placenta praevia would be useful, and that the suggestion that there is a sharp aetiological distinction between central and non-central types requires investigation. The present inquiry has been undertaken with this object. The method used was a comparison between a large series of cases of placenta praevia and a control group of births selected at random from the population in which the cases occurred. Also considered were the association with twinning and congenital malformations and the incidence of previous abortions and of ante-partum haemorrhage.