Fertility and reproductive history of mothers of mongoloid defectives.

In a previous communication (Smith and Record, 1955) it was observed that there appeared to be an association between mongolism and primogeniture which invites investigation of the fertility of mothers of mongols. Such a study is by no means new. It has been attempted by many previous workers, prompted perhaps by the observations that the risk of producing a mongol increases sharply with advancing maternal age and that fertility of women becomes impaired as age increases. Various indices of fertility have been used. The one most easily recorded is the size of the family at the end of the reproductive period. Shuttleworth (1909) considered that families containing a mongol tended to be larger than usual, but Brousseau (1928) regarded family size as unimportant; in neither case was comparison with a control group attempted. Penrose (1949), using fraternities containing nonmongoloid mental detectives as controls, found that mean sibship size was raised in families containing mongols. This observation could easily result from the higher maternal age at which mongols are born; by controlling this variable, Jenkins (1933) showed that the mongols of Shuttleworth's and Brousseau's series came from families rather smaller than average. The matter cannot be regarded as settled, however, since the control group used by Jenkins (U.S. births in 1927) probably differed considerably from the population from which the cases were drawn. A further objection is that the cases were diagnosed some time after birth; the increased risk of infection to which members of large fraternities are exposed, and the high mortality of mongols from infectious disease, would tend to eliminate at an early age mongols born into large families. Another measure of fertility is the pregnancy-free interval preceding birth. The view that this interval was longer than usual was first recorded by Langdon Down (1909). Although no comparison with a control group was attempted, Beidleman (1945), Benda (1946, 1949), and Engler (1949) supported this view, but Penrose (1934) and Bleyer (1938) considered it to be unimportant. The effect of maternal age appears to have been ignored except by Jenkins (1933), who found that the mean pregnancyfree interval preceding mongol births was longer than that shown by the general population of comparable age. This result, however, cannot be unreservedly accepted in view of the unrepresentativeness of his control group. The pregnancy-free interval before mongol births has also been compared with the interval before the birth of unaffected sibs. By this technique, 0ster (1953) showed that the period preceding the mongol conception appeared to be longer. The method presents many difficulties, one of which (referred to by Record and McKeown, 1950) arises from the fact that the larger, and therefore more fertile, families are unduly represented among the normal sibs. 0ster avoided this error by restricting comparison to the same birth-rank intervals, but was consequently prevented from matching for maternal age, since in each fraternity he chose unaffected sibs born before the mongol. Two sources of error were thus introduced: