Sex Differences In Length Of Gestation In Mammals

SEX DIFFERENCES IN LENGTH OF GESTATION IN MAMMALS
By THOMAS McKEOWN From the
and

BRIAN MacMAHON

Department of Social Medicine, University of Birmingham (Received 6 September 1955)

Observations on length of gestation of males and females are presented for man, the guinea-pig, sheep, pig and rabbit; from the literature, data are also available for the cow, horse, camel and goat.Pregnancy is longer for males than for females in the cow, horse and possibly the sheep and camel; it is longer for females than for males in man and possibly the guinea-pig. No definite conclusion is reached about the pig and rabbit, in which litter-size is normally large, or about the goat, in which the number of observations on record is small. The sex differences in length of gestation are reflectedin the sex ratios of offspring born after different periods of gestation, and in the percentage distribution of the sexes by duration of gestation. The difference in length of gestation can be attributed to a sex difference in foetal weight in man, but not in the guinea-pig or in species such as the cow, in which gestation is longer for males than for females. It is suggested that in the cow thelonger male gestation may be due to the fact that the proportion of males conceived is higher if mating takes place early, rather than late, in heat. (The only alternative explanation is a sex difference, attributable to something other than weight of foetus, in the period between fertilization and birth.) In man the earlier birth of males is apparently due to their greater weight, attributablewholly or in part to a sex difference in rate of prenatal growth. This observation does not exclude the possibility that in man, as in the cow, the proportion of males conceived is higher if mating occurs early in the cycle, since the method of recording length of gestation (from onset of menstruation) gives no information about time of mating.

Recorded sex differences in length of gestation inmammals are small, and no parti¬ cular significance has hitherto been attached to them. In man, for example, the mean period of gestation is shorter for males than for females, but the difference is only a fraction of a day, and has been generally regarded as of statistical rather than biological interest. Two observations suggested that the matter might be worth further investigation. The first wasa fairly marked association, in the guinea-pig and in man, between length of gestation and sex ratio of offspring. The second was the fact that the relationship between sex and gestation is not the same in different species ; in the cow, for example, males are born later than females. This last observation suggested that in one species at least the sex difference in time of parturition could notbe attributed to a sex difference in foetal weight (perhaps the most obvious explanation of the earlier birth of the human male foetus), and might be due to variation in the time of mating, rather than to variation in the interval between fertilization and birth.
MATERIAL

(a) Man Human data are derived mainly from three sources : (i) An investigation of all births (23,970) in Birmingham during1947 [Gibson & McKeown, 1950] ; sex and duration of gestation (from onset of the last menstrual period preceding pregnancy to birth) were known for 17,071 single births, (ii) Examination of 1227 consecutive single preg-

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T. McKEOWN AND B. MacMAHON

nancies in the county borough of Smethwick [Gibson & Dougray, 1953]. In 763 it was possible to estimate the period between ovulationand birth, by using the length of the menstrual cycle (recorded for 1 year after delivery) to calculate the interval between onset of menstruation and ovulation, (iii) Histories of 400 mothers whose first- and second-born children were delivered in the same Birmingham maternity hospital, and for whom obstetric notes were available. Examination is, however, restricted to 214 mothers (428...