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Editor,—Grech et al 1 reported that sex ratio (proportion male) at birth declines highly significantly with increase in geographical latitude in Europe. I offered evidence that this may be attributable to variation of maternal hormone levels with latitude.2Grech,3 while acknowledging this possibility, proposes that the effect with latitude may be secondary to an effect with temperature. Accordingly he writes : “an interesting study would be the analysis of seasonal variations of sex ratios at birth, for individual countries. A latitude effect would thus be excluded, and any variations in the birth sex ratio would be more likely to be caused by a temperature variation effect”. If Grech were correct, conceptions in cold seasons should produce an excess of female births. I have reviewed 17 studies of seasonality of sex ratio at birth.4The results of the smaller studies were inconclusive, but the largest studies found that in the USA, over half a century, sex ratios at birth tended to be high in early summer, and low in autumn and winter. This was so for black and white births, for urban and rural births and for stillbirths. The variation was very slight—that is, from a peak of 0.5140 in June, to a trough of 0.5116 in February. Thus there is a tendency for a low sex ratio at birth to be associated with spring and summer conceptions (rather than winter ones). In short, the available evidence is against Grech's suggestion. However, the variation of sex ratio with season and latitude (and a number of other variables, for example, maternal age, paternal age, birth order, race, social class, smoking) is so slight as probably to preclude useful search for the causes. In contrast, large sex ratio variations are associated with various forms of placental pathology. Highly substantial and significant male excesses are associated with abruptio placenta, placenta praevia, fatty liver of pregnancy, and toxaemia: highly significant female excesses are found with placenta accreta and extrauterine pregnancy.5 I suggest that workers should pursue this variation because it is clear that some of these disorders are not caused by the (sex of the) fetus. I have hypothesised instead that they are caused by maternal hormone profiles that predate the disorder and are responsible both for the disorder and the sex of the fetus. The point could be tested, among others, by examining the extent to which hormone concentrations control fallopian tube motility.
Author's reply: More on sex ratios at birth
Editor,—The proposed theory—that is, that maternal hormone profiles are responsible for reproductive disorders that may produce an excess of one gender over another—is very interesting.1-1
However, the difference in gender ratio by latitude noted in our study, although small, was highly significant (p<0.001) because of the large number of live births included (over 16 million) over the relatively short period studied (1990–95).1-2 While a large literature review by James in 1987 failed to find any strong association between ambient temperatures and M/F,1-3 a more recent study by Lerchl in 1999, which included approximately 50 million live births, showed that in Germany, over the period 1946–1995, significantly more male children were born in April to June. Naturally this peak was conceived in July to August, the warmest part of the year.1-4
How could maternal hormone profiles and the environment be tied together? What might influence maternal hormone levels in different latitudes, and hence different countries? Would such hormonal differences be innate, perhaps because of interracial differences, or might external influences, such as diet or sunlight be responsible?
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