Galton Laboratory Department of Biology University College London London, United Kingdom NW1 2HE
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INTRODUCTION |
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Demissie et al. (1), on the basis of their own data and of a meta-analysis of previous studies, concluded that an excess of sons is associated with placenta previa. They wrote: "The mechanism for this association remains undetermined, but it may provide clues to the causes of low placental implantation (1
, p. 829)."
I drew the same conclusions (2) and offered a hypothesis on this point. Very substantial quantities of data have been adduced to suggest that parental hormone levels around the time of conception are associated with the sexes of the resulting offspring (3
). Ex hypothesi, high levels of estrogen are associated with boys and low levels with girls.
In my article (2), the high sex ratio (proportion of boys) associated with placenta previa was juxtaposed with the low offspring sex ratio reportedly associated with ectopic pregnancy. I wrote:
The ovum is normally fertilized at the upper end of the Fallopian tube. It is helped in its journey to the uterus by smooth muscle contractions of the tube, and it has been shown that these contractions are dependent on estrogen concentrations (Jansen 1984 [4]). If estrogen concentrations are below optimum, these contractions may be presumed to be insufficient to discharge the ovum into the uterus before it "hatches" from the zona. Under these conditions one may expect nidation in the tube itself and an ectopic pregnancy. In contrast, in cases of placenta previa, one may suspect an undue degree of hormonally induced tubal motility. In such cases the fertilized ovum would be discharged into the uterus some days before it was due to "hatch" and nidate. Under these circumstances, one would expect an excess of nidations in the vicinity of the cervix as a consequence of either random movement within the uterus, or any slow escape of fluid from the uterus through the cervix. In short, it is suggested that placenta previa is due to high maternal concentrations of estrogen and that (some) ectopic pregnancies are caused by low maternal estrogen. Thus explanations are available for both these phenomena and for the unusual sex ratios associated with them. (It is not claimed that anomalous degrees of tubal motility are the only cause of either condition) (2
, pp. 14045).
The point could be tested by examining the extent to which hormone concentrations control Fallopian tube motility.
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REFERENCES |
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