By 'sex' we are referring to fetal gender, not any activities that may have lead to the pregnancy, sorry. We wrote a paper on fetal sex determination with ultrasound that appeared in the New England Journal of Medicine in 1983. Almost nobody was providing parents with that information, if they wanted it, then, and now, it has become a routine part of prenataldiagnosis.
All embryoes try to become female, and when a Y chromosome is present, development detours towards male. Moms have two X chromosomes and can only provide an X to their offspring, while dads provide an X or a Y, and hence determine fetal sex genetically. Sperm with Y chromosomes are supposed to swim faster than those with X chromosomes, but X bearing sperm may survive longer - which has lead to some misleading notions that try to relate fetal sex selection to specific intercourse positions or to the timing of relations versus the day of ovulation. External genitals start to differentiate at about 10 weeks after conception. Prior to this time, there is no external difference in boys and girls, eventhough their gender identity has been set since the moment of conception. |
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Armed with some basic info about
X and Y chromosomes, it's easy to blame or congratulate a dad for fetal gender,
but that is probably not correct. Some couples seem to make one model, which
is hard to reconcile with millions of X and Y sperm, randomly competing to
fertilize an X ovum. We have a suspicion that there are maternal factors within
the genital tract that influence which of those little guys gets through,
and perhaps, Moms are in control of this part of development too. This image
is a 3D magnification view of the clitoris and labia of a little girl of 18
weeks GA. It is pretty easy to tell if it is a boy or girl with a quick ultrasound look between the legs after 14 weeks GA. |
