The sex of babies is determined at conception, when exactly half of embryos should be girls and half boys.
The first half of that is wrong by modern claims. Sex is, of course, observed at that stage.
It’s also wrong by the older, truer, standards in the second half. Because there are more conceptions of boys that girls. Can’t recall the number but 10% is calling from dim memory. The equity is 50/50 at puberty and since male embryos are more likely to abort, male children more likely to die, there are more conceptions of males than females.
Sigh, that’s pretty good, being wrong about both modern and real science in the once sentence.