One of the questions asked by medieval thinkers was how did the sex of a baby come about? Why did couples sometimes produce a boy and sometimes a girl?
There were lengthy discussions on the matter such as the 'questiones' analyzing Avicenna's chapter 'On the Generation of the Embryo' and then there were short recipes in folk lore and other medical treates that didn't have a detailed explanation. The Trotula suggest one way of conceiving a male child was for a woman to have her husband take the womb and vagina of a hare, dry them to a powder, mix with wine and drink. The woman should take the testicles of a hare and do the same thing, and as soon as she had finished menstruating, lie with her husband, after which they would conceive a male child...
Boys were seen as the more desireable gender to be conceived and some writers of the time, thought that a girl was in fact a failed boy and that the father's seed was defective in that it had failed to form the mother's contribution to the pregnancy into a boy. There was also the idea that if a girl was born, then the mother's influence had won out over the man's because she had a 'masculine heart.' One wonders if Louis VII thought this about Alienor of Aquitaine!
Factors influencing conception included the nature and humours of the parents, their relationship to each other, the circumstances of intercourse and the astrological situation at the time of conception including the phase of the moon.
Giles of Rome, a late 13thC scholar and promoter of the theories of Aristotle made this ten point tick list for determining what sex a child would be.
1. Quality of male sperm: warm produces a male child
2. Quantity of male sperm: more produces male child
3. Age of father: a man just past his youth produces a male child
4. Place and disposition of testicles: a warm right testicle produces a male child.
5. Fetus's place in the womb. The right side produces a male child
6. Complexion of menstruum: warm produces a male child
7. Food and drink (of father or of both parents) before intercourse: warm produces a male child
8. Air: cold season or region produces a male child.
9. Winds - the north wind produces a male child.
10. Celestial influence will determine whether the child is male or not.
There are a variety of systems at work here. While warmth in the body produces a male child, cold seasons and strong north winds were seen as favouring the masculine, but men with 'cold and dry' testicles...rarely or never generate because their seed is useless for generation.' (Trotula).
The right side of the womb was viewed as the warmer side and better for producing a male child because of the heat of the nearby liver, while the left side, closer to the colder spleen (in the way medievals understood the humours and composition of the body) would produce a female child.
There was a theory, promulgated by the medical schools at Salerno that the womb was divided into 7 cells. Three on the left, three on the right. Any fetus occupying one of the left cells would be female, any to the right would be male. The one in the middle would produce a hermaphrodite.
You could tell what sex a child was going to be at birth by observing a pregnant woman. More swollen on the right meant a boy, and also if she started walking by using her right foot the same.
Where sheep were concerned, if they faced north when mating, then the lamb would be a male, if south, female. One supposes that shepherds, intent on increasing their flocks, would be hoping for the latter!