Some French beliefs tied to being a godmother or a godfather (taken from Eloise Mozzani's Le Livre des Superstitions/The Book of Superstitions. Has a LOT of things. Doesn't always know when to make a choice and has a wide but dubious range of sources, but still is VERY complete in both good and bad)
The godfather and godmother must not be chosen before the birth of the baby, else the baby will be the subject of divine wrath and be doomed to die. One must wait until the baby let out its first cry before choosing them.
Up until the 16th century, especially in wealthy and/or noble families, one could have several godfathers and godmothers - in Venice, sometimes people had up to FIFTY godfathers and godmothers). To limit these excesses, it was decided that someone could only have two godfathers and two godmothers are most.
If a little girl has a two godfathers, or a little boy two godmothers, the child will marry at least twice in their life. However this belief doesn't hold anymore as later the limit was put to one godfather and one godmother per child.
The godmother must not be pregnant during the baptism of her godchild, else she could "dirty" the fate of her godchild and endanger her own baby with upcoming death. It seems to be based on the logic that a future mother should not be thinking/loving/dedicating herself to the child of another... But then you also have the belief that if a woman expects twins she can be a godmother while pregnant no probs.
In general there was a LOT of criteria about the godmother, who not only could not be pregnant when chosen, but also should not be grieving for anybody, and could not a godmother again if a previous godchild of hers died. The "ideal" godmother had to be a single, virgin woman, or at most married but without children. In some regions it went as far as the parents' duty being to make sure the godparents didn't have any "physical or moral abnormality", else it would be passed onto the child. A common saying in the Vosges for example was "A parrain niais, filleul stupid" (For a naive godfather, a stupid godson). However, another common saying is that boys inherit the behavior/temper of their godmothers and girls the one of their godfathers. In Belgium, if your child dies and you have a new one, you can't reuse the same godparents.
To be a godparent for the first time brings good luck - especially if the first godchild is a girl for a man, a boy for a woman. In Bretagne it is thought that when you are a godparent for the first time, all the cats of the hamlet/town will go watch the baptism.
While in some regions of France it is believed that being a godmother favors marital matters (a woman whose first godchild is a boy will be married fast, if it is a girl she will only ever be able to give birth to girls), in Belgium two lovers or engaged people who end up as godparents of the same child are sure never to be married. (It is probably because early on, before the Reformation, the Church forbade the godmother and godfather from marrying). Also, in France the reverse belief is held that two godparents are more likely to get married to each other - but in the 15th century it was also considered that, every time a godfather had sex with his godmother (his "commère"), their "carnal union" would cause a storm.
Before, it was deemed that a godfather could not marry his goddaughter. It was a monstrous, almost incestuous union. A tale told of how a girl who married her godfather gave birth not to a human baby, but to twelve hedgehogs.
If on the morning of a baptism a godfather urinates in front of the su, his godchild will never suffer bladder problems.