The effect of sexual position on baby gender is unproven, but most speculation about it is based around the theory that "male" sperm are small and fragile but quick, whereas "female" sperm are larger and tougher but slow.

What decides the gender of a baby?

A child's biological sex (male or female) is determined by the chromosome that the male parent contributes. Males have XY sex chromosomes while females have XX sex chromosomes; the male can contribute the X or Y chromosome, while the female must contribute one of their X chromosomes.

How can I increase my chances of having a baby girl?

According to this method, to increase the chance of having a girl, you should have intercourse about 2 to 4 days before ovulation. This method is based on the notion that girl sperm is stronger and survives longer than boy sperm in acidic conditions. By the time ovulation occurs, ideally only female sperm will be left.