Sex chromosomes are one pair among the 23 pairs of chromosomes, determining the baby's gender. The other 22 pairs are called autosomes, which determine the baby's genetic makeup.
There are two types of sex chromosomes: the X chromosome and the Y chromosome. An egg contains only an X chromosome, while sperm contain either an X or a Y chromosome. When an egg and sperm unite, their chromosomes also combine.If both chromosomes are X, the baby will be female. If one is X and the other is Y, the baby will be male. In essence, the baby's gender is entirely determined by the father: if the sperm carries an X chromosome, the baby will be female; if it carries a Y chromosome, the baby will be male. Some analyses suggest that sperm carrying the Y chromosome swim faster, while those carrying the X chromosome have greater vitality. Therefore, the ratio of boys to girls remains roughly equal.
Below are other potential influencing factors for your reference, though it's important to note these are merely suggestions with probabilities far lower than the actual variation rate.
Relationship with male work stress. Higher work stress in men can subject the testes to significant changes in pressure, temperature, or water pressure, making the more fragile Y chromosome sperm more likely to die off first. This may increase the likelihood of having a girl.
Relationship with acid-alkaline environment. Sperm carrying the X chromosome swim fastest in acidic conditions, while Y-chromosome sperm thrive in alkaline environments. Thus, the mother's internal pH level plays a role.
Relationship with orgasm. The normal vaginal environment is acidic, while semen is alkaline. However, when a woman reaches orgasm, the acidity in the vagina gradually weakens, creating a more favorable environment for Y-chromosome sperm to thrive.