While there are fertility theories like this all over the place, there is no scientific evidence to link time of conception and the sex of the baby.
For example, one popular theory claims that you can up your odds of conceiving a baby girl by making love no closer than two to three days prior to ovulation, and that you can increase your chances of conceiving a baby boy by timing intercourse as close to ovulation as possible.
Remember that the woman always supplies an X chromosome. The gender of the baby is determined by the chromosome supplied by the sperm, an X for a girl and a Y for a boy. Male sperm are more fragile and don’t live as long as female sperm, but they tend to swim faster. So some people believe that you can increase your chance of giving birth to a boy or a girl when you are trying to conceive simply by timing conception.
Then there’s the Chinese fertility calendar, which purportedly determines the gender of your child by the lunar age of the mother and the month the child is conceived. But that’s an old wives’ tale at best.
Even ultra high-tech methods of sex selection, which go with in vitro fertilization methods when you’re trying to conceive, don’t offer couples much better than 75 to 25 odds, so you’ve gotta be dreaming if you expect some of these low-tech sex selection techniques to deliver up a baby of the “right” sex.
So if you’re trying to conceive and you have your heart set on either a boy or a girl, there’s probably little you can do to guarantee the gender of your baby.