I've heard you can determine a baby's sex by having intercourse at a certain time in your cycle. Is this true?
Perhaps. While some studies have shown a 70 percent to 75 percent success rate in using timed intercourse to determine gender, other research has shown no influence.
Here's the theory behind timed intercourse: A child's gender is determined by a pair of chromosomes: XX for a female and XY for a male. Since a woman's eggs contain only an X and the man's sperm contain either an X or a Y, the sperm is the de facto decision maker regarding gender.
Studies indicate that sperm carrying the Y chromosome are faster swimmers, while those with the X chromosome seem to live longer in the woman's reproductive tract. Therefore, to conceive a boy, it is advised that sex take place close to ovulation—on days 12 to 14 of a 28-day cycle—to let those speedy swimmers reach the egg first (and before expiring). If you are hoping for a girl, intercourse should occur well before ovulation—on days 9 to 11—to allow the longer-living X-chromosome-bearing sperm the necessary time to reach the egg. (Sperm carrying the Y chromosome will have expired by the time the egg is released.)