Dear Dr. Kate,
When trying to conceive, are there things you can do to try to tip the scales in the direction of one sex or the other, i.e. so if you wanted a baby girl you could increase your odds and vice versa?
-- Gender Bender
There are some theories about trying for a girl or a boy, famously laid out in Dr. Shettles' book, How to Choose the Sex of Your Baby. These ideas haven't been tested in large-scale studies (though that would be interesting!), so I'd file this advice in your doesn't-hurt-to-try-it folder.
Male sperm are thought to be faster but weaker swimmers, so to try for a boy, you want to have sex as close to the time of ovulation as possible. Have sex the day before and the day of ovulation, if you can. And the best positions are ones that get the deepest penetration-him entering her from behind, or on her back with her legs pulled back-to deliver the sperm closest to the uterus. Best of all, female orgasm helps move those little guy swimmers towards the egg, so she should try to climax around the time that he does.
Female sperm, on the other hand, are slower but heartier, so to try for a girl, you want the male sperm to have swum off (or died off!), leaving only the female ones behind. The best times to have sex are two or three days before you ovulate. Your guy should pull out slightly when he comes; this makes the sperm have to travel the length of the vagina, increasing the chances that the stronger female sperm will make it. And the woman should try not to have an orgasm when trying for a girl (oh, the irony).