The medical literature is rife with all sorts of claims about unusual ways to predict the sex of an unborn baby.
One that has been around for decades, and has even gained some acceptance, is the idea that fetal heartbeat is faster among girls. Rates above 140 beats per minute, it is said, are typical for girls; below that, look for a boy. How this belief came about is not entirely clear, but studies that have looked into it over the years have traced it to folklore.
They have also found that the belief holds little water. Typically, the embryonic heart rate starts out at about 85 beats per minute and then accelerates roughly 3 beats per minute each day during the first month. After the rate reaches an average of about 175 beats per minute, studies show, the acceleration reverses; by the middle of pregnancy, the rate averages 120 to 160 beats per minute.
In a study published in The British Journal of Obstetrics and Gynecology, researchers studied fetal heart-rate variations in 79 women, looking for differences between male and female fetuses. They could not find any.