The rather miraculous process of human reproduction is surrounded in mystery and myth. Theories abound regarding how to improve fertility, treat morning sickness and induce labor. Predicting a baby's gender is another area filled with knowing opinions based largely in folklore and intuition.
I have recently heard it suggested that sperm for male children come from one testicle, and sperm for female children come from the other one. The evidence for this concept? A man who had damage to one testicle (from mumps) had five sons, supposedly because his "female testicle" was not working. This myth follows a basic misunderstanding about gender determination (not to mention that when mumps causes inflammation of the testicle, it usually affects them both).
Although there is no such thing as a "female testicle" or a "male testicle," a baby's gender is determined by his or her father. If the father provides an X chromosome, the baby will be female, and if he supplies a Y chromosome, the baby will be male. The body has no mechanism to segregate the X sperm from the Y sperm.
Because sperm containing X chromosomes and sperm containing Y chromosomes are nearly equal in number and because the chances for either type of sperm to fertilize an egg are roughly equal, each naturally occurring conception leads to an equal chance of having a boy or a girl.
Mothers can provide only an X chromosome, so they do not have the same influence over gender.
Myths regarding how baby boys or girls are determined probably have their roots in at least two faulty assumptions. One is the concept that past events alter future ones. For example, the chances that a couple will have five boys in a row is 1 in 32 (or about 3 in 100); but with each individual child, the chances of having a boy are 1 in 2 (50%), even if the couple already has four sons.
Another assumption is that when something unusual happens (for example, having five boys in row), there must be an explanation that is separate from the normal state of affairs. Although unusual things do happen, they do not imply that the laws of nature should be repealed!
It is perhaps easy to see how the notion of "testicular gender" might take hold, since men determine gender, there are two genders and two testicles. For a father who has five sons, it may be tempting to assume that a single functioning testicle has led to this otherwise unlikely situation, yet men who have had a testicle removed surgically (for example, because of an injury or a tumor) still, on average, father male and female babies in equal proportions.
Despite what you may have heard, there is relatively little a couple can do — through naturally occurring conception — to alter the chances of conceiving a boy or a girl. Many people believe that position during intercourse, diet, the lunar cycle or gender patterns in the couple's families all contribute to the sex of the baby. In fact, these factors have no well-established effect. In 2008, researchers in England published a study suggesting that higher calorie intake, higher potassium intake and eating cereal every day for breakfast could increase the chances of having a boy. However, the effect was rather small — 56% of the women with the highest calorie intake had boys. And these findings have not been confirmed by other researchers. In fact, studies using birth data during times of war and famine have not found a consistent effect of mothers' diets on babies’ gender frequencies.
The timing of intercourse does seem to have an effect. Sperm carrying Y chromosomes apparently swim faster but do not survive as long as their X-carrying counterparts. So if a couple has intercourse right at the time of ovulation, the faster-swimming male sperm have an advantage, and a male baby is slightly more likely — but only very slightly. And, if the timing is a day or two earlier, more of the male sperm die out, and a female baby is a bit more likely.
It’s not a particularly precise way to affect your baby’s gender. It's often difficult to time ovulation accurately, and your odds of determining gender do not change much. Without the application of advanced (and sometimes invasive and expensive) technology, such as in vitro fertilization, the chances of conceiving a boy or a girl remain close to equal.
If you have a strong preference for a boy or a girl, your options are limited. You could plan to have enough babies so both genders are eventually represented. Or it may be better to rethink your position and hope for a healthy baby of either sex.
Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.