There are plenty of old wives' tales regarding how to tell whether a pregnant woman is going to have a boy or a girl. But one thing that doctors know for certain is that in the second and third trimesters, women having girls display higher levels of a hormone known as maternal serum HCG (MSHCG) than do women pregnant with boys. Now new research suggests that such hormonal differences appear less than three weeks after conception. The findings, published today in the journal Human Reproduction, may help explain how girls and boys exert control over their mother's hormones.

Yuval Yaron of the Genetic Institute at Sourasky Medical Center in Tel Aviv and colleagues followed 347 pregnancies achieved through in vitro fertilization. The researchers tested the mothers' MSHCG levels between 14 and 20 days after fertilization and detected some differences as early as day 16. Three weeks into pregnancy, women carrying girls exhibited hormone levels 18.5 percent higher than those of their boy-carrying counterparts, regardless of factors such as previous pregnancies or maternal age.

Finding this gender-related difference so early in pregnancy may help explain how it occurs. Previous research into the variance suggested that either some genes on the X chromosome that regulate protein expression may become over-expressed in the presence of a female fetus or hormones from male fetuses could suppress MSHCG. Because the glands that produce fetal hormones do not develop within the first three weeks of pregnancy, the authors conclude that "there is a differential expression of genes by the placentas of female compared with male fetuses."

Such differences alone cannot successfully predict the sex of a baby so early on, however. According to Yaron, the proportion of pregnant women with MSHCG levels either high or low enough to allow for a successful prediction at three weeks is small. "It would be possible to predict the sex of a fetus," he explains, "if we can identify other markers that also demonstrate early gender-related differences." Until then, parents-to-be will just have to wait a bit longer to find out what color to paint the nursery.