In a human reproduction study conducted by a team of Israeli researchers led by Dr. Yuval Yaron, 1,325 pregnant women were tested at 10–13 weeks to compare hCG levels between women carrying female fetuses and women carrying male fetuses. The findings were conclusive: women who had a female fetus had significantly higher hCG levels than women carrying males.

The authors noted that the reason for the gender-related difference in maternal serum hCG has remained elusive since the phenomenon was first described in 1965. Based on these studies, some have speculated that it may be possible for woman carrying a male fetus to have a delayed positive urine pregnancy test result because of the lower level of hCG. However, Dr. Yaron and his colleagues concluded that while the gender-related difference in maternal hCG levels is statistically significant, it has little value in predicting fetal sex because of the small proportion of pregnant women with serum HCG concentrations that are high or low enough to allow a prediction with high probability