Maternal serum HCG is higher in the presence of a female fetus as early as week 3 post-fertilization

Background: Maternal serum HCG (MSHCG) is higher when the fetus is a female than when it is male. This has been demonstrated in the second and third trimesters of pregnancy, and recently at 10-14 weeks gestation. In this study we assessed whether this gender-related difference can be detected as early as week 3 post-fertilization. Methods: The IVF setting was chosen because it provides precise dating of gestational age and early sonography for the number of gestational sacs. The study included 347 IVF cycles from 335 patients. Only pregnancies with a single implanted embryo that resulted in a single live birth of known gender were included. MSHCG was measured on days 14-20 post-fertilization, and levels were expressed as gestational age-corrected multiples of the median (MoMs). The log(10) MSHCG MoMs were compared according to fetal gender. Results: MSHCG levels were significantly higher (18.5%) in week 3 post-fertilization in the presence of a female fetus (P<0.0002). Conclusion: Because a fetal gender-related difference in MSHCG can be demonstrated as early as week 3 post-fertilization, the difference may be attributed to placental factors and not to the effects of the fetal hypothalamic-hypophyseal-gonadal axis.