Differences between boys and girls show less than three weeks into pregnancy

Female embryos exert a greater influence than male embryos over the hormone that nurtures early pregnancy, and the difference can be detected as little as 16 days after conception, according to new research published (Wednesday 30 January) in Europe’s leading reproductive medicine journal Human Reproduction.*

Israeli scientists have demonstrated that levels of maternal serum HCG (MSHCG) – the hormone that reveals in blood or urine tests whether or not a woman is pregnant – are nearly a fifth higher less than three weeks into pregnancy if a woman is carrying a girl than if she is carrying a boy.

HCG’s role at the beginning of pregnancy is to maintain the function of the corpus luteum – a temporary structure formed in the ovary after an egg is shed that produces oestrogen and progesterone until the placenta can take over their production.

“Other research has already shown that MSHCG is significantly higher in the presence of a female fetus in the second and third trimesters of pregnancy. But we’ve discovered that levels were increased as early as 16 days after fertilisation,” said lead researcher, Dr Yuval Yaron, Director of the Prenatal Genetic Diagnosis Unit in the Genetic Institute at Tel Aviv Medical Center.

He said that their finding that MSHCG levels increase with female fetuses before the development of the glands that produce the fetal hormones means that the higher level must be due to the way that the placenta expresses proteins in female pregnancies and not to male hormones suppressing MSHCG, as has been suggested by other researchers. Genes that the researchers believe are potential candidates for regulating protein expression in the placenta have been mapped to the X (female) chromosome.

The study involved 347 IVF pregnancies with women being measured for MSHCG levels between one and three times from day 14 to day 20. There were 184 female fetuses and 163 male fetuses.
MSCHG levels were 18.5% higher by three weeks’ gestation in the women who had girls.

But, Dr Yaron pointed out that the findings could not be used alone to predict the sex of a baby.

“Although the gender differences are statistically significant, the proportion of pregnant women with serum HCG concentrations high or low enough to allow a prediction with high probability is too small. It would be possible to predict the sex of a fetus if we can identify other markers that also demonstrate early gender-related differences. We are working on this now and hope to have some results soon.”

The ability to identify the sex of a baby very early in a pregnancy could help parents who carry inherited serious sex-linked conditions and who have to face the difficult choice of whether or not to continue with a pregnancy.

* Maternal serum HCG is higher in the presence of a female fetus as early as week 3 post-fertilization. Human Reproduction. Vol 17. No 2. pp485-489.