By Judy Siegel


TEL AVIV (January 31) - Israeli researchers have shown for the first time that it is possible to differentiate between a male or female human embryo in the uterus from as early as 16 days after conception.

The team at Tel Aviv's Ichilov Hospital, headed by Dr. Yuval Yaron, said the level of serum HCG - the hormone tested to determine whether a woman is pregnant - is higher in the mother when the embryo is female than when it is male.

Until now, researchers have shown that HCG is higher for female embryos only at three months' gestation, and thought the hormone results from the embryo's own sex organs.

However, the fact that the sex is detectible at only 16 days - when the embryo's gonads and sex glands have not yet formed - indicates that the difference is genetic, apparently resulting from the female's two X chromosomes compared to the male's single X chromosome. Yaron said the higher HCG level must be due to the way that the placenta expresses proteins in female pregnancies and not to male hormones suppressing HCG in the mother's blood.

The Ichilov team's discovery was published in the latest issue of Europe's leading reproductive medicine journal, Human Reproduction, which is published in England.

Yaron, who wrote the article along with Ofer Lehavi, Avi Orr-Urtreger, Ilan Gull, Joseph B. Lesssing, Ami Amit and Dalit Ben-Yosef, told The Jerusalem Post that he "stumbled upon" the discovery. "We were looking for how early one could see the difference in HCG and what causes the difference," he said. Knowing the sex of an embryo so early could be important in screening those affected by sex-linked genetic diseases such as Duchenne's muscular dystrophy, hemophilia, various metabolic disorders, and types of mental retardation, most of which occur in boys because they are linked to the Y chromosome.

However, Yaron insisted that even if an embryo produced by a couple with a history of such genetic diseases were found to be male, there would be a 50 percent chance that the baby would be unaffected, thus the test could not be the basis for an abortion.

In 1965, researchers S. Brody and G. Carlstrom determined that HCG levels are higher in women bearing female babies during the third trimester of pregnancy. But until Yaron's work, which involves complicated mathematical calculations, no one was able to show this any earlier than the third month of pregnancy. 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 estrogen and progesterone until the placenta can take over their production.

In the new research, the levels of maternal serum HCG was found to be nearly a fifth higher if the woman is carrying a girl than if she is carrying a boy - but the condition is that the exact day of conception must be known. If the date is off even 24 hours, the results are not relevant said Yaron, a gynecologist, geneticist, and director of the prenatal genetic diagnosis unit in the hospital's genetic institute.

Source: Jerusalem Post 01/31/2002