Embryonic machismo.

Even from their earliest moments, men seem to be trying to get ahead. Men typically inherit one Y and one X chromosome, while women get two X chromosomes. Geneticists seeking to understand how this initial difference plays out during development have discovered that at least one gene on the Y chromosome turns on quite early -- even before the DNA from the egg and sperm have fully consolidated.

Moreover, XY (male) embryos grow and divide faster than their XX (female) counterparts, says Eugene Pergament of Prentice Women's Hospital in Chicago. This early difference may one day provide an easy way to determine the sex of embryos used for in vitro fertilization (IVF), he adds.

Pergament studied gene expression in embryos created through IVF. Because of abnormalities, these embryos could not be implanted into the mother. When a sperm first fertilizes an egg, only messages from maternal genes are active. Some of the embryo's genes then take over right away, while others don't get going until the embryo reaches the 10-cell stage, Pergament finds. The gene that codes for the male-determining factor, called sry, turns out to be one of the earliest to begin exerting its influence. No one knows exactly what the factor does, but once sry is turned on, it seems to stay active, he adds.

He and his colleagues then examined medical records for 36 IVF procedures. In IVF, physicians implant eggs 48 to 52 hours after fertilization, counting the number of cells in each implant. Pergament averaged these cell counts and matched that number against the sex of the resulting baby. He concludes that when the mother receives embryos that average four or more cells, she is six times more likely to have a boy.

Males of other species also tend to jump the gun in the womb, he notes. Male rat fetuses are heavier then female fetuses after 12 days, and male mice have more cells early on than females.