Medicine: Got a Nickel?

Once the embryo has entered the uterus (see above), it is still nine months before anyone knows for sure what sex it will be. Doctors have not had much luck at predicting; tossing a coin has proved as good a test as any ("Heads it's a boy; tails it's a girl"). At the gynecological congress, Belgium's Dr. Pierre A. Rosa told of a surefire test discovered by accident during some research on the placenta.

Both male & female babies slough off many different types of cells into the amnion ("bag of waters") in which they are carried. After the seventh month, a...