Q. Are more boy babies born, or more girls?

Concord

A. Boys have the edge at the start. But nature has a feminine preference from then on.

There are approximately 105 boys born for every 100 girls. There is little regional variation in that ratio around the world.

Reproductive biologists aren't sure why more males are conceived, but some believe it has to do with how much the sperm for each gender weighs. The sperm that will create a female carries an X chromosome. The `male' sperm has a Y.

Think of that Y as an X that's missing a leg. That leg has a tiny but real weight. So the sperm that will make a male is lighter. Scientists think that may help the male sperm swim faster, or use less energy and last longer, as millions of sperm compete to be the one that will fertilize the egg.

But if the ``guys'' get to the egg first, how do any females get conceived? Dr. Machelle Seibel, of the Faulkner Center for Reproductive Medicine, says what matters most isn't how fast the sperm gets to the finish line. It's more an issue of when the egg gets there to greet them.

The egg is viable for about 18 hours after it is released from its follicle in the ovary. If the introduction of sperm takes place before that 18 hour window begins, the sperm that get to the fallopian tube first find no egg. They get there too soon. But if ovulation occurs several hours afterward, the steady, heavier, slower female sperm have time to catch up and even the odds.

From fertilization on, however, the chances of surviving are better if you're female. Dr. Selwyn Oskowitz of the Boston In Vitro Fertilization Clinic, says more male fetuses die during pregnancy. ``For some reason we don't understand, males are not as strong or viable.''

After birth, more male children die of illness than females, and by puberty, the population is gender neutral. And women live longer than men, too, by two to three years.