LONDON - Couples are more likely to have a girl than a boy if either of the partners smoked heavily while they were trying to conceive, new research suggests.

Some scientists consider the ratio of male to female births to be an indicator of a population's health, because male sperm and embryos are more fragile than their female counterparts.

The study published this week in The Lancet medical journal is the first to propose that smoking may play a role.

Normally, boys have a slight edge over girls, with almost 52 percent of all babies born worldwide being male. The balance tends to even out later in life because females are better at survival.

However, the comparative number of males has been declining in several industrialized countries over the past few decades and researchers suspect toxic substances may be partly to blame.

Dr. Henrik Moller, who has conducted extensive research on sex ratios but was not connected with the latest study, said the findings "fit with what is already known about certain exposures, certainly in the male."

The sex ratio is the proportion of one sex to the other in a given population and is expressed as the number of males for every 100 females.

The sex ratio at birth in most countries is between 104 and 106 males to 100 females. It fluctuates within a narrow range from time to time in some areas but the general trend of the last 20 years has been downward.

"The decline is absolutely minuscule, but it's there. It's genuine," said William James, a researcher at University College in London who has also studied sex ratios.

The proportion of males "has declined in most developed countries. In the United States it has gone down in the white population but up in the black population," said James, who was not involved in the study. "It has gone down in Italian cities, but up in the Italian provinces. It's moving all over the place and I think nobody really knows why."

In the latest study, Japanese scientists recorded the sex of 11,815 newborns delivered in their clinic between December 2000 and July 2001.

Each mother was questioned about her daily cigarette consumption and that of her partner around the time of conception — from three months before her last menstrual period until the time the pregnancy was confirmed.

The overall sex ratio among babies in the study was 104 boys to 100 girls. That equates to about 52 percent male.

However, when the couples were grouped according to their smoking habits, the ratio changed.

When neither the mother nor the father smoked, there were 121 boys for every 100 girls, or 55 percent male infants.

When both partners were pack-a-day smokers the ratio was 82 boys to 100 girls, or 45 percent male.

When one partner smoked, the ratio favored girls, but wasn't quite as low as when both were smokers.

Moller, a professor at Imperial College in London, said there is no way to tell whether the father's smoking or the mother's smoking has more of an influence on the sex of the baby, nor how smoking exerts its power.

"It's quite speculative how these things might work," he said. "You might think that it happens by selectively knocking out the sperm cells that give rise to a son. You could also think of other mechanisms, such as the probability of implantation of a male fertilized egg and a female fertilized egg would be different, or the probability of an early loss of a male embryo could be increased. It could be anything."

The researchers hypothesize that sperm cells carrying the Y chromosome — responsible for male children — are more sensitive to damage caused by smoking than sperm cells with an X chromosome.

On the Net:

The Lancet, http://www.thelancet.com

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