Something odd has been happening to the weights of baby boys born in Canada compared to the weights of baby girls.

Male babies on average have always been modestly larger than girls, but over the past two decades, the difference in size between the sexes has been diminishing. The shrinkage in this unusual gender gap hasn't been huge, with the relative difference declining by about half of one per cent, but it has been large enough for researchers to detect it.

The finding – described by the scientists who discovered it as the first time the trend has been observed anywhere in the world – is the result of a study that compared the weights at birth of almost all of the approximately five million baby boys and girls born in Canada from 1981 to 2003.

The researchers, from the University of Montreal, McGill University and the Public Health Agency of Canada, cannot explain the trend, although they suspect chemical pollutants that interfere with hormones may affect males during fetal development more severely than girls.

There are no other plausible factors “that we can think of, but we have no direct evidence that any environmental chemicals are the cause,” said Michael Kramer, a professor at the department of pediatrics at McGill and one of the researchers.

The finding will appear in next month's issue of the journal Epidemiology, and is part of a growing body of scientific literature suggesting that when it comes to pollutants, males may be the weaker sex, being at more risk from chemical contaminants than females.

“This is just one more indication [that] something is going on in our fetuses that is not good,” observed Frederick vom Saal, a biologist at the University of Missouri and a leading U.S. researcher on chemicals that interfere with normal hormone function.

He said the idea that pollutants in the environment might be behind the trend “is very plausible” because genetic factors that might cause the gap in girl and boy weights to change wouldn't arise that quickly.

Dr. Kramer and his colleagues made one other finding that many women who've had babies recently may suspect already. Children at birth over the past couple of decades have been getting ever larger. Boys in 1981 clocked in at an average of 3.391 kilograms and by 2003 tipped the scales at 3.507 kg. Over the same period, baby girls went from 3.248 kg to 3.375 kg, weighing nearly as much on average in 2003 as the boys did back in 1981.

The figures were for full-term births, and consequently excluded the extremely tiny babies that modern medicine is able to rescue through the use of incubators.

A number of factors explain the trend to plumper babies, according to Dr. Kramer. Among them are that women are getting heavier. They're gaining more weight while pregnant, but also contributing has been a reduction in smoking during pregnancy and an older average age among women giving birth. Such birth-size- inhibiting factors as teen pregnancies are also much less common than they used to be.

But the fact that girls have been gaining in weight relative to boys is harder to explain.

One reason that scientists suspect pollutants is that some chemicals block the action of testosterone, the key male hormone. Testosterone is an anabolic steroid that helps fetuses put on weight in the same way that some athletes use knock-offs of it as growth-promoting compounds to put on more muscle mass.

Among the most widely used chemicals that undermine testosterone production are the plasticizers known as phthalates. One possible explanation for the weight finding is that fetuses may be getting less of the crucial male hormone as a result of their mothers' exposure to the chemicals.

Another reason scientists are looking at hormone disruption is that there is a rare genetic disorder highlighting the importance of these biological compounds in determining birth weight.

A small number of children are born with the characteristic manly pair of XY chromosomes, but who have a genetic mutation that makes them insensitive to male hormones. These children develop anatomically with the external genitalia and other features characteristic of females, although genetically they are male and have testes hidden away in their abdomens.

Babies with this condition have similar weights to girls, suggesting that whatever is causing the gap between males and females to shrink is likely linked to problems with getting enough male hormones.

Dr. vom Saal said the Canadian finding suggests metabolic abnormalities are occurring during fetal development. He said this could predispose babies to conditions such as obesity later in life.

However, Dr. Kramer discounted these worries, and said the change in weight difference “is far too small to be of clinical or public health importance.”