CHICAGO (AP) _ The search for Mr. Right may be getting even harder.

The ratio of boys to girls born in the United States and Canada dipped ever so slightly between 1970 and 1990, and a study suggests environmental factors _ such as prenatal exposure to pesticides _ may be why.

The declines began even earlier in several other industrialized countries and corresponded with increases in some male birth defects and prenatal exposure to pesticides and industrial chemicals, said Devra Lee Davis, an environmental epidemiologist at the World Resources Institute, a Washington-based policy-research institute.

Davis and her colleagues examined data on birth ratios and increases in male birth defects, such as misplaced urinary openings, and testicular cancer.

They concluded the reduction in the proportion of males born may indicate that ``some as yet unrecognized, environmental health hazards are affecting the sex ratio of births as well as other unexplained defects in male reproduction.'' Their analysis appears in Wednesday's issue of The Journal of the American Medical Association.

Some studies have indicated male fetuses are more vulnerable than females to prenatal exposure to toxic substances, and either die before birth or are born with defects or a susceptibility to cancer. Other studies have suggested environmental factors can cause harm even before gender has been determined and may block embryos from developing into males.

Although the death rate for males tops that of females at nearly every age, the worldwide human sex ratio hovers around 106 male births for every 100 female births. That is, about 51.5 percent of births are males.

In the United States, the percentage dropped from about 51.34 percent to 51.21 percent between 1970 and 1990, the researchers reported. That's a decrease of one male birth per 1,000 live births, or 38,000 males over the 20-year period, the researchers reported.

In Canada, the loss was 2.2 male births per 1,000 live births, or 8,600 males, during the 20-year period.

Similar declines were noted in the Netherlands, Scandinavia, Finland and Germany; some began as early as 1950 and lasted into the mid-1990s.

The authors are conducting research to see if the U.S. trend continued beyond 1990.

Dr. Robert Mittendorf, director of health studies in the obstetrics and gynecology department at the University of Chicago, called the article provocative but ``a stretch.''

``It doesn't really hammer down the fact that we may or may not be able to use (birth ratios) as an indicator of environmental safeness in regard to pollutants,'' he said.