Sep. 15, 1999 CHAPEL HILL, N.C. - Medical scientists have known for decades that high levels of lead in the body often cause spontaneous abortions, but now a new study shows that lower lead levels can produce that result too.
The risk of spontaneous abortion nearly doubles for every increase of five micrograms of lead per deciliter of blood, the research revealed.
Dr. Irva Hertz-Picciotto, professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health, Dr. Victor H. Borja-Aburto of Mexico's National Institute of Public Health and colleagues conducted the study. A report appears in the Sept. 15 issue of the American Journal of Epidemiology.
"We did this work because results of previous studies of low-level lead in pregnant women and spontaneous abortion were inconclusive," Hertz-Picciotto said. "The earlier research was not designed well enough to answer the question. This study is the best-designed so far and, we believe, does answer it. While further work is needed to confirm our results, it does appear that low to moderate lead can be a problem during pregnancy."
The project took place in Mexico City from 1994 to 1996. Investigators enrolled 668 women, who were interviewed, contributed blood specimens and were followed through home visits or telephone calls during their pregnancies. Mexico's respected National Institute of Neurology and Neurosurgery, which participates in the U.S. Centers for Disease Control and Prevention Blood Lead Proficiency Testing Program, analyzed the blood samples for lead. Another Mexican laboratory conducted analyses for signs of infection. Women averaged about 11 micrograms of lead per deciliter of blood, which is similar to average levels in the United States in the 1970s.
Researchers recorded whose pregnancies failed and whose did not and compared those records with subjects' blood lead levels. Thirty-six of 562 subjects still participating at five months gestation already had lost their babies.
"We did not find many infections that could cause spontaneous abortions, but we did find a dose response for lead," said Hertz-Picciotto, also a fellow at UNC-CH's Carolina Population Center. "That means that the more lead the women had in their systems, the greater the chance that they would spontaneously abort. The levels of lead in blood in our women were all below the acceptable standard for occupational exposures."
Most lead that Mexican women absorb comes from consuming food from ceramic cookware glazed with lead and from air pollution, she said. Another source may be husbands whose clothes become contaminated at work or whose semen is contaminated.
Since lead was eliminated from gasoline in the United States in the 1970s, levels of the toxic metal in U.S. residents have dropped over the past several decades except in certain subgroups, Hertz-Picciotto said. Among the chief sources of continued exposure in this country are air and soil pollution, lead-based paints in older homes and apartments and certain traditional ethnic remedies.
The new study indicates that exposures comparable to those of the U.S. general population in the 1970s and to many populations worldwide today can boost the risk of spontaneous abortion, the scientist said.
"These are far lower than exposures encountered in some occupations," she said.
The Mexican government funded the study. Scientists consider the research a landmark study since investigators measured blood lead levels before any pregnancies failed and avoided limitations of earlier work.
Lead at both high and low doses already has been shown to damage the nervous system and at high doses to injure the kidneys and reproductive systems, Hertz-Picciotto said. The metal easily crosses the placenta to developing infants.
Borja-Aburto earned his Ph.D. in epidemiology at UNC-CH's School of Public Health in 1995 and now directs the National Center of Environmental Health in Metepec, Mexico.
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