Spring and early summer is the nation's season of risk for conceiving a child with birth defects, the leading cause of infant mortality in the United States, a new study finds.

The reason appears to be that the pesticide load is highest in the air, water and elsewhere from April through July, due to the agricultural growing season and increased pest control efforts in urban areas at that time, the researchers say.

They found that the birth defect rate was 3 percent higher for babies conceived in the April to June period than the other months of the year.

Birth defects affect one in every 33 babies (3 percent) born in this country, according to the Centers for Disease Control and Prevention (CDC). The 22 defects studied are all fairly serious, ranging from cleft lip to deformities in the heart. For instance, about 1 in every 100 to 200 babies is born with a heart deformity, according to the CDC, and heart defects make up one third to one fourth of all birth defects.

Death from birth defects vary by defect and medical access. Heart defects cause half of all deaths from birth defects in infants in some parts of the world. Other defects are common and not life-threatening but require corrective surgery, such as hypospadias where the urethra is misplaced such that urine comes out the wrong part of the penis.

The study calculated monthly U.S. birth defect rates from 1996 to 2002 using CDC data on 22 types of birth defects and 30 million cases in all. The result: a statistically significant association between the rates of defects for babies conceived between April and June and monthly concentrations of nitrates, atrazine and other pesticides as calculated from U.S. Geological Survey water quality data. The association held when the researchers controlled for alcohol use, smoking and diabetes in the mothers, and also held when looking individually at each of 11 specific defects, including spina bifida; circulatory, tracheal, gastrointestinal, urogenital and musculoskeletal anomalies; cleft lip; adactyly (missing hands, fingers, toes and the like); clubfoot; and Down syndrome.

"What we wanted to do in this study is ask the big question: If all live births and all birth defects were analyzed across the United States for 12 years time, would we find a season of risk for birth defects? Our finding was a robust one, the answer was yes," said study leader and neonatologist Dr. Paul Winchester of Indiana University School of Medicine.

Evidence linking agricultural chemicals, such as nitrates and pesticides, with embryonic development problems has accumulated for decades in animals and in humans in high-risk occupations such as agricultural workers in cornfields or orchards. Now the link has been extended, for the first time Winchester said, nationwide regardless of the mother's occupation.

A causal link between birth defects and pesticides and nitrates remains "plausible but not proven" by this study, the authors write in the April issue of the journal Acta Paediatrica.

University of Florida biologist Louis J. Guillette did not work on the study but said he thinks the association between seasonal exposure to pesticides and birth defects rates will hold up over time.

"I believe that we will learn that this association might be even worse then we think now," Guillette said.

"There is clear evidence from lab animals and wildlife studies that pesticides and nitrates at environmentally relevant concentrations alter the developing embryo."

Pesticides get around

The USGS data used in the study provided measures of pesticides found in streams and groundwater, but Winchester said this is not just a water study.

"Surface water is a major indicator of environmental contamination," he told LiveScience from an office adjacent to a neonatal unit he oversees at St. Francis Hospital in Indianapolis, housing 21 infants at the time, mostly born premature but some of whom had serious birth defects. "Several studies have now confirmed that pesticides applied in a corn field end up in rain, snow, dust and most importantly, in the bodies of reproductive couples." The latest studies show that 99 percent of Americans have pesticides hanging out in their body tissues, but the scientific question is whether these are harmful, he said. And some 80 percent to 100 percent of fetuses in the United States are exposed to pesticides and most are exposed to mixtures of several of them, according to a review of studies cited by Winchester and his colleagues. Winchester's study is about as strong as you can get in humans, Guillette said. "The patterns observed are powerful," and causal data would be impossible to obtain as it is unethical to do experiments on pregnant women. Also, he pointed out, more and more research shows that the underlying molecular and cellular biology that underpins the development of the embryo's nervous system, limbs, reproductive organs etc. are almost identical among fish, amphibians, alligators, birds, and mammals (including mice, rats and humans). So studies linking pesticides with deformities in other animals and their embryonic development also apply to humans. Limitations and what to do The study's limitations include that vital records on birth defects can be unreliable, but defects tend to be underreported, so better records would likely strengthen the association. In fact, 12 states still have no active surveillance programs for birth defects, meaning that officials relied only on reports by mothers at birth and never followed up to confirm defects that showed up later. Winchester said there are also studies that have found seasonal pesticides exposure to be associated with premature births, academic achievement, learning disabilities and life span. Some people "look at a pregnant woman and judge her harshly, saying, 'You're too fat. You smoke,'" Winchester said. "But if you are unlucky enough to conceive in certain months, we can make predictions about the outcome." It's relatively easy to reduce your exposure to pesticides, he said. One approach is to eat organic food. Totally eliminating pesticides exposure is much harder, because they "are in everything," he said.