Sometimes, new scientific knowledge comes at a terrible price. That's what happened in the 1960s with thalidomide, a drug that was prescribed to pregnant women to combat morning sickness. To be sure, thalidomide met screening and safety standards of the time. But then babies of women who had taken the drug were born with deformed limbs and other defects. Scientists discovered too late that, contrary to a long-held scientific theory, the placenta does not act as a barrier to protect the developing fetus from harmful chemicals.

Fortunately, scientific understanding of how chemicals affect prenatal and postnatal development has come a long way since then. Important advances in molecular biology and genetics have given researchers a much more thorough understanding of how normal development occurs. But still, major developmental defects such as neural tube and heart deformities occur in approximately 120,000 of the 4 million infants born in this country each year. And many more subtle behavioral changes such as delays in growth, attention-deficit disorders, or hyperactivity often go undetected. Exposure to toxic chemicals - both manufactured and those that occur naturally in the environment - causes about 3 percent of all developmental defects, and 25 percent may be the result of a combination of genetic and environmental factors.

That's why it is so important for scientists to be able to use new discoveries in developmental biology and genetics when they study chemicals for their potential to cause birth defects. So far, only a fraction of manufactured and naturally occurring chemicals has been adequately evaluated. New efforts are needed to determine how specific chemicals could disrupt human development. The government, the private sector, and the scientific community must seize the opportunity to ensure that new scientific knowledge is used effectively to reduce developmental defects.

For example, scientists recently have discovered the critical ways in which cells in embryos communicate, ultimately activating proteins that turn particular genes on and off. These so-called "signaling pathways" regulate development and are used repeatedly in various combinations at different times and locations in the embryo and fetus. Fortunately, strikingly similar pathways are found in a wide variety of animals, including humans. They have changed very little over time. Studying the effects of chemicals on signaling pathways in animal models should help facilitate understanding of abnormal development in humans. Relatively simple assessments using models such as the roundworm and fruit fly could be used more effectively to provide clues about how exposures to varying amounts and combinations of chemicals might affect development. The models also could be used to study how the occurrence of defects is altered by factors such as age, nutrition, disease history, and genetic constitution. Based on findings from this research, more extensive studies could be conducted on animals whose biological systems more closely resemble those of humans.

In addition, recent breakthroughs in mapping the human genome should provide important information for scientists who study developmental toxicology. New data on genetic variability in humans is key to understanding how the relationship between genes and the environment leads to developmental defects. Using that information, researchers could identify genetic variations - known as polymorphisms - among individuals. Studies have suggested that people with specific polymorphisms who also are exposed to certain chemicals in the womb have a higher incidence of developmental defects than the general population.

While prenatal studies are extremely important, all developmental periods are potentially susceptible to disruption by toxic agents. That is why all stages of human development from conception through puberty should be examined in studies of chemical toxicity. And a wider range of adverse outcomes, including growth retardation or behavioral effects, should be examined.

Testing thousands of chemicals is bound to generate vast amounts of data from industry, academia, and government researchers. Information on amounts of chemicals to which people are exposed and other factors that might affect susceptibility to developmental problems also is needed. New databases should organize this information in a way that is useful for risk assessment.

For too long, research has been hindered by a lack of collaboration among scientists from different fields. Educational programs and professional workshops should be organized to encourage such interaction among researchers in developmental toxicology, biology, genomics, and epidemiology.

The remarkable scientific advances of recent decades have the potential to alleviate much human suffering and improve public health - but only if they are applied appropriately. It's time to make sure that the best science is used to help reduce developmental defects.