Environmental toxicants and fetal development

Did You Know?

• Organizations such as the American College of Obstetricians and Gynecologists and the American College of Nurse-Midwives recommend precautions to minimize exposure to relevant environmental toxins in pregnancy.
• To decrease exposure to pesticides, recommendations include washing all produce thoroughly, peeling the skin from fruits and vegetables, or buying organic produce if possible.
• If living in a home built before 1978, checking whether lead paint has been used is advised; crumbling or peeling paint should not be touched, and a professional should remove it.

Effects

Environmental toxicants can be grouped by the harm they cause to a developing fetus, including structural defects, growth changes, functional impairments, cancers, or death. In the United States, one in ten infants is born preterm, and about 5% have low birth weight. Preterm birth, meaning delivery before 37 weeks of pregnancy, is a leading cause of infant death throughout childhood. Exposure to toxins like lead, tobacco smoke, and DDT has been associated with higher risks of miscarriage, low birth weight, or preterm delivery. Substances that cause structural birth defects are called teratogens. These are agents outside the embryo or fetus that increase the chances of malformations, cancer, mutations, functional problems, poor growth, or pregnancy loss. Teratogens fall into four categories: drugs taken during pregnancy (including recreational and pharmaceutical), infections passed from mother to child, radiation such as X-rays, and mechanical forces like oligohydramnios. They affect the fetus through several mechanisms: interfering with how fast cells multiply, altering biochemical pathways, disrupting normal cell or tissue interactions, acting as external factors, interacting with genes at a threshold level, or through pollution’s effects on brain plasticity, which can lead to neurodevelopmental disorders. Autism spectrum disorder (ASD) is one such disorder, marked by difficulties in social interaction and communication, with symptoms ranging from alexithymia and aggression to trouble regulating emotions, repetitive behaviors, and reduced empathy. ASD has multiple influences, including epigenetic, genetic, and environmental factors. However, these findings are debated, as many experts believe rising diagnosis rates in some regions result from better screening and more accurate diagnostic methods.

Toxicants and their effects

Certain substances are known to be especially dangerous during pregnancy, including lead, which can be stored in a mother's bones; cigarette smoke; alcohol; mercury; carbon dioxide; and ionizing radiation. When a pregnant person drinks alcohol, it can cause a group of conditions called fetal alcohol spectrum disorders, with fetal alcohol syndrome being the most severe form. These disorders can produce a variety of physical, cognitive, neurodevelopmental, and neurobehavioral problems, and the specific effects depend on how much alcohol is consumed and when during pregnancy it is used. Physical signs of fetal alcohol syndrome may include distinct facial features, poor growth, and problems with central nervous system development, while behavioral and emotional issues are also common. Exposure to tobacco smoke before birth can lead to stillbirth, placental problems, premature delivery, lower birth weight, physical defects like cleft palate, reduced lung function, and a higher chance of infant death. Mercury comes in two forms—elemental and methylmercury—that can cause poisoning during pregnancy. Methylmercury, found in seafood and freshwater fish worldwide, harms the developing nervous system, especially the brain. Eating fish is the main way people are exposed to mercury, and some large fish contain enough to damage a fetus's developing nervous system, sometimes resulting in learning disabilities.

Natural gas development

Living near natural gas wells may pose risks during pregnancy. A study in rural Colorado found that mothers living within ten miles of natural gas development had a higher chance of giving birth to infants with congenital heart defects and neural tube defects. There was also a slight link between the density and closeness of these wells and lower average birth weight. Possible harmful substances from these sites include benzene, solvents, polycyclic aromatic hydrocarbons, and other air pollutants like toluene, nitrogen dioxide, and sulfur dioxide.

In Pennsylvania, the number of unconventional natural gas wells jumped from zero in 2005 to 3,689 by 2013. A 2016 study involving over 9,000 mothers and nearly 11,000 newborns in the Geisinger Health System found that living near this type of drilling during pregnancy was tied to preterm birth and pregnancies labeled high-risk by doctors. Another study in southwestern Pennsylvania linked living close to unconventional gas drilling with lower birth weight. Researchers could not determine whether the exposure came through air, soil, or water, and called for more and larger studies.

Endocrine disruptors are chemicals that can interfere with normal hormone levels and development. They can attach to hormone receptors, change hormone concentrations, trigger incorrect hormone responses, and disrupt enzyme function. Oil and gas extraction releases these disruptors into the environment, mainly through contamination of groundwater and surface water, as well as increased air pollution. This problem is often underreported, so its true impact is likely underestimated. A 2016 study highlighted the need to include an endocrine health component in assessments of oil and gas drilling in populated areas, stressing that such evaluations should consider how these chemicals affect hormone function and overall health, as well as the environment.

Role of the placenta

The placenta, while healthy, acts as a semipermeable membrane that blocks most pathogens and some foreign substances. However, its primary job is to deliver nutrients for growth, making it an imperfect barrier by design. Small, fat-soluble molecules cross via passive diffusion, while larger or charged substances rely on active transport. Some toxic chemicals can also be actively transported. The amount of a substance reaching the fetus depends on how much crosses the placenta and how quickly it is metabolized and eliminated. Because a fetus has an immature metabolism, it cannot efficiently detoxify substances. Consequently, any toxicants the mother encounters are passed to the fetus, potentially affecting development. Substances with a high likelihood of crossing the placenta include carbon dioxide, lead, ethanol, and components of cigarette smoke. To identify developmental hazards, scientific evidence is necessary. In 2004, Brent outlined criteria for recognizing causes of congenital malformations, which also apply to general developmental toxicity: consistent findings from well-conducted epidemiology studies linking exposure to specific effects; data trends showing a relationship between changing exposure levels and the effect; and animal studies supporting the correlation between substance exposure and particular outcomes.