This week, in honor of National Birth Defects Prevention Month (January), the National Birth Defects Prevention Network has published several warnings against the dangers of smoking during pregnancy. After years of coordinated efforts by lawmakers, public health officials, clinicians, and researchers, smoking rates in the general population have halved since the mid-1950s. But smoking remains one of the deadliest public health problems, and animal research has recently uncovered even more ways in which it can harm expecting mothers and their infants.
In 2014, the most recent year for which data is available, more than one of every five pregnant women in the United States smoked cigarettes during her pregnancy. Worse, more than one of every ten women smoked in the last three months of her pregnancy, further increasing the chance that her newborn will develop a birth defect. It could range in severity from premature birth or minor growth retardation to a cleft palate or lung deformity. Alarmingly, maternal smoking has also been shown to raise the risk of Sudden Infant Death Syndrome by as much as 300 percent. (Researchers believe SIDS may be caused by a birth defect in the brain.)
Animal research is responsible for much of what is now conclusively known about the many chemicals, substances, and other factors that can cause birth defects. Though smoking dangers are generally well-understood, in the past few years animal studies have uncovered even more ways in which nicotine can harm a developing fetus. As recent discoveries in animal research have shown, birth defects can result from behaviors and factors including:
Smoking hookah is especially popular in parts of South Asia and the Middle East, as well as areas in the United States with large communities of immigrants from those regions. Waterpipe smoke is produced by vaporizing flavored tobacco before it passes through a water basin and then is inhaled.
In a 2015 study, pregnant rats were placed in inhalation exposure chambers filled with waterpipe smoke for two hours each day. Researchers linked the animals’ exposure to birth defects including low birth weights, growth retardation, and even neonatal death.
In another rat study conducted in 2017, researchers mimicked secondhand smoke exposure by extracting the compounds of cigarette smoke. Then, they delivered the compound to the rats, via implanted pumps, in three cycles: (1) before conception, (2) in early pregnancy, and (3) in late-stage pregnancy. The study’s results indicate maternal exposure to secondhand smoke even prior to conception caused impaired function in their offspring’s brain circuits responsible for memory, learning, emotional behavior, and mood.
The team does not know exactly how exposure to secondhand tobacco smoke led to these defects. Metabolic function, hormones, and developmental changes to the fertilized egg—each of these, or some combination, might be responsible for the adverse outcomes observed in the rats’ offspring. Additional animal studies could help determine whether and to what extent these factors play a role; but either way, the researchers determined secondhand smoke plays a role even before conception.
One of the most common birth defects, cerebral palsy, is caused by a restriction in the amount of oxygen and blood that can reach the developing brain. This leads to cellular death in the regions of the brain that govern movement and memory. Children born with cerebral palsy will experience lifelong problems with motor skills and coordination, as well as cognitive impairment and intellectual disability. Symptoms can present simultaneously and they can be quite severe.
In 2017, researchers placed pregnant mice in inhalation exposure chambers filled with cigarette smoke. Newborn mice from the litter were made to perform behavioral tests, and then their brains were analyzed. Researchers identified many of the same developmental disabilities characteristic of cerebral palsy in humans, and they found the animals’ cells were unable to clear up a toxic buildup of chemicals caused from their mothers’ exposure to cigarette smoke. Drawing from some earlier work involving rhesus monkeys, they successfully explained how smoking causes cellular death, which produces symptoms resembling cerebral palsy in mice.
Each year in the United States, more than 8,000 newborn infants are diagnosed with cerebral palsy, and the condition is identified in about 1,200 to 1,500 young children postnatally. (In these latter cases, symptoms are generally recognized by the time the children reach preschool age.) The link between smoking and cerebral palsy identified in this mouse study have profound implications for human health.
Several animal studies have linked nicotine exposure, alone, to birth defects and adverse pregnancy outcomes. By delivering the chemical to pregnant rats via implanted pumps, researchers found fetal growth was restricted even in the absence of inhaled tobacco smoke or water vapor. Other studies have found that nicotine delivered to rats, mice, and guinea pigs by these means during their gestational cycles (pregnancies) caused hyperactive behavior in the animals’ offspring. Researchers have likened this to conditions like Attention Deficit Hyperactivity Disorder (ADHD) in humans.
Substantial evidence from animal research has shown that maternal exposure to nicotine, alone—even absent any exposure to firsthand or secondhand tobacco smoke—can increase the risk of Sudden Infant Death Syndrome (SIDS). In human infants, problems with autoresuscitation reflexes—like the failure to gasp for air when deprived of oxygen—are thought to be characteristic of SIDS. Researchers have observed this problem in newborn mice whose mothers were exposed to nicotine (by ingesting the chemical in powder form) during their pregnancies.
Each of these findings suggest the need for more research. More importantly, studies like these demonstrate the tremendous need for animals in preclinical basic science. New understandings about the risks associated with maternal exposure to certain chemicals could alleviate suffering and improve the health of mothers and infants around the world.
Progress in the fight against infant mortality, birth defects, and other threats to maternal/infant health, depends on data collected via studies with laboratory animals. More research will lead to new conclusions about the relationship between maternal exposure to certain chemicals or substances and fetal development issues. It’s a responsibility to future generations that’s shared by all—to facilitate scientific and medical advancement by supporting and promoting research with animals.