Pregnancy hypertension risk increased by traffic-related air pollution

A new report from the National Toxicology Program (NTP) suggests that traffic-related air pollution increases a pregnant woman’s risk for dangerous increases in blood pressure, known as hypertension.

NTP scientists evaluated published research on the link between traffic-related air pollution, or TRAP, and hypertensive disorders broken down by pollutant measurements of TRAP, such as particulate matter (PM2.5). PM is the term for a mixture of solid particles and liquid droplets found in the air, and PM2.5 refers to fine inhalable particles, with diameters that are generally 2.5 micrometers or smaller. The average human hair is about 70 micrometers in diameter, about 30 times larger than the largest fine particle.

“What we found when we reviewed the literature is that exposure to PM2.5 from traffic emissions was associated with development of hypertensive disorders in pregnant women,” said Brandy Beverly, Ph.D., lead scientist and researcher at the National Institute of Environmental Health Sciences, part of the National Institutes of Health. “When these women are exposed to PM2.5 during their entire pregnancy, the likelihood of developing preeclampsia increases by about 50%.”

Other components of TRAP that NTP evaluated included nitrogen oxides, carbon monoxide, black carbon, and elemental carbon, along with parameters like traffic density and mothers’ proximity to main roads.

For example, the literature suggests that women who live within a quarter of a mile of a major roadway or in high traffic density regions may be at an increased risk for developing hypertensive disorders of pregnancy.

TRAP comes from the combustion of fossil fuels by motor vehicles. These vehicle emissions are mixtures of gases and particles that are easily inhaled and have adverse health effects. TRAP is known to be a major risk factor for cardiovascular disease, including hypertension.

Hypertensive disorders of pregnancy complicate more than 10% of pregnancies worldwide and are a leading cause of maternal and fetal illness and death. According to the American College of Obstetrics and Gynecology, mothers with hypertension during pregnancy are more likely to have a pre-term delivery. Their infants are at greater risk for low birthweight and a range of long-term health problems associated with pre-mature birth.

“Hypertensive disorders of pregnancy refer to a range of clinical conditions, all of which include high blood pressure during pregnancy,” said Beverly. “The disorders are classified into four distinct types, based on differences in the timing and onset of the symptoms.”

More than one day of early-pregnancy bleeding linked to lower birthweight

NIH study cautions that more research is needed to determine if this small difference in weight poses a health risk.

Women who experience vaginal bleeding for more than one day during the first trimester of pregnancy may be more likely to have a smaller baby, compared to women who do not experience bleeding in the first trimester, suggest researchers at the National Institutes of Health. On average, full-term babies born to women with more than one day of bleeding in the first trimester were about 3 ounces lighter than those born to women with no bleeding during this time. Additionally, infants born to women with more than a day of first trimester bleeding were roughly twice as likely to be small for gestational age, a category that includes infants who are healthy but small, as well as those whose growth has been restricted because of insufficient nutrition or oxygen or other causes.

The study appears in Obstetrics & Gynecology and was conducted by researchers at the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and other U.S. research institutions.

The authors caution that the decrease in birthweight of infants born to women with vaginal bleeding was small. More studies are needed to determine if these infants are at risk for any additional health risks in infancy or later in life.

“The good news is that only one day of bleeding was not significantly associated with reduced growth,” said the study’s senior author, Katherine L. Grantz, M.D., an investigator in the NICHD Epidemiology Branch. “But our results suggest that even if bleeding stops before the second trimester, a pregnancy with more than one day of bleeding is at somewhat of a greater risk for a smaller baby.”

According to the study authors, first trimester vaginal bleeding occurs in 16 to 25 percent of pregnancies. Earlier studies on whether bleeding is associated with adverse pregnancy outcomes have been inconclusive. The researchers analyzed data from the NICHD Fetal Growth Study, which enrolled women ages 18 to 40 at 12 hospitals in the United States. The study used ultrasound to track fetal growth throughout pregnancy. Of the roughly 2,300 women in the analysis, 410 (17.8 percent) had bleeding in the first trimester. Of these, 176 bled for one day and 234 bled for more.

The researchers also analyzed birthweight outcomes in a secondary analysis limited to 2,116 women with birthweight data available. Compared to women with no bleeding, fetuses of women with more than one day of bleeding were 68 to 107 grams (approximately 2 to 4 ounces) lighter in weeks 35 to 39 of pregnancy. The average birth weight of these babies who were full-term was about 3 ounces lighter than infants born to women who did not bleed at all. Infants who were small for gestational age were delivered to 148 women in the non-bleeding group (8.5 percent), nine women in the one-day group (5.7 percent), and 33 women (15.7 percent) in the group that had more than a day of bleeding.

Dr. Grantz added that when ultrasound exams indicate that the fetus is small for gestational age, physicians typically increase the number of exams to monitor the pregnancy more closely.

NICHD conducts and supports research in the United States and throughout the world on fetal, infant and child development; maternal, child and family health; reproductive biology and population issues; and medical rehabilitation. For more information, visit https://www.nichd.nih.gov.

NIH, the nation’s medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.