They are also more likely to skip prenatal doctor visits, especially early in the pregnancy, the study found.
These findings suggest that the chronic stress and time demands of a long commute can lead to adverse birth outcomes — a worrisome matter, given that the average time Americans spend commuting to work is on the rise.
“The finding that low birth weight might be associated with a source of stress like long-distance commuting is somewhat expected, since chronic strain has been found to be linked to adverse birth outcomes,” said study co-author Muzhe Yang, a professor of economics at Lehigh University, in a released statement.
“However, it was surprising to find an association with under-use of prenatal care among pregnant women commuting long-distance,” he added.
A single-state study
For the study, Yang and his co-author, Yang Wang, an assistant professor of public affairs at the University of Wisconsin-Madison, used birth-record data collected by the New Jersey Department of Health for 2014 and 2015.
The data included the addresses of each new mother’s home and workplace, so the researchers were able to determine how far the women traveled to work each day — not as the crow flies, but as the roads meander. The data also included details about each woman’s pregnancy and birth, including the growth rate of the baby in the womb (as determined by ultrasound), the baby’s weight at birth and whether the baby was premature.
A low-weight baby is considered one that weighs 5.5 pounds or less, and a premature baby is one born before the 37th week of gestation.
For this study, the researchers looked specifically at the data for pregnant New Jersey women whose commute was at least 50 miles (100 miles round-trip — the U.S. Census Bureau’s definition of a long-distance commute), but no greater than 100 miles. (The researchers reasoned that anybody who has to travel more than 100 miles is likely telecommuting.) They also included only those women who had given birth to a single child.
The average trip to work among the long-distant commuters in the study was 64 miles, or 78 minutes by car.
New Jersey, by the way, has one of the longest average commute times of any state in the country — 31.7 minutes, according to data from the U.S. Census Bureau. The national average commute time is 26.6 minutes. Minnesota’s average is 23.5 minutes.
Long-distance commutes are not unusual in the U.S., as background information in the study points out. About 2.2 million American workers travel at least 50 miles to work each way, and about 1.7 million workers spend at least 90 minutes commuting in each direction.
The analysis of the New Jersey data showed that the longer the commute for a pregnant woman, the greater her risk of having a low birth-weight baby. Specifically, for every 10 miles of travel distance, the probability of having a low birth-weight baby rose by almost one percentage point when compared to the national average.
The women with long commutes were also at higher risk of their unborn babies being smaller than expected for the number of weeks of pregnancy, a condition known as intrauterine growth restriction. For every 10 miles of commute, that risk rose by 0.6 percentage points.
The researchers believe the higher risk of adverse birth outcomes is most likely the result of the chronic stress induced by the long, daily commutes to work, although they didn’t measure the women’s stress in this study.
But the data also revealed another possible reason for the poorer birth outcomes: fewer pre-natal visits to doctors. Every 10 miles of commuting distance in the study was associated with a 2.5 percent reduction in the number of prenatal checkups made by the women.
The women with the 50-mile-plus commutes averaged 11 prenatal visits during their pregnancies, when the national average is between 10 and 15, say the study’s authors.
The women were particularly less likely to see a doctor during the first three months of their pregnancy. Other research has shown that prenatal visits early in pregnancy are especially important for healthy birth outcomes.
“These results suggest a self-reinforcing mechanism,” said Wang in a released statement. “Those who are in greater need of prenatal care because of the potential adverse effects of stress triggered by long commutes are under-using prenatal care, which could lead to even worse birth outcomes.”
Needed: expanded leave
The study’s findings are only observational. They show a correlation between longer commutes and poorer birth outcomes, but they don’t prove that the commutes are the direct cause of those poorer outcomes. Other factors may be involved.
In addition, the study didn’t look at what forms of transportation the women used to get to work. That factor may affect the level of stress experienced during a commute.
Still, the study’s findings are, as Yang noted, not entirely unexpected given what we know about the effects of stress on pregnancy.
Yang says that the study underscores the need to expand maternity leave to cover pregnancy — if only to give women time to get to all their prenatal doctor visits.
“The findings highlight a subpopulation of pregnant women who are likely in greater need of prenatal maternity leave to facilitate prenatal visits,” he said. “Having the needed time off during the prenatal period can be particularly important for pregnant women who are long commuters.”
Whether policymakers and employers will take steps to help pregnant women get the time off work for those visits is another matter, however. Despite improvements under the Affordable Care Act, the United States remains one of the worse countries in the world regarding paid pregnancy and maternity leave.
FMI: An abstract of the study can be found at the Economics & Human Biology website, but the full study is behind a paywall.