Pregnant women who go to work with nagging guilt — that working somehow hurts their babies — can rest a little easier.

Women who work during pregnancy are no more likely to have premature or low birth-weight babies than those who don't, according to University of Minnesota researchers who studied more than 1,500 women who gave birth in 2005.

"It should provide reassurance to women who are employed ­during their pregnancies that they are not harming their babies by working," said Katy Kozhimannil, lead researcher.

Those who have high stress ­levels at work or perform physical labor might still be at higher risk of poor birth outcomes, the researchers say. But Kozhimannil said this study should refocus the debate away from the health threat of work, and toward specific work hazards.

Minnesota's rate of preterm births increased over the past decade from 9 percent to more than 10 percent of all births, according to the March of Dimes.

Kozhimannil said the study, published in the journal Women's Health Issues, is an important step in finding the causes.

The results come as a new book by Facebook COO Sheryl Sandberg has rekindled a long-running ­argument about the balance between work and family, and as Congress is debating the Pregnant Workers Fairness Act. The bill would require employers to accommodate the health needs of pregnant women — such as allowing those on airline ground crews to handle ticketing instead of baggage.

Babies born at low birth weights or before 37 weeks gestation are at increased risk for a variety of ailments, from hearing and vision problems to such muscular conditions as cerebral palsy to infant death.

Causes include rising rates of obesity, births by older mothers, and multiple births from assisted reproduction — along with a persistent ­minority of women who smoke during pregnancy. But other environmental influences, such as employment, have been poorly understood or analyzed.

The study was based on survey responses from more than 1,500 women who gave birth. Researchers controlled for ethnic, economic, social and educational traits, and then compared the women based on their employment status.

"It's not necessarily the fact that they are employed, but the circumstances of their work or their lives that contributed to that increased risk," she said.

The study also confirmed other known risk factors for preterm births, such as race and poverty.


The issue is particularly ­relevant in Minnesota, which has one of the nation's highest rates (around 78 percent) of two-parent family households with two incomes.

Katie Blozis, 30, of Edina said she always knew she would work during the pregnancies of her two children, now 2 years old and 10 months. Her recruiting business required her to work odd hours and evenings, when pregnancy left her exhausted. But she said the pros outweighed the cons.

"Kind of a no-brainer for me," she said. "It definitely helped to pass the time."

Courtney Wetternach of Mound said her first pregnancy, in 2009, came at a stressful time — she was directing emergency management for a hospital and dealing with the H1N1 influenza epidemic. But she wasn't sure in her first ­trimester if the bad nausea was typical for a pregnancy or ­exacerbated by her work.

Either way, it didn't dissuade the 34-year-old from working through her second pregnancy. Her second son is due later this month.

"I'm not lifting things, and I'm not having to do anything concerning as far as activity level," she said.

Kozhimannil took no position on the federal legislation, but said she hopes employers would interpret her study as good news. If work itself isn't a problem, then employers might find it worthwhile to invest in ways that support their pregnant workers and reduce their stress or activity levels when appropriate, she said.

"One of the most expensive things that can happen to an employer is having [an employee with] a preterm birth or a low birth-weight baby," she said. "Let's ensure that those other factors are not impacting women adversely."