The results reinforce the findings of other studies that link seat belts with better chances of keeping both mother and baby alive.
"The worst thing you can do is have the mother get hurt, and the best way to protect the mother and protect the baby is to have the mother wear a seatbelt," said Kathleen DeSantis Klinich, a researcher at the University of Michigan Transportation Research Institute, who was not part of the study.
Thousands of pregnant women are hospitalized each year after motor vehicle accidents.
"One thing we're always concerned about is (educating) patients on seatbelt use," said Dr. Haywood Brown, the chair of Obstetrics and Gynecology at Duke University Medical Center and senior author of the new study.
"Nonetheless, like all individuals, some choose and some do not choose to wear their seatbelt," he added.
To get a better sense of which women don't use restraints and how that affects the outcome of their pregnancies, Brown and his colleagues searched through a trauma registry at Duke University Hospital.
They found 126 cases of women in their second and third trimesters who had been in a car crash and cared for at the hospital between 1994 and 2010.
Three fetuses - or 3.5 percent - died among the 86 mothers who were wearing a seatbelt during the accident.
Another three fetuses - 25 percent in this case - died among the 12 mothers who were not wearing a seatbelt.
"The bottom line is, you've got to wear your restraint because it decreases the risk not only for your injuries but injury to your child," Brown told Reuters Health.
The American College of Obstetricians and Gynecologists recommends that seat belts should be worn at all times, and the lap belt should be fitted low across the hip bones, below the belly.
Women without a seatbelt were more likely to be first-time mothers than those who wore a seatbelt.
Brown said it's possible that the habit of buckling in children might prompt mothers to put on their own seatbelt.
Airbags came out in 17 of the accidents, and in those cases the mother was more likely to experience the placenta separating from the uterus - a condition that can be fatal for the mother or the fetus.
Catherine Vladutiu, a researcher at the University of North Carolina who was not involved with this research, said it's likely the airbag itself is not to blame for such serious consequences.
"I think the airbag is a function of how severe the crash was, so it's hard to tease out whether that had any direct effect on fetal demise. I would err on the side of (considering it) an indicator of crash severity," Vladutiu told Reuters Health.
Brown said some women will disarm the airbag for fear that it will damage the baby in case of a crash, but "it's not the smart thing to do because it will save your life if the airbag comes out."
In her own research, "we found that airbags, if anything, are beneficial because they help protect the mother," said Klinich.
Another study, from researchers in Washington state, found that airbags did not increase the risk of most pregnancy-related injuries (see Reuters Health story of December 22, 2009 here: reut.rs/jdVDeD).
Brown said he's following up on his study with a survey exploring why some pregnant women don't wear seatbelts, in an effort to identify those women at risk and help them to change their behavior.
SOURCE: http://bit.ly/10eypd8 American Journal of Obstetrics and Gynecology, online February 25, 2013.