Seatbelts do a pretty good job of protecting an unborn fetus in the event of a car accident, says a new study.

But only if expectant mothers wear them.

According to researchers from the University of Michigan, the lives of 200 of the roughly 370 fetuses killed yearly in U.S. vehicle crashes would have been spared if all pregnant women wore seat belts.

According to lead author Dr. Mark Pearlman, the clear lesson is that every time a pregnant woman gets into a motor vehicle, she should wear a seat belt.

Pearlman and colleagues analyzed 57 crashes involving women who were at least 20 weeks pregnant, in which twelve fetuses were killed. They found that the pregnant women who wore seat belts reduced the risk of their fetus being killed or injured in a vehicle crash by 84 %.

Non-fatal injuries caused by the accidents included the woman's placenta prematurely separating from the uterine wall, preterm birth prior to 32 weeks of pregnancy and direct fetal injury.

The researchers hope the findings will dispel once and for all what they call the "myth" that wearing a seat belt is somehow bad for the fetus.

Dr. Pearlman advises pregnant women to wear seat belts under the belly as much as possible, and across the hips with the shoulder strap placed between the breasts. He says seat belt straps should be below the belly button as low they can comfortably go; they should also be snug.

Pearlman says about 82 percent of people in the United States wear seat belts regularly and 6 percent to 7 percent of pregnant women are involved in some type of car crash during their pregnancy.

The study also showed that air bags were not harmful to fetuses.

The research is published in the April, 2008 issue of the American Journal of Obstetrics & Gynecology.