James Pivarnik of Michigan State University in East Lansing advises pregnant women to get at least 150 minutes of moderate-intensity aerobic activity per week.

"There has been quite a dramatic change in regards to pregnancy and exercise," Pivarnik said in a statement. "While it used to be thought that avoiding exercise meant avoiding harm to the fetus, research now shows physical activity can not only improve health of the mother but also provide potential long-term benefits for the child."

Pivarnik, who has studied the topic for more than 20 years, led a team that has written new physical activity guidelines for pregnant women as part of the U.S. Department of Health and Human Services 2008 Physical Activity Guidelines.

"In the time I have been performing research on exercise during pregnancy, we have shifted from cautious conservatism to encouraging women to be active throughout gestation," Pivarnik said.

The move toward measuring recommended exercise by overall minutes as opposed to specific time and frequency is related to research recently published by Pivarnik and Michigan colleagues in the journal Medicine and Science in Sports and Exercise.