Violence between couples during pregnancy makes parenting harder once a baby is born, U.S. researchers said. Mark E. Feinberg of Pennsylvania State University and colleagues interviewed 156 expectant couples -- before the baby was born, six months after the birth of the child and when the baby was approximately 13 months old. The interviews determined the degree of physical violence between couples prior to the birth of the baby and how well couples were able to act as a team while parenting after the baby was born. "The results suggest that working with couples to curtail or prevent violence in their relationships before the birth of their child may have positive implications for the development of co-parenting relationships after the child is born," the researchers said in a statement. The study published in the Journal of Family Issues found 29.8 percent of mothers had acted violently at least once in the past year, while 17.3 percent of fathers acted violently. Finding mothers more violent than fathers is not an uncommon discovery in average community samples, the researchers said. "In our sample it seemed to be the 'common couple' type of violence -- shoving, slapping and hitting and is usually not intended to control the partner but occurs out of frustration in the middle of an argument -- that occurred, not the controlling and severe abuse that people think of when they think of domestic violence," said Marni L. Kan, now a psychologist with RTI International in Research Triangle Park, N.C.