Lead researcher Dr. Steven Bloom said that further study must be done to determine if bladder problems were permanent.

"Oftentimes, it's best for the patient to do what's more comfortable for her," said Bloom, of the University of Texas Southwestern Medical Center in Dallas.

The study involved 320 women at Parkland Memorial Hospital who gave birth for the first time and had uncomplicated pregnancies and did not receive epidural anesthesia.

They were randomly assigned, with both groups tended by nurse midwives. Of the two groups, 163 were coached to push for 10 seconds during a contraction, and 157 told to "do what comes naturally."

The coached women had smaller bladder capacity and a decreased "first urge to void."

The findings are published in the American Journal of Obstetrics and Gynecology.