The study, published in the American Journal of Obstetrics & Gynecology, found 24 percent of mothers of premature newborn infants had experienced physical abuse during pregnancy compared with 8 percent of mothers of full-term newborn infants.

Violence was associated with preterm birth even after controlling for age, marital status, education, income, parity, planned pregnancy, antenatal care, smoking, alcohol and illicit drugs use.

Using the Abuse Assessment Screen questionnaire, researchers from the University of Porto Medical School and the Hospital S. Joao in Porto, Portugal, surveyed 2,660 Caucasian mothers of newborns at the hospital to assess their experiences during pregnancy.

"The main strengths of this study are its large sample size; the high participation rate; the inclusion of women from virtually all social and economic strata and a broad range of pregnancy and delivery risk profiles," study co-author Dr. Teresa Rodrigues said in a statement. "Assessment of violence exposure took place with guaranteed privacy, through face-to-face interviews that used highly trained personnel."