Prenatal exposure to methamphetamine may also be a problem in the United States. Although according to the NSDUH, less than 1 percent of pregnant women aged 15-44 had used methamphetamine in the past year, any use among this population is of concern. Unfortunately, our knowledge of the effects of methamphetamine during pregnancy is limited. The few human studies that exist have shown increased rates of premature delivery, placental abruption, fetal growth retardation, and heart and brain abnormalities. However, these studies are difficult to interpret due to methodological issues, such as small sample size and maternal use of other drugs. Ongoing research is continuing to study developmental outcomes such as cognition, social relationships, motor skills, and medical status of children exposed to methamphetamine before birth.