A couple with infertility has decided to adopt an infant and receive a call several months later that an infant is available. The social worker gives them the history of the biological mom, a teenager who tried to conceal her pregnancy by dieting and taking laxatives. However, at 26 weeks' gestation, the teenager shared her pregnancy with the family and received prenatal care. Given this history, the couple should be counseled to observe the infant for which possible abnormality?