A healthy, 44-year-old woman, G2, P1, has a screening ultrasound at 18 weeks' gestation that shows no fetal anomalies. There is decreased fetal movement at 32 weeks' gestation, and ultrasound shows fetal growth restriction with relative sparing of the fetal head. The placenta appears normally positioned in the lateral fundus, but appears small, and the amniotic fluid index is reduced. Maternal blood pressure is normal. Which of the following conditions is most likely to be present?