A 29-year-old pregnant client at 32 weeks presents to the prenatal clinic for a routine visit. Fundal height measures 4 cm smaller than expected for gestational age. An ultrasound is performed and reveals a fetus with estimated weight below the 10th percentile for gestational age, confirming intrauterine growth restriction (IUGR). Which underlying complication is the most likely cause of the fetal growth restriction (IUGR)?