【单选题】19-year-old G1P0 presents to her obstetrician's office for a routine OB visit at 32 weeks gestation. Her pregnancy has been complicated by gestational diabetes requiring insulin for control. She has been noncompliant with diet and insulin therapy. She has had two prior normal ultrasounds at 20 and 28 weeks gestation. She has no other significant past medical or surgical history. During the visit, her fundal height measures 38 cm. Which of the following is the most likely explanation for the discrepancy between the fundal height and the gestational age?