A 24-year-old primigravida presents for her first antenatal ultrasound at 18 weeks of gestation. On transabdominal sonography, the fetal biometry shows: biparietal diameter (BPD) 38 mm, femur length (FL) 28 mm, and head circumference (HC) 134 mm. The abdominal circumference is 112 mm. Additionally, there is a small amount of fluid in the fetal stomach, and the four-chamber view of the heart appears normal. However, the sonographer notes absence of the fetal stomach bubble on repeat scanning. What is the most likely diagnosis?