The primipara, 29 years old, 2 pregnant, 0 pregnant 35 weeks, 24 weeks of pregnancy, induced labor due to spina bifida. The early pregnancy went smoothly. Ultrasound examination at 32 weeks of gestation found polyhydramnios, fetuses were greater than the number of gestational weeks, and no obvious deformity was seen. Pregnant women are obese and have symptoms of polydipsia, polyphagia and polyuria recently.The first disease considered in this case is ( )
选项：

A:placental abruption;
B:Maternal and child blood types are incompatible;
C:gestational diabetes;
D:Placenta previa