Polyhydramnios

Polyhydramnios in the 2nd or early 3rd trimester is found in about 1 per 200 pregnancies. 

There are essentially two major causes of polyhydramnios:

Reduced fetal swallowing, which may be due to neuromuscular disorders, facial or neck tumors, gastrointestinal obstruction, compressive pulmonary defects, or narrow thoracic cage due to skeletal dysplasias.

Increased fetal urination, which is observed in maternal diabetes, hyperdynamic fetal circulation (due to fetal anemia, fetal and placental tumors, arteriovenous malformations), or twin-to-twin transfusion syndrome.