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What is Polyhydramnios and explain in detail?
Polyhydramnios is a medical condition describing an excess of amniotic fluid in the amniotic sac. It is seen in about 1% of pregnancies.  It is typically diagnosed when the amniotic fluid index (AFI) is greater than 24 cm.
There are two clinical varieties of polyhydramnios: chronic polyhydramnios where excess amniotic fluid accumulates gradually, and acute polyhydramnios where excess amniotic fluid collects rapidly.
The opposite to polyhydramnios is oligohydramnios, not enough amniotic fluid.

Presentation
Associated conditions
Fetuses with polyhydramnios are at risk for a number of other problems including cord prolapse, placental abruption, premature birth and perinatal death. At delivery the baby should be checked for congenital abnormalities.

Causes
In most cases, the exact cause cannot be identified. A single case may have one or more causes, including intrauterine infection (TORCH), rh-isoimmunisation, or chorioangioma of the placenta. In a multiple gestation pregnancy, the cause of polyhydramnios usually is twin-to-twin transfusion syndrome. Maternal causes include cardiac problems, kidney problems, and maternal diabetes mellitus, which causes fetal hyperglycemia and resulting polyuria (fetal urine is a major source of amniotic fluid).
A recent study distinguishes between mild and severe polyhydramnios and showed that Apgar score of less than 7, perinatal death and structural malformations only occurred in women with severe polyhydramnios. 
In another study, all patients with polyhydramnios, that had a sonographically normal fetus, showed no chromosomal anomalies.(M/C for polyhydramnio is fetal anomalies Williams Obstetrics, 24th Edition – Cunningham,  Leveno, Bloom et al. Table 11–2)
but these anomalies include:

gastrointestinal abnormalities such as esophageal atresia and duodenal atresia (causing inability to swallow amniotic fluid), anencephaly, facial cleft, neck masses, tracheoesophageal fistula, and diaphragmatic hernias. An annular pancreas causing obstruction may also be the cause.
Bochdaleks hernia, in which the pleuro-peritoneal membranes (especially the left) will fail to develop and seal the pericardio-peritoneal canals