USG at 28 weeks of gestation shows oligohydramnios is likely due to :

Etiology of oligohydramnios: Intrauterine growth restriction Post-term pregnancy Premature rupture of membranes Drugs like ACE inhibitors and prostaglandin inhibitors Renal anomalies Renal Agenesis Infantile polycystic kidney disease urinary tract obstruction Ref: Sheila 2e pg 216.