A pregnant woman presents for a routine antenatal checkup at 18 weeks of gestation. On abdominal examination, the uterine size was found to correspond to 16 weeks of gestation. On USG, oligohydramnios was found. Which of the following is a likely cause ?

Explanation

## Correct Answer: D. Renal agenesis Renal agenesis (bilateral) is the classic cause of severe oligohydramnios in the second trimester. The fetus produces approximately 800–1200 mL of urine daily by 20 weeks of gestation, which constitutes the major source of amniotic fluid in the second and third trimesters. Bilateral renal agenesis eliminates fetal urine production entirely, leading to profound oligohydramnios or anhydramnios. This is pathognomonic for bilateral renal disease. The clinical presentation—small-for-dates uterus (16 weeks size at 18 weeks) combined with oligohydramnios on ultrasound—is the hallmark of bilateral renal pathology. Unilateral renal agenesis does not cause oligohydramnios because the contralateral kidney compensates. The diagnosis is confirmed on USG by absence of both fetal kidneys and bladder. This condition is incompatible with postnatal life (Potter syndrome), but the antenatal recognition allows counseling regarding poor prognosis and perinatal management. In the Indian obstetric setting, renal agenesis accounts for approximately 1 in 3000–4500 births and is a leading cause of second-trimester oligohydramnios when bilateral. ## Why the other options are wrong **A. Bartter syndrome** — Bartter syndrome is a rare inherited tubulopathy causing salt wasting and hyperreninism, but it does NOT cause oligohydramnios. It presents postnatally with polyuria, hypokalemia, and metabolic alkalosis. There is no mechanism by which Bartter syndrome reduces amniotic fluid volume in utero. This is an NBE distractor exploiting confusion between renal disorders and amniotic fluid pathology. **B. Anencephaly** — Anencephaly causes polyhydramnios (excess amniotic fluid), not oligohydramnios, because the exposed neural tissue allows increased transudation of fluid across fetal membranes. Additionally, anencephalic fetuses have intact kidneys and produce normal urine. Anencephaly is detected on USG by absence of skull vault and cerebral tissue. This option reverses the amniotic fluid pathology to trap students unfamiliar with neural tube defect associations. **C. Fetal anemia** — Fetal anemia (from hemolytic disease, intrauterine infection, or blood loss) causes polyhydramnios due to high-output cardiac failure and increased transudation, not oligohydramnios. Anemia does not impair renal function or urine production. The anemia may be accompanied by hydrops fetalis with ascites and pericardial effusion, but amniotic fluid volume increases. This is a classic NBE trap pairing a fetal condition with the wrong amniotic fluid abnormality. ## High-Yield Facts - **Bilateral renal agenesis** is the most common cause of severe oligohydramnios in the second trimester; fetal urine accounts for 800–1200 mL/day of amniotic fluid by 20 weeks. - **Potter syndrome** (bilateral renal agenesis) presents with characteristic facies (low-set ears, micrognathia, hypertelorism), pulmonary hypoplasia, and is incompatible with life. - **Unilateral renal agenesis** does NOT cause oligohydramnios because the contralateral kidney compensates; bilateral disease is required. - **Oligohydramnios with small-for-dates uterus** on second-trimester USG should prompt immediate evaluation for bilateral renal pathology (agenesis, severe dysplasia, or obstruction). - **Anencephaly causes polyhydramnios**, not oligohydramnios; fetal anemia and Bartter syndrome do not affect amniotic fluid volume. ## Mnemonics **KIDNEY = Amniotic Fluid Source** **K**idneys produce **urine** → **I**s the major source of **A**mniotic **F**luid in 2nd/3rd trimester → **N**o kidneys = **N**o urine = **O**ligohydramnios. Use when you see oligohydramnios + small uterus in second trimester. **Bilateral Renal Disease Rule** **Bilateral** renal agenesis/dysplasia → oligohydramnios. **Unilateral** → normal AFV. Remember: one kidney can do the job; two kidneys cannot be replaced. ## NBE Trap NBE pairs oligohydramnios with non-renal fetal conditions (anemia, Bartter syndrome) to test whether students understand that amniotic fluid volume in the second trimester is primarily determined by fetal urine output, not systemic fetal pathology. The distractor anencephaly reverses the amniotic fluid abnormality (polyhydramnios instead of oligohydramnios). ## Clinical Pearl In Indian antenatal clinics, when oligohydramnios is detected in the second trimester alongside a small-for-dates uterus, bilateral renal agenesis must be ruled out urgently via detailed fetal renal ultrasound (absence of both kidneys and bladder). Early diagnosis allows counseling about perinatal mortality and preparation for neonatal resuscitation decisions, which is critical in resource-limited Indian settings where neonatal dialysis is not widely available. _Reference: DC Dutta's Textbook of Obstetrics (8th ed.), Ch. 12 (Antenatal Diagnosis); Harrison's Principles of Internal Medicine, Ch. 297 (Genetic Disorders of the Kidney)_