A pregnant woman was prescribed indomethacin. Which of the following effects does the drug have on the fetus?

Correct Answer: B. Premature closure of ductus arteriosus

Indomethacin is a non-selective NSAIDs that inhibits both COX-1 and COX-2 enzymes, thereby blocking prostaglandin synthesis. Prostaglandins, particularly PGE₁ and PGE₂, are critical for maintaining ductal patency in the fetus. The ductus arteriosus is a fetal vascular shunt that diverts blood from the pulmonary artery to the descending aorta, bypassing the non-functional fetal lungs. When indomethacin crosses the placental barrier and reaches fetal circulation, it suppresses prostaglandin production, removing the vasodilatory stimulus that keeps the ductus arteriosus patent. This leads to premature constriction and closure of the ductus arteriosus in utero, a serious complication that can result in fetal pulmonary hypertension, right ventricular strain, and hydrops fetalis. This is why NSAIDs, particularly indomethacin, are contraindicated in pregnancy—especially in the third trimester. Indomethacin is occasionally used therapeutically (off-label) to close a patent ductus arteriosus (PDA) in preterm neonates, exploiting this same mechanism, but in utero closure is iatrogenic and harmful. The effect is dose- and duration-dependent, with prolonged exposure carrying the highest risk.

Why the other options are wrong

A. Premature closure of ductus venosus — The ductus venosus is a fetal vascular shunt in the liver that diverts umbilical venous blood away from hepatic circulation. While NSAIDs affect prostaglandin-dependent vascular tone, the ductus venosus is not primarily prostaglandin-dependent for patency in the way the ductus arteriosus is. This is an anatomically plausible but pathophysiologically incorrect distractor that tests whether students confuse fetal shunt physiology. C. Keeps ductus arteriosus patent — This is the opposite of the true effect and represents a common NBE trap. Students who know NSAIDs affect the ductus arteriosus but misremember the direction of effect may select this. In reality, indomethacin closes the ductus arteriosus by blocking prostaglandins; it does not keep it patent. This option exploits incomplete or reversed recall. D. Closure of fossa ovalis — The foramen ovale (not fossa ovalis, though the terms are sometimes conflated) is an atrial septal opening maintained by pressure gradients between right and left atria during fetal life. Its closure is a postnatal event driven by hemodynamic changes after birth, not by prostaglandin-dependent mechanisms. NSAIDs do not directly affect foramen ovale patency, making this a pure anatomical distractor.

High-Yield Facts

Mnemonics

PGE keeps DA open (PGE = Prostaglandin E = Ductus open) PGE₁/PGE₂ → Vasodilation → Ductus Arteriosus OPEN. Block PGE (with NSAIDs) → Vasoconstriction → Ductus CLOSED. Use when recalling fetal ductal physiology. NSAID in pregnancy = DA closure (Not Safe in pregnancy = Ductus Arteriosus closure) NSAIDs are teratogenic and cause ductal closure. Remember: NSAIDs are contraindicated in pregnancy, especially third trimester, because they close the ductus arteriosus.

NBE Trap

NBE pairs indomethacin with "ductus arteriosus" to test whether students know the direction of effect. Option C (keeps ductus arteriosus patent) is a reversal trap—students who recall NSAIDs affect the ductus but misremember the mechanism may select the opposite answer.

Clinical Pearl

In Indian obstetric practice, NSAIDs are avoided in pregnancy, particularly in the third trimester. Indomethacin is occasionally used therapeutically in neonatal ICUs to close PDA in preterm infants—the same mechanism that makes it dangerous in utero. A pregnant woman presenting with pain should be counseled toward paracetamol or safe analgesics rather than NSAIDs.

_Reference: KD Tripathi Pharmacology Ch. 18 (NSAIDs and Autacoids); Harrison Ch. 297 (Pregnancy and Drug Teratogenicity)_