A 28-year-old primigravida at 16 weeks of gestation presents to the antenatal clinic. She has a history of epilepsy and is currently on antiepileptic therapy. Which of the following antiepileptic drugs is the most common cause of fetal malformations when used during the first trimester?

Explanation

## Antiepileptic Drugs and Teratogenicity in Pregnancy ### Phenytoin — The Highest Teratogenic Risk **Key Point:** Phenytoin is the most common antiepileptic drug (AED) associated with fetal malformations, causing the well-described **fetal hydantoin syndrome**. ### Fetal Hydantoin Syndrome — Clinical Features | Feature | Description | |---------|-------------| | **Facial dysmorphism** | Broad nasal bridge, short nose, wide-set eyes, micrognathia | | **Skeletal anomalies** | Hypoplasia of distal phalanges, nail hypoplasia, growth retardation | | **Cardiac defects** | Atrial septal defect (ASD), ventricular septal defect (VSD), patent ductus arteriosus (PDA) | | **Cleft palate** | Occurs in 1 in 500–1000 exposed infants | | **CNS effects** | Intellectual disability, developmental delay | | **Incidence** | 5–10% of exposed pregnancies; risk highest in 1st trimester | ### Why Phenytoin Is Most Teratogenic 1. **Metabolism via epoxide pathway** — Phenytoin is metabolized to toxic epoxide intermediates that accumulate in fetal tissues. 2. **Placental transfer** — Crosses placenta freely; fetal levels approach maternal levels. 3. **First-trimester exposure** — Critical period for organogenesis (weeks 6–9); highest malformation risk. 4. **Dose-dependent effect** — Higher maternal doses correlate with increased fetal risk. **High-Yield:** Phenytoin is the **classic teratogenic AED** — this is a high-frequency NEET PG recall question. ### Comparison with Other AEDs | Drug | Teratogenic Risk | Key Malformations | Pregnancy Category | |------|------------------|-------------------|--------------------| | **Phenytoin** | **Very High** | Fetal hydantoin syndrome, cleft palate | D | | **Levetiracetam** | Low | Minimal reported; emerging as safer option | C | | **Lamotrigine** | Low–Moderate | Cleft palate (rare); generally safer | C | | **Lacosamide** | Insufficient data | Limited experience in pregnancy | C | | **Valproate** | **Very High** | Neural tube defects, developmental delay, autism spectrum | X | | **Carbamazepine** | Moderate | Fetal carbamazepine syndrome (similar to hydantoin) | D | **Clinical Pearl:** Valproate is even more teratogenic than phenytoin (especially for neural tube defects), but the question asks for the **most common cause of fetal malformations** — phenytoin has the longest clinical history and highest documented incidence of the classic syndrome. ### Management in Pregnancy 1. **Pre-conception counselling** — Discuss risks; optimize seizure control on monotherapy if possible. 2. **Preferred AEDs in pregnancy** — Levetiracetam and lamotrigine are now preferred; phenytoin should be avoided if alternatives available. 3. **Folic acid supplementation** — 4–5 mg daily to reduce neural tube defect risk. 4. **Monitoring** — Increased seizure frequency may occur; AED levels should be monitored (especially phenytoin, which has non-linear kinetics). **Warning:** Do not confuse phenytoin teratogenicity with valproate — valproate is contraindicated (Category X), but phenytoin is the classic "most common" teratogenic AED in older literature and exam questions.