A 32-year-old woman with bipolar I disorder becomes pregnant. She is maintained on lithium. At which gestational period is the risk of Ebstein's anomaly (tricuspid valve malformation) highest?

• A Second trimester (weeks 14–20)
• B First trimester (organogenesis, especially weeks 5–10) ✓
• C Third trimester (weeks 28–36)
• D Peri-partum period only

Explanation

Lithium exposure during the first trimester, especially during cardiac organogenesis (weeks 5–10), is associated with Ebstein's anomaly (apical displacement of the tricuspid valve). The absolute risk is small (approximately 1–2 per 1,000 live births, vs. ~1 per 20,000 in the general population), but the first trimester is the critical window for cardiac morphogenesis. If lithium is continued, fetal echocardiography at 16–20 weeks is recommended. The risk does not primarily arise from second or third trimester exposure.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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