Carbamazepine Teratogenicity — MRCPsych Paper B MCQ

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HardMood DisordersCarbamazepine TeratogenicityMRCPsych Paper B

A 30-year-old woman with bipolar I disorder, maintained on carbamazepine monotherapy 600 mg daily, is found to be 8 weeks pregnant and has taken the drug continuously since before conception. She asks how likely it is that her baby will have a serious birth defect. Using current UK regulatory and product safety data for first-trimester carbamazepine monotherapy exposure, which figure best approximates the prevalence of major congenital malformations in exposed pregnancies (background population prevalence 2–3%)?

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Correct answer: C — Approximately 5%

Explanation lettering: E = shown as B · B = shown as D · D = shown as E

C is correct. UK product information quotes a pooled prevalence of 4.93% major congenital malformations after first-trimester carbamazepine monotherapy, and the MHRA antiepileptic pregnancy safety review concludes 4–5% — roughly double the 2–3% background rate, with dose dependence (lower risk below 400 mg/day). Neural tube defects, especially spina bifida, are characteristic, alongside craniofacial, cardiac, genitourinary and skeletal anomalies. B is the background population figure the stem explicitly supplies, so it cannot also be the exposed-pregnancy risk; A falls below background and is implausible for a recognised teratogen; D approximates high-dose valproate rather than carbamazepine; E grossly overstates risk. Practically, NICE advises against carbamazepine for mental health indications in pregnancy, so review of prophylaxis and high-dose folic acid/specialist referral are needed.

Reference: Tegretol 200mg Tablets, Summary of Product Characteristics, section 4.6 Fertility, pregnancy and lactation (electronic medicines compendium, current version) — https://www.medicines.org.uk/emc/product/7845/smpc (corroborated by MHRA, Antiepileptic drugs: review of safety of use during pregnancy, January 2021 — https://www.gov.uk/government/publications/public-assesment-report-of-antiepileptic-drugs-review-of-safety-of-use-during-pregnancy/antiepileptic-drugs-review-of-safety-of-use-during-pregnancy)