A 32-year-old woman at 8 weeks gestation is found to have active tuberculosis. She is to be started on anti-TB therapy. Before initiating treatment, you need to assess for baseline fetal structural abnormality, particularly given the teratogenic potential of some first-line agents. Which investigation is most appropriate to establish baseline fetal anatomy?

Explanation

## Baseline Fetal Assessment Before Anti-TB Therapy in Early Pregnancy ### Clinical Context At 8 weeks gestation, the critical period for organogenesis is ongoing. While first-line anti-TB drugs (isoniazid, rifampicin, ethambutol, pyrazinamide) are generally considered safe, establishing baseline fetal anatomy is essential to distinguish pre-existing anomalies from drug-induced teratogenicity. ### Why Transvaginal Ultrasound? **Key Point:** Transvaginal ultrasound at 8 weeks provides the highest resolution assessment of early fetal anatomy, including cardiac structure, neural tube closure, and crown-rump length. This establishes a baseline against which any later abnormalities can be attributed. **High-Yield:** At 8 weeks: - Cardiac activity is visible and can be assessed for basic structural integrity - Neural tube closure can be confirmed (critical window: 22–28 days post-conception) - Gestational age is accurately determined (±3 days) - Transvaginal approach offers superior image quality compared to transabdominal at this early stage **Mnemonic: SCAN** — **S**tructure (cardiac, neural), **C**rown-rump length, **A**natomic survey, **N**ormal viability. ### Why Not the Other Investigations? | Investigation | Why Not Appropriate | |---|---| | **Maternal chest X-ray** | While needed to assess TB severity, it does not assess fetal anatomy. It is a maternal diagnostic tool, not a fetal baseline assessment. | | **Maternal hCG & progesterone** | These assess viability and hormonal status, not fetal structural anatomy. They do not establish baseline anatomy. | | **CVS for karyotyping** | CVS at 8 weeks carries a 0.2–0.5% miscarriage risk and is reserved for high-risk pregnancies (advanced maternal age, abnormal screening). It is not indicated for baseline anatomy assessment in a low-risk pregnancy. | **Clinical Pearl:** First-line anti-TB drugs are relatively safe in pregnancy. Isoniazid and rifampicin have decades of safety data. Establishing baseline anatomy allows safe continuation of TB therapy without fear of undetected drug effects.