A 28-year-old primigravida at 32 weeks gestation presents with vaginal bleeding and mild uterine contractions. Speculum examination shows blood pooling in the vagina but no cervical lesions. Fetal heart rate is reassuring. Which is the most appropriate next investigation to confirm the diagnosis of preterm labor?

## Investigation of Choice to Confirm Preterm Labor **Key Point:** Transvaginal ultrasound (TVU) for cervical length measurement is the most appropriate next investigation to **confirm** preterm labor in a symptomatic woman at 32 weeks gestation. ### Why Cervical Length Measurement is the Answer In a patient presenting with vaginal bleeding and uterine contractions, the clinical question is whether true preterm labor (cervical change) is occurring. Transvaginal ultrasound directly assesses the cervix for: - **Cervical shortening:** Length < 25 mm at 24–34 weeks is strongly associated with imminent preterm delivery - **Cervical funneling (beaking):** Indicates early cervical dilation from the internal os - **Cervical dilation:** Objective evidence of active labor **High-Yield:** - TVU cervical length is the **structural/anatomical gold standard** for confirming cervical change in symptomatic preterm labor - A cervical length **< 20 mm** in a symptomatic patient has high positive predictive value for delivery within 7 days - A cervical length **> 30 mm** has excellent negative predictive value, allowing safe expectant management - TVU is safe even with vaginal bleeding (placenta previa must be excluded first by transabdominal scan, but speculum exam here showed no placenta previa) ### Why fFN is NOT the Best Answer Here Fetal fibronectin (fFN) is a **biochemical risk-stratification tool**, not a diagnostic confirmatory test for preterm labor. Critically: - **fFN is invalidated by vaginal bleeding** — blood in the vagina causes false-positive results, rendering the test unreliable in this clinical scenario - fFN predicts *risk* of delivery within 7–14 days but does not confirm cervical change (i.e., active labor) - ACOG and RCOG guidelines note that fFN should not be collected in the presence of significant vaginal bleeding ### Comparison of Investigations | Investigation | Role | Limitation in This Case | |---|---|---| | **TVU cervical length** | Directly confirms cervical change = preterm labor | **Best answer** — objective, safe, diagnostic | | fFN test | Biochemical risk stratification | **Invalidated by vaginal bleeding** (false positives) | | Transabdominal ultrasound | Placental location, fetal biometry | Does not confirm preterm labor; already implied by reassuring FHR | | Amniocentesis | Fetal lung maturity, infection | Invasive; not first-line for diagnosis | **Clinical Pearl:** Per ACOG Practice Bulletin on Preterm Labor, TVU cervical length is the preferred objective test to confirm preterm labor in symptomatic women. fFN is complementary but is unreliable when vaginal bleeding is present, as in this case. **High-Yield Mnemonic: CERVIX** — **C**onfirms labor, **E**valuates funneling, **R**eliable despite bleeding, **V**aginal route safe, **I**mmediate result, **X**cludes false positives from blood. *(Reference: ACOG Practice Bulletin No. 171, Preterm Labor; Williams Obstetrics, 25th ed., Chapter 42)*