A 28-year-old primigravida at 32 weeks gestation presents with vaginal bleeding and lower abdominal pain. Speculum examination reveals pooling of fluid in the posterior fornix. Which is the most appropriate next investigation to confirm rupture of membranes?

Explanation

## Diagnosis of Rupture of Membranes in Preterm Labor ### Clinical Presentation Pooling of clear fluid in the posterior fornix is a classic sign of rupture of membranes (ROM). The patient's presentation with vaginal bleeding and abdominal pain raises concern for preterm labor complicated by ROM. ### Investigation of Choice: Ferning Test **Key Point:** The ferning test (crystallization test) is the gold standard bedside confirmatory test for ROM. Amniotic fluid contains electrolytes that crystallize in a characteristic fern-like pattern when dried on a glass slide. **High-Yield:** Ferning is: - Highly specific (>98%) for amniotic fluid - Performed on pooled vaginal fluid obtained by speculum examination - Immediate result (within minutes) - Inexpensive and non-invasive - Does NOT require digital cervical examination (which is contraindicated in preterm labor with ROM due to infection risk) ### Mechanism of Ferning Amniotic fluid contains sodium chloride and other electrolytes. When the fluid dries on a glass slide, these salts crystallize in a branching, fern-like pattern under low-power microscopy. This pattern is pathognomonic for amniotic fluid and distinguishes it from other vaginal fluids (cervical mucus, urine). ### Supporting Tests (Secondary) | Investigation | Role | Timing | |---|---|---| | Nitrazine paper | pH >6.0 suggests amniotic fluid (but not specific—vaginal infections also raise pH) | Bedside | | Ultrasound (AFI) | Assesses amniotic fluid volume; oligohydramnios supports ROM diagnosis | After ferning confirms | | Nonstress test | Fetal well-being assessment; performed after ROM confirmed | After ferning confirms | **Clinical Pearl:** In this case, pooling on speculum exam + ferning test is diagnostic. If ferning is negative but clinical suspicion remains high (e.g., history of fluid loss, oligohydramnios on ultrasound), consider repeat testing or MRI of amniotic fluid. ### Management After Confirmation Once ROM is confirmed at 32 weeks: 1. Administer antenatal corticosteroids (betamethasone 12 mg IM × 2 doses, 24 hours apart) 2. Prophylactic antibiotics (ampicillin + erythromycin or cefixime) 3. Monitor for chorioamnionitis (fever, elevated WBC, uterine tenderness, fetal tachycardia) 4. Plan delivery at 34 weeks or earlier if signs of infection or fetal compromise [cite:Williams Obstetrics 26e Ch 42]