Assessing Second-Trimester Ferning in Diagnosing Preterm Prelabor Rupture of Membranes

Objective Preterm prelabor rupture of membranes (PPROM) diagnosis is made through visualization of amniotic fluid (pooling), nitrizine testing, sonographic low amniotic fluid, and microscopic detection of amniotic fluid arborization (ferning). Data exist on the specificity and sensitivity of ferning detection but have not focused on the second trimester. Our objective is to evaluate the presence of ferning in transvaginally collected amniotic fluid in pregnancies with known second-trimester PPROM to determine if there is a difference in ferning based on gestational age and sample drying time. Study Design This was a prospective study evaluating amniotic fluid in individuals undergoing termination of pregnancy between 15 and 24 weeks gestation. A control vaginal swab was collected prior to rupture of membranes and a sample vaginal swab was collected at the time of rupture of membranes at a termination procedure. A 10-mL sample of amniotic fluid was collected too and then centrifuged to separate blood. Slides were analyzed at five different drying time intervals after rupture of membranes, on both the vaginal and centrifuged samples, and examined by two blinded investigators to assess for ferning. Maternal demographics, obstetrical history, and termination information were collected. Statistical analysis was performed using descriptive statistics and regression analyses. Results A total of 99 individuals consented and 93 were included. The mean gestational age at the time of the termination was 19.5 +/- 2.5 weeks. There was a significant effect of drying time with the odds of observing ferning increasing with longer drying time, up to 10 minutes ( p < 0.001). Gestational age did not impact ferning detection ( p = 0.09). Centrifuging increased ferning detection by 15% at 10 minutes compared to the vaginal swab. Conclusion In cases of known second-trimester PPROM, ferning was detected more often after 10 minutes of sample drying, and centrifuging the amniotic fluid to remove blood. Key Points The detection of ferning in the second trimester was improved with longer drying times. The highest detection rate was seen after at least 10 minutes of drying time. Gestational age did not impact the rates of ferning. Blood contamination impacts the ability to see ferning on microscopic examination.