Transabdominal scanning of the cervix at the 20-week morphology scan: comparison with transvaginal cervical measurements in a healthy nulliparous population.

BACKGROUND Healthy, nulliparous women at low risk for preterm birth would not usually undergo transvaginal scanning at the 20-week morphology scan. The study aimed to determine whether transabdominal cervical measurement would be sufficient to exclude a short cervix in this population. AIMS To investigate the relationship between transabdominal (TA) and transvaginal (TV) ultrasound measurements of the cervix at 20 weeks' gestation. METHODS At 20 weeks' gestation, TA and TV cervical length was measured in 203 healthy nulliparous participants in the Screening for Pregnancy Endpoints (SCOPE) study. The TA and TV measurements were correlated and examined for variance. RESULTS Paired measurements were achieved in 203 cases. The shortest cervical length on TV scanning was 22 mm, the longest was 59 mm, with TA equivalents of 21 mm and 56 mm respectively. The mean TV cervical length was 39.1 (SD 6.2) mm and mean TA 36.6 (SD 5.8) mm. The average difference between the measurements was 2.6 (SD 5.2) mm, the TA length being the shorter of the two. A TA on the 25th percentile (33 mm length) was associated with a 25th percentile TV length of 36 mm. The intraclass correlation coefficient between TV and TA measurements was 0.77, but the actual difference between the two measurements was not constant. CONCLUSIONS Transabdominal measurements are consistently less than TV measurements. As the measurements are correlated, TA scanning could be used to assess cervical length in most cases initially. Where the TA length is < 5th percentile (27 mm), this measure could be used as an indication to perform a TV scan as this correlates with a 5th percentile TV measurement of 28 mm.