Change in cervical length in cases resulting in threatened preterm labor

Purpose To determine the predisposing changes in cervical length (CL) and the critical range of CL in which significant uterine contractions emerge resulting in threatened preterm labor (TPL). Methods Sixty-eight uncomplicated singleton pregnancies where the CL was <25 mm before 31 weeks were divided into cases with TPL ( n = 23) or without ( n = 45). CL and uterine contractions were monitored sequentially starting between 16 and 20 weeks. The gestational ages when a CL of <25 or <15 mm was first observed, the interval between these two measurements, and the CL value at TPL diagnosis were analyzed retrospectively. Results (1) The gestational ages when a CL of <25 and <15 mm was first detected were lower in the TPL group (25 (median); 18–30 (range) and 28; 25–33 weeks, respectively) than in the non-TPL group (27; 20–30 and 33; 26–35 weeks; P = 0.030 and P < 0.001). (2) The interval between the two measurements was shorter in the TPL group (2.5; 0–15 weeks) than in the non-TPL group (5.5; 0–13 weeks, P = 0.034). (3) The CL value at TPL diagnosis was 13 mm (median), ranging from 7 to 18 mm. Conclusion Cases with early onset and subsequent rapid CL shortening before 31 weeks resulted in TPL when CL decreased below the range 7–18 mm.