Changes in shear wave speed pre- and post-induction of labor: a feasibility study

Abstract

Objective: To explore the feasibility of using shear wave speed (SWS) estimates to detect differences in cervical softening pre- and post-ripening in women undergoing induction of labor.

Methods: Subjects at 37-41 weeks' gestation undergoing cervical ripening before induction of labor were recruited (n = 20). Examinations, performed prior to administration of misoprostol and 4 h later included Bishop score, transvaginal ultrasound measurement of cervical length, and 10 replicate SWS measurements using an ultrasound system equipped with a prototype transducer (128 element, 3 mm diameter, 14 mm aperture) attached to the clinician's hand. Subjects were divided into two groups, 'not-in-labor' and 'marked-progression', based on cervical evaluation at the second examination. Measurements were compared via individual paired hypotheses tests and using a linear mixed model, with the latter also used to compare groups. Spearman's rank correlation coefficient was used to compare SWS with Bishop score. The linear mixed model can take into account clustered data and accommodate multiple predictors simultaneously.

Results: The Wilcoxon signed-rank paired test established a significant difference in pre- and post-ripening SWS, with mean SWS estimates of 2.53 ± 0.75 and 1.54 ± 0.31 m/s, respectively (P < 0.001) in the not-in-labor group (decrease in stiffness) and 1.58 ± 0.33 and 2.35 ± 0.65 m/s for the marked-progression group (increase in stiffness). The linear mixed model corroborated significant differences in pre- and post-ripening measurements in individual subjects (P < 0.001) as well as between groups (P < 0.0001). SWS estimates were significantly correlated with digitally-assessed cervical softness and marginally correlated with Bishop score as assessed by Spearman's rank correlation coefficient.

Conclusions: In-vivo SWS estimates detected stiffness differences before and after misoprostol-induced softening in term pregnancies. This ultrasonic shear elasticity imaging technique shows promise for assessing cervical softness.

Keywords: cervix; induction of labor; shear wave speed.

Figure 1

Transducer was attached to the clinicians hand (a) and inserted into a sterile glove filled with gel (b).

Figure 2

Boxplot of replicate shear wave speed (SWS) estimates for each patient ID for the Not In Labor and Marked Progression groups. The boxes represent interquartile range (IQR), the interior line is the median, whiskers are maxima and minima within 1.5*IQR, and cross markers show outliers beyond the 1.5*IQR, and cross markers show outliers beyond the 1.5*IQR.

Figure 3

Boxplot of Post-Pre SWS medians for each patient for the Not in Labor (NIL) and Marked Progression (MP) groups. For the NIL group, all SWS medians decreased for each patient indicated by values < 0 and all patients in the MP group had an increase in SWS median from pre to post measurement.