Human uterine wall tension trajectories and the onset of parturition

Abstract

Uterine wall tension is thought to be an important determinant of the onset of labor in pregnant women. We characterize human uterine wall tension using ultrasound from the second trimester of pregnancy until parturition and compare preterm, term and twin pregnancies. A total of 320 pregnant women were followed from first antenatal visit to delivery during the period 2000-2004 at the John Hunter Hospital, NSW, Australia. The uterine wall thickness, length, anterior-posterior diameter and transverse diameter were determined by serial ultrasounds. Subjects were divided into three groups: women with singleton pregnancies and spontaneous labor onset, either preterm or term and women with twin pregnancies. Intrauterine pressure results from the literature were combined with our data to form trajectories for uterine wall thickness, volume and tension for each woman using the prolate ellipsoid method and the groups were compared at 20, 25 and 30 weeks gestation. Uterine wall tension followed an exponential curve, with results increasing throughout pregnancy with the site of maximum tension on the anterior wall. For those delivering preterm, uterine wall thickness was increased P < 0.05 compared with term. For twin pregnancies intrauterine volume was increased compared to singletons (P < 0.001), but wall thickness was not. There was no evidence for increased tension in those delivering preterm or those with twin gestations. These data are not consistent with a role for high uterine wall tension as a causal factor in preterm spontaneous labor in singleton or twin gestations. It seems likely that hormonal differences in multiple gestations are responsible for increased rates of preterm birth in this group rather than increased tension.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Flowchart of study enrolment and exclusions.

Figure 2. Examples of ultrasonographic measurements taken with an Aspen Advanced Ultrasound System.

(A) Longitudinal Diameter (LD) of the uterus using Freestyle™ software; (B) Transverse Diameter (TD) of the uterus using Freestyle™ software; (C) Anterior-Posterior Diameter (APD) and Uterine Wall Thickness (UWT) shown.

Figure 3. Intrauterine pressure at gestational age 17 to 40 weeks.

IUP was interpolated and extrapolated from Fisk et al. .

Figure 4. Uterine wall thickness trajectories for term singleton, preterm singleton and twins.

(A) term singleton trajectories and their 5th, 50th and 95th centiles; (B) preterm singletons 5th, 50th and 95th centiles shown against corresponding term singletons centiles; (C) twins 5th, 50th and 95th centiles shown against corresponding term singletons centiles. Series of blue dots fitted with blue solid lines, term singleton trajectories; black bold dashed lines 5th, 50th and 95th centiles of term singletons; red dotted lines 5th, 50th and 95th centiles of preterm singletons; green dot-dashed lines 5th, 50th and 95th centiles of twins.

Figure 5. Intrauterine volume for term singleton, preterm singleton and twins.

(A) term singleton trajectories and their 5th, 50th and 95th centiles; (B) preterm singletons 5th, 50th and 95th centiles shown against corresponding term singletons centiles; (C) twins 5th, 50th and 95th centiles shown against corresponding term singletons centiles. Series of blue dots fitted with blue solid lines, term singleton trajectories; black bold dashed lines 5th, 50th and 95th centiles of term singletons; red dotted lines 5th, 50th and 95th centiles of preterm singletons; green dot-dashed lines 5th, 50th and 95th centiles of twins.

Figure 6. Maximum uterine wall tension for term singleton, preterm singleton and twins.

(A) term singleton, preterm singleton and twin trajectories; (B) preterm singletons 5th, 50th and 95th centiles shown against corresponding term singletons centiles; (C) twins 5th, 50th and 95th centiles shown against corresponding term singletons centiles. Series of blue dots fitted with blue solid lines, term singleton trajectories; series of red dots fitted with red solid lines, preterm singleton trajectories; series of green dots fitted with green solid lines, twin trajectories; black bold dashed lines 5th, 50th and 95th centiles of term singletons; red dotted lines 5th, 50th and 95th centiles of preterm singletons; green dot-dashed lines 5th, 50th and 95th centiles of twins.

Figure 7. The developed equations for human uterine wall tension show a developing human uterus growing throughout gestation.

Maximal tension occurs at the anterior and posterior of the uterus. The , , axes are in meters, and the color bar describes uterine wall tension. (A) gestational age of 19 weeks; (B) gestational age of 25 weeks; (C) gestational age of 30 weeks; (D) gestational age of 36 weeks.