Objective: Morphologic studies suggest dramatic, asymmetric uterine growth during pregnancy that is caused by muscle cell hypertrophy. This growth is most marked at the fundus. Our objective was to evaluate sonographically the in vivo changes in myometrial thickness during active labor, second-stage labor, and after delivery.
Study design: Abdominal ultrasound scans were performed on 52 term pregnant women to investigate the dynamic changes in myometrial thickness during the active and second stages of labor and immediately after delivery. Twenty-six women (mean +/- SEM gestational age, 39.09 +/- 0.3 weeks) were in active labor (cervical dilatation >4 cm with regular uterine contractions). An additional 26 nonlaboring women (gestational age, 39.8 +/- 0.2 weeks) provided control measurements. The myometrium was defined sonographically as the echo homogeneous layer between the serosa and the decidua. Myometrial thickness was measured at the low segment and mid anterior, fundal, and posterior uterine walls by the same observer. Myometrial thickness was also measured during uterine contractions and after artificial rupture of the amniotic membranes. All laboring women had uncomplicated labor patterns when studied and were delivered spontaneously.
Results: The myometrium was significantly thinner during active labor compared with nonlabor at each site studied: midanterior (mean [+/-SEM] myometrial thickness, 5.8 +/- 0.27 vs 8.83 +/- 0.51 mm; t test, P <.001), fundus (mean myometrial thickness, 6.78 +/- 0.32 vs 8.49 +/- 0.35 mm; P =.0015), and posterior (mean myometrial thickness, 6.22 +/- 0.34 vs 8.12 +/- 0.30 mm; P <.001). However, myometrial thickness did not differ among sites within the two groups. The thickness of the low segment was not affected by labor status (nonlabor, 4.68 +/- 0.48 vs labor, 4.66 +/- 0.37 mm; P =.97). Similarly, the myometrial thickness of the anterior uterine wall was unaffected by contractions (no contractions, 5.56 +/- 0.2 vs contractions, 5.68 +/- 0.22 mm; t test, P =.654). There was no change in myometrial thickness measured immediately before and after rupture of the amniotic membranes, despite a significant decrease of the amniotic fluid index. There was significant thickening of the anterior and fundal myometrium during the second stage of labor after the fetal head descended to +3 station by digital examination (anterior, 12.99 +/- 0.60 vs 5.8 +/- 0.27 mm; t test, P <.001; fundus, 10.61 +/- 1.63 vs 6.78 +/- 0.32 mm; t test, P =.04). Valsalva maneuver (pushing) during contractions did not affect myometrial thickness at the fundus (between contractions, 10.61 +/- 1.63 vs pushing, 10.76 +/- 1.95 mm; t test, P =.99). Immediately after delivery, the myometrial thickness at the placental insertion site was the thinnest. After completion of the third stage of labor, the uterine fundus remained significantly thinner than the anterior and posterior walls (fundus, 27.37 +/- 3.5 mm vs anterior, 40.94 +/- 3.5 vs posterior, 42.34 +/- 2.44; one-way analysis of variance, P =.02).
Conclusion: There is significant and widespread thinning of the myometrium during active labor. Descent of the fetal head during the second stage of labor is associated with a significant relative thickening of the anterior and fundal myometrium. After delivery, the relationship reverses. These findings suggest the directionality of the expulsive force vectors (fundal dominance) is not determined by asymmetric myometrial growth but, rather, may be a function of increased "myometrial mass" that results from increased surface area at the fundus.