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Uterine anomalies, induction of labor, and uterine rupture.

Gerard G. Nahum, MD I this issue, Drs. Samuels and Awonuga1 examine a case of uterine rupture associated with a second-trimester pregnancy within a congenital rudimentary uterine horn after administration of misoprostol. It is important because it emphasizes 1) the increased dangers associated with pregnancies that reside within congenital rudimentary uterine horns, 2) the importance of identifying uterine anomalies in gravid women, and 3) the potentially lethal dangers of misoprostol use during pregnancy. Uterine rupture in pregnancy is associated with a high rate of fetal and maternal morbidity and mortality.2 Fortunately, the normal, unscarred uterus ruptures infrequently. In a study by Gardeil et al,3 the overall rate of unscarred uterine rupture during pregnancy was 1 in 30,764 (0.0033%). No case of uterine rupture occurred among 21,998 primigravidas, and only 2 occurred among 39,529 multigravidas without a prior uterine scar (0.0051%). Several factors increase the risk of uterine rupture during pregnancy. Aside from the well-known risk attributable to preexisting uterine scars, the following factors are also associated with a higher rate of uterine rupture: grand multiparity, malpresentation, breech extraction, uterine instrumentation, uterine trauma, dystocia, fetal macrosomia, labor induction, and the presence of a uterine anomaly. The first issue pertains to the risk of uterine rupture associated with pregnancies occupying anomalous uteri. Congenital uterine malformations complicate 1:594 pregnancies.4 Importantly, the walls of congenitally abnormal uteri are thinner than for normal uteri.5,6 Moreover, their myometrium tends to diminish in thickness as gestation advances and can be inconsistent over different aspects of the uterus.5–7 Furthermore, additional wall thinning can occur as a result of uterine contractions.6 Unicornuate uteri constitute 5% of uterine malformations.4 However, most of these so-called “unicornuate” uteri possess a rudimentary horn of contralateral müllerian origin (74%).4 More than 50% of these horns are cavitary, and the majority are noncommunicating (72%–85%), meaning that they contain an endometrial cavity with no communication to the cervix, vagina, or the contralateral hemiuterus (Fig. 1).4,8 All pregnancies occupying such “blind” rudimentary uterine horns result from transperitoneal migration of sperm or fertilized ova, which typically occurs in 50% of human pregnancies.9 Because these horns have no cervical or vaginal outlet, they have an unusual propensity to rupture. Rudimentary uterine horn pregnancies occur in 1:76,000 gestations.6 Of these, 85% occupy noncommunicating horns.8 Even for the 15% with connection to the cervix, there is an unacceptably high rate of uterine rupture. During the 20th century, 47% of reported noncommunicating uterine horn pregnancies ruptured (358 of 419 cases), as did 52% of communicating rudimentary horns (29 of 56 cases) (no significant differSee related case report on page 1160.