Introduction
Uterine myomas (also known as leiomyomata, fibroids, fibromyomas, leiomyofibromas, and fibroleiomyomas) are benign tumors that originate from clonal proliferation of smooth muscle cells of the uterus; they are common among women of reproductive age, with a prevalence of 20–60%.1–4The prevalence of uterine myomas in pregnant women has been reported as 0.4–10.7%; however, conflicting evidence is available regarding the obstetric outcomes in this patient group.1,2 Although some studies have reported no increased risk of adverse pregnancy outcomes (APOs),5–7 other studies found that uterine myomas during pregnancy increased the risk of preterm births (PTB).1,8,9
In children, PTB accounts for 75% of perinatal mortality and more than half of long-term morbidity.10–12 PTB can occur spontaneously or may be induced medically.13 A major cause of spontaneous PTB is premature labor triggered because of unknown origin.13 Preterm premature rupture of membranes (pPROM), which is often followed by an intrauterine infection (II), is a risk factor associated with PTB.14,15 Gestational hypertension (GH) is a common maternal complication and a risk factor for medically-induced PTB.12,13
PTB prevention regarding particular causal factors is paramount to improving neonatal outcomes; detailed evaluation of women at risk of PTB is required for tailored treatment.12 Therefore, accurate assessment of risk factors for PTB and its causal factors in women with uterine myomas are relevant to patients and physicians because of the frequency of uterine myomas. The majority of previous studies on PTB risks in women with uterine myomas were retrospective and involved small samples,7,16 yielding conflicting results on the association between uterine myomas and pPROM, II, and GH.1,2,5–8,17 Furthermore, the biochemical mechanisms of PTB in women with uterine myomas and of the effect of uterine myomas on the course of pregnancy remain unclear.1,2,8
Therefore, in the present study, we aimed to clarify the effects of uterine myomas on APOs; PTB, pPROM, II, and GH relative to the general population and to evaluate the effects of II on the incidence of PTB, and pPROM in women with uterine myomas. The goal was to clarify how uterine myomas affect APOs, using data from a nationwide Japanese prospective birth-cohort study.