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.