Placenta previa can have serious adverse consequences
for both mother and baby, including an increased risk of
maternal and neonatal mortality[1-3], fetal growth
restriction and preterm delivery[4], antenatal and intrapartum
hemorrhage[5-7], and women may require a
blood transfusion[8] or even an emergency hysterectomy.
It is a relatively uncommon condition, with an
overall incidence in England of 6.3 per 1000 births[9],
but incidence rates are higher among women with
advanced maternal age, multiple gestation, high parity,
or who smoke or use illegal drugs[10]. The risk of
placenta previa is also reported to be higher among
women with previous uterine surgery, including cesarean
section[11].
In England, cesarean sections constituted 25% of
National Health Service (NHS) deliveries during 2010,
and the rates have been rising for both primary and
emergency CS[9]. The risk of placenta previa in a pregnancy
after a CS delivery has been reported to be
between 1.5 and 6 times higher than after a vaginal
delivery. A meta-analysis of studies published before
2000 of previous CS as a risk factor for placenta previa
found an overall odds ratio of 2.7 [10]. However, the
overall odds ratio was lower in studies that had better
adjustment for confounders[10]. A recent study from
the USA that was not included in the meta-analysis, and
which used a population-based cohort of 11 million pregnancies, found an adjusted odds ratio of 1.8 for all
pregnancies[12] and an adjusted odds ratio of 1.5 for
second births only[13].
Evidence about the risk of placenta previa following a
previous CS in UK women is limited to results published
25 years ago[14]. We used the Hospital Episode
Statistics (HES), an administrative database of all admissions
to NHS hospitals in England, to define a population-based
cohort and to quantify the association
between CS at first birth and the risk of developing placenta
previa in the subsequent pregnancy. We also performed
a meta-analysis of the reported results in peerreviewed
articles since 1980.