Cigarette smoking increases the relative risk of placenta previa at least twofold (Usta, 2005). It has been postulated that carbon monoxide hypoxemia causes compensatory placental hypertrophy and more surface area. Smoking may also be related to decidual vasculopathy. Last, uterine leiomyomas are a risk factor for previa Oenabi, 2017).
Several clinical characteristics also raise previa risks. Foremost, women with one or more prior cesarean deliveries are at greater risk for subsequent placental disorders that include placenta previa, abruption, or morbidly adherent placenta (Gibbins, 2018; Klar, 2014). The cumulative risks for placenta previa that accrue with the increasing number of cesarean deliveries are extraordinary. The risk rises even further if there was a prior prelabor cesarean delivery (Downes, 2015). In one MFMU Network study of 30,132 women undergoing cesarean delivery, the incidence was 1.3 percent for those with only one prior cesarean delivery, but it was 3.4 percent if there were six or more prior cesareans (Silver, 2006). In a retrospective cohort of nearly 400,000 women who were delivered of two consecutive singletons, those with a cesarean delivery for the first pregnancy had a 1.6-fold greater risk for previa in the second pregnancy (Gurol-Urganci, 2011). hese same investigators reported a 1.5-fold higher risk from six similar population-based cohort studies. The likelihood of previa is increased more than eightfold in women with parity greater than four and who have more than four prior cesarean deliveries (Gesteland, 2004; Gilliam, 2002).
Importantly, women with a prior uterine incision and placenta previa have an elevated likelihood that cesarean hysterectomy will be necessary because of an associated morbidly adherent placenta (Wei, 2014). In one study, 6 percent of women with a primary cesarean delivery for previa required a hysterectomy. his rate was 25 percent for women with a previa undergoing repeat cesarean delivery (Frederiksen, 1999).
Maternal serum aphaetoprotein (MSAFP) levels, if abnormally elevated for otherwise unexplained reasons during prenatal screening, raise the risk for previa and a host of other abnormalities. Moreover, women with a previa and comorbid MSAFP level :2.0 multiples of the median (MoM) at 16 weeks' preterm birth (Chap. 14, p. 283).
Last, assisted reproductive technoloy AR) used for concep tion elevates previa risks. Some of this association may derive from overlapping efects. For example, older women comprise a signiicant portion of ART patients (Luke, 2017). In addition, multifetal gestation is a well-known risk of both in vitro fertil ization and previa. However, even adjusting for these overlap ping elements, ART is still associated with higher previa rates (Romundstad, 2006).
Painless bleeding is the most characteristic event with placenta previa. Bleeding usually does not develop until near the end of the second trimester or later, but it can begin even before mid pregnancy. And undoubtedly, some late abortions are caused by an abnormally located placenta. Bleeding from a previa usu in a woman who has had an uneventful prenatal course. his so-called sentinel bleed is rarely so profuse as to prove fatal. Usu ally it ceases, only to recur. However, in perhaps 10 percent of women, particularly those with a placenta implanted near but not over the cervical os, there is no bleeding until labor onset.
Bleeding at this time varies from slight to profuse, and it may clinically mimic placental abruption.