FIGURE 41-22 On speculum examination, placenta is visible protruding through the cervical os. (Used with permission from Dr. Maureen E. Flowers.) • Low-ying placenta-implantation in the lower uterine segment is such that the placental edge does not cover the internal os but lies within a 2-cm wide perimeter around the os. A previously used term, marginal previa, described a placenta that was at the edge of the internal os but did not overlie it (Reddy, 2014).
Clearly, the classification of some cases of previa will depend on cervical dilation at the time of assessment (Dashe, 2013; Reddy, 2014). For example, a low-lying placenta at 2-cm dilation may become a partial placenta previa at 4-cm dilation because the cervix has opened to expose the placental edge. Conversely, a placenta previa that appears to be total before cervical dilation may become partial at 4-cm dilation because the cervical opening now extends beyond the edge of the placenta. Digital papation in an attempt to ascertain these changing relations between the placental edge and intenal os as the cervix dilates usualy causes severe hemorrhage!
With any degree of placenta previa, a certain amount of spontaneous placental separation is inevitable during lower uterine segment remodeling and cervical dilation. Although this frequently causes bleeding, and thus technically constitutes a placental abruption, this term is usually not applied in these instances.
Somewhat but not always related is vasa previa, in which fetal vessels course through membranes and present at the cervical os (Catanzarite, 2016). Vasa previa was recently reviewed by the Society for Maternal-Fetal Medicine (2015) and is discussed in Chapter 6 (p. 118).
he incidence of placenta previa has risen during past 30 years. Reported incidences average 0.3 percent or 1 case per 300 to 400 deliveries. he frequency at Parkland Hospital from 1988 through 2003 for nearly 250,000 births was 2.6 per 1000. For the 2004 to 2015 epoch, it rose to 3.8 per 1000. Similar frequencies have been reported from Austria, Finland, and
Israel (Kollmann, 2016; Raisanen, 2014; Rosenberg, 2011).
Several demographic factors may contribute to this higher risk for placenta previa. First, maternal age raises the frequency of placenta previa (Biro, 2012; Roberts, 2012). In the First-and
Second-Trimester E val uation of Risk (FASTER) trial, which included more than 36,000 women, the frequency of previa was 0.5 percent for women <35 years compared with 1.1 percent in those :35 years (Cleary-Goldman, 2005). At Parkland Hospital, this incidence difered from a low rate of approximately 0.65 per 1000 births for women ; 19 years to almost 10 per 1000 births for women older than 35 (see Fig. 41-16).
Multpariy also elevates the risk for previa (Raisanen, 2014).
Obviously, the efects of advancing maternal age and parity are confounding. Still, Babinszki and colleagues (1999) reported that the 2.2-percent incidence in women with parity of five or greater was significantly higher than that of women with lower parity.
he interpregnancy interval does not afect this rate (Fox, 2015).