Beginning with the use of sonography in obstetrics, the term placental migration was coined to describe the apparent movement of the placenta away from the internal os (King, 1973). Obviously, the placenta does not move per se, and the mechanism of apparent movement is not completely understood. To begin with, migration is clearly a misnomer, because decidual invasion anchors chorionic villi at the cervical os.
Explanations of placental migration are likely additive.
First, apparent movement of the low-lying placenta relative to the internal os is related to the imprecision of two-dimensional sonography. Second, as pregnancy progresses, growth of the lower and upper uterine segments difers. With greater blood low in the upper uterus, placental growth is more likely directed toward the fundus-trophotropism. Many of those placentas that "migrate" most likely never were circumferentially implanted with true villous invasion that reached the internal cervical os. Importanty, a low-ying placenta or placenta previa is less likey to "migrate JJ if there is a prior cesarean delivey scar.
he frequency of placental migration has been quantiied. Sanderson and Milton (1991) studied 4300 women at midpregnancy and found that 12 percent had a low-lying placenta. Of placentas not covering the internal os, previa did not persist, and none subsequently had placental hemorrhage. Conversely, approximately 40 percent of placentas that covered the os at midpregnancy continued to do so until delivery. Thus, placentas that lie close to but not over the internal os up to the early third trimester are unlikely to persist as a previa by term (Heller, 2014; Parrott, 2015). However, other evidence from Bohrer and associates (2012) showed that a second-trimester low-lying placenta was associated with antepartum admission for hemorrhage and increased blood loss at delivery.
The likelihood that placenta previa persists after being identiied sonographically at given epochs before 28 weeks' gestation is shown in Figure 41-20. For twin pregnancies, similar findings are reported until 23 weeks, after which the previa persistence rate is much higher (Kohari, 2012). Staford and coworkers
FIGURE 41 -21 Placenta previa showing that copious hemorrhage could be anticipated with any cervical dilatation.
• Previa, prior CD ) • Previa, no prior CD � • 1-5 mm,prior CD )S 75 • 1-5 mm,no prior CD
FIGURE 41-20 Likelihood of persistence of placenta previa or low-lying placenta 1 to 5 mm from the internal os at delivery. These are shown as a function of sonographic diagnosis at three pregnancy epochs. CD = cesarean delivery. (Data from Oyelese, 2006.) (2010), but not Trudell and colleagues (2013), found that a previa and a third-trimester cervical length <30 mm elevated the risks for hemorrhage, uterine activity, and preterm birth. Friszer and associates (2013) showed that women admitted for bleeding had a greater chance of delivery in the subsequent 7 days when the cervical length was <25 mm, although Trudell (2013) again did not confirm this.
Terminology for placenta previa has evolved, and from a Fetal Imaging Workshop sponsored by the National Institutes of Health (NIH), the following classiication was recommended: • Placenta previa-the internal os is covered partially or completely by placenta (Figs. 41-21 and 41-22). In the past, these were further classiied as either total or partial previa.