Color Doppler US can help differentiate lower uterine-segment implantation from a gestational sac of a failed pregnancy in the process of expulsion by demonstrating loss of circumferential blood flow in the failed pregnancy. Furthermore, applying pressure to the anterior surface of the uterus will result in downward movement of the gestational sac of a failed pregnancy.9
Not all gestational sacs that implant in the lower uterine segment lead to PAS: Subsequent normal pregnancies have been reported in this circumstance. In such cases, a normal thick myometrium is noted anterior to the gestational sac.7 A patient with lower uterine-segment implantation without evidence of anterior myometrial thinning remains at risk for third-trimester placenta previa.7
Cesarean scar pregnancy carries significant risk of PAS. In these cases, the gestational sac is typically implanted within the scar, resulting in a thin anterior myometrium and significantly increased vascularity of the placental–myometrial and bladder–uterine wall interfaces (FIGURE 2).9 Differentiating cesarean scar pregnancy from a lower uterine-segment implantation is easier to perform before the eighth week of gestation but becomes more difficult as pregnancy advances. Although it might be useful to distinguish between true cesarean scar pregnancy and lower uterine-segment implantation adjacent to or involving the scar, both carry considerable risk of PAS and excessive hemorrhage, and the approach to treating both conditions is quite similar.
Lacunae, with or without documented blood flow on color Doppler US, are the third marker of PAS in the first trimester.8 Although some retrospective series and case reports describe the finding of lacunae in the first trimester of patients with diagnosed PAS, more recent literature suggests that these spaces are seen infrequently and at a similar frequency in women with and without PAS at delivery.7
Second- and third-trimester markers
Multiple diagnostic sonographic markers of PAS have been described in the second and third trimesters.
Placental location is a significant risk factor for PAS. Placenta previa in the setting of prior CD carries the highest risk of PAS—as high as 61% in women with both placenta previa and a history of 3 CDs.10 An anterior placenta appears to be a stronger risk factor for PAS than a posterior placenta in women with prior CD; the location of the placenta should therefore be evaluated in all women in the second trimester.
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