The clinical outcome of cesarean scar pregnancies implanted "on the scar" versus "in the niche"

BACKGROUND: The term cesarean scar pregnancy refers to placental implantation within the scar of a previous cesarean delivery. The rising numbers of cesarean deliveries in the last decades have led to an increased incidence of cesarean scar pregnancy. Complications of cesarean scar pregnancy include morbidly adherent placenta, uterine rupture, severe hemorrhage, and preterm labor. It is suspected that cesarean scar pregnancies that are implanted within a dehiscent scar ("niche") behave differently compared with those implanted on top of a well-healed scar. To date there are no studies that have compared pregnancy outcomes between cesarean scar pregnancies implanted either "on the scar" or "in the niche." OBJECTIVES: The purpose of this study was to determine the pregnancy outcome of cesarean scar pregnancy implanted either "on the scar" or "in the niche." STUDY DESIGN: This was a retrospective 2-center study of 17 patients with cesarean scar pregnancy that was diagnosed from 5-9 weeks gestation (median, 8 weeks). All cesarean scar pregnancies were categorized as either implanted or "on the scar" (group A) or "in the niche" (group B), based on their first-trimester transvaginal ultrasound examination. Clinical outcomes based on gestational age at delivery, mode of delivery, blood loss at delivery, neonate weight and placental histopathologic condition were compared between the groups with the use of the Mann-Whitney U test. Myometrial thickness overlying the placenta was compared among all the patients who required hysterectomy and those who did not with the use of the Mann-Whitney U test. Myometrial thickness was also correlated with gestational age at delivery with the use of Spearman's correlation. RESULTS: Group A consisted of 6 patients; group B consisted of 11 patients. Gestational age at delivery was lower in group B (median, 34 weeks; range, 20-36 weeks) than in group A (median, 38 weeks; range, 37-39 weeks; P = .001). In group A, 5 patients were delivered via cesarean delivery (with normal placenta), and 1 patient underwent a cesarean-hysterectomy for placenta accreta. In group B, 10 patients had a cesarean-hysterectomy for placenta increta/percreta, and 1 patient underwent gravid-hysterectomy for vaginal bleeding at 20 weeks gestation. Blood loss was increased, but not significantly higher in group B (median, 1200 mL; range, 600-4000 mL) than in group A (median, 700 mL; range, 600-1400 mL; P = .117). Myometrium was statistically significantly thinner in the patients group that require hysterectomy (median, 1 mm; range, 0-2 mm) than in the group that did not (median, 5 mm; range, 4-9 mm; P = .001). Myometrial thickness showed a positive correlation with the gestational age (r = 0.820; P<.0005). CONCLUSION: Patients with cesarean scar pregnancy implanted "on the scar" had a substantially better outcome compared with patients in whom the cesarean scar pregnancy implanted "in the niche." Myometrial thickness <2 mm in the first-trimester ultrasound examination is associated with morbidly adherent placenta at delivery.