Natural history of early first-trimester pregnancies implanted in Cesarean scars

Abstract

Objective: To describe the ultrasound findings and natural history of pregnancies implanted within or on Cesarean section scars in the first trimester of pregnancy.

Methods: This was a prospective observational study of 10 women diagnosed with a pregnancy implanted in or on a Cesarean section scar in the first trimester, who declined medical intervention because of their desire to continue the pregnancy. The study population comprised women at < 12 weeks' gestation who were seen in our early pregnancy unit between January 2011 and September 2013. Nine women were followed up by serial ultrasound examinations and had detailed care plans for delivery at King's College Hospital (KCH). One woman was followed up and delivered at another teaching hospital. The first-trimester ultrasound findings were compared with the clinical outcome of the pregnancy.

Results: The nine patients who were followed up at KCH developed ultrasound findings of morbidly adherent placenta (MAP) in the second and third trimesters. All 10 patients were diagnosed with MAP at the time of delivery by Cesarean section. The gestational age at delivery ranged from 26 to 38 weeks. The uterus was conserved in five patients, and Cesarean hysterectomy was performed in the remaining five. All three women with complete implantation of the gestational sac within the scar and two of three cases with placental lakes in the first trimester had hysterectomies. The two cases with bulging of the gestational sac out of the uterine contour had a preterm emergency hysterectomy due to placenta percreta. Histology confirmed placenta accreta in the five hysterectomy specimens. There were no fetal or neonatal complications.

Conclusions: Implantation of a pregnancy on or in a Cesarean section scar is a precursor of MAP; however, the degree of morbidity associated with this implantation is variable and difficult to predict based on first-trimester ultrasound findings only. The assessment of ongoing pregnancies implanted in Cesarean scars is most beneficial when performed between 7 and 9 weeks' gestation. Complete implantation within the myometrial defect, bulging of the trophoblast from the uterine contour and large placental lakes in the first trimester are ultrasound findings that may predict severe placenta accreta or percreta and consequently a poor outcome.

Keywords: Cesarean section scar pregnancy; morbidly adherent placenta; placenta accreta; placenta previa; ultrasonography.