Clinical meaning
Placenta previa involves ectopic implantation in the lower uterine segment where the decidua basalis is thinner and less vascularized than the fundal region. This results in abnormal trophoblastic invasion and a higher rate of placenta accreta spectrum disorders (accreta, increta, percreta), particularly in patients with prior uterine scars. The lower segment's limited contractile ability after placental separation contributes to hemorrhage that is unresponsive to uterotonics alone. The clinician must integrate diagnostic imaging, classify the previa, manage expectant versus active management protocols, prescribe corticosteroids for fetal lung maturity, order blood products, and determine delivery timing in consultation with maternal-fetal medicine.
Diagnosis & workup
Diagnostics & workup: - Order transvaginal ultrasound for definitive diagnosis (superior to transabdominal for os-placenta distance) - Classify: complete previa (covers os), marginal previa (edge within 2 cm), low-lying placenta (edge 2-3.5 cm from os) - Order MRI if placenta accreta spectrum is suspected (loss of retroplacental clear space, myometrial thinning) - Order serial CBC every 4-6 hours during active bleeding to track hemoglobin trends - Order comprehensive coagulation panel (PT, PTT, fibrinogen, D-dimer) to assess for DIC - Order type and crossmatch with 4 units PRBCs available for complete previa - Order Kleihauer-Betke test for Rh-negative patients to calculate RhoGAM dosing - Schedule follow-up transvaginal ultrasound at 32 and 36 weeks to reassess placental location