Reproductive · Obstetric Hemorrhage
The facts most likely to be tested
Placenta previa presents classically as painless, bright red vaginal bleeding occurring after 20 weeks gestation.
The gold standard for diagnosis is transvaginal ultrasound, which accurately identifies the placental edge in relation to the internal cervical os.
Digital cervical examinations are strictly contraindicated in patients with known or suspected placenta previa due to the risk of catastrophic hemorrhage.
Major risk factors include prior cesarean section, advanced maternal age, multiparity, and a history of placenta previa in previous pregnancies.
Patients who are hemodynamically stable are managed with pelvic rest, avoidance of intercourse, and corticosteroids for fetal lung maturity if preterm.
Delivery is indicated via scheduled cesarean section between 36 0/7 and 37 6/7 weeks gestation to prevent the onset of labor.
Placenta previa is strongly associated with placenta accreta spectrum, particularly in patients with a history of prior uterine surgery.
Vignette unlocked
A 32-year-old G3P2 at 31 weeks gestation presents to the emergency department after an episode of painless, bright red vaginal bleeding. She has no history of abdominal pain or uterine contractions, and fetal heart rate monitoring is reassuring. Her obstetric history is significant for a prior cesarean section performed for breech presentation. Physical examination reveals a soft, non-tender uterus, and the patient is hemodynamically stable. A transabdominal ultrasound suggests a low-lying placenta, but the view is limited.
What is the most appropriate next step in the management of this patient?
Transvaginal ultrasound
The patient presents with classic signs of placenta previa; a transvaginal ultrasound is the gold standard for definitive diagnosis and is safe to perform when done by an experienced provider, whereas digital cervical exams are contraindicated.
Full handout
High yield triage
Etiology / Epidemiology
Associated with prior C-section, advanced maternal age, and multiparity. Results from abnormal implantation over the internal os.
Clinical Manifestations
Presents as painless, bright red vaginal bleeding in the third trimester. Sentinel bleed is common.
Diagnosis
Transvaginal ultrasound is the gold standard. Avoid digital exams if suspected.
Treatment
Stabilize with corticosteroids for lung maturity. Digital pelvic exam is contraindicated.
Prognosis
High risk of placenta accreta spectrum. Delivery via scheduled C-section at 36-37 weeks.
Full handout
Epidemiology & Etiology
Incidence increases with prior uterine surgery, including C-sections and myomectomies. Other major risk factors include smoking, multiple gestations, and a history of placenta previa in prior pregnancies. It is a leading cause of antepartum hemorrhage.
Pertinent Anatomy
The placenta implants in the lower uterine segment, covering or encroaching upon the internal cervical os. This anatomical placement places the placenta directly in the path of cervical dilation.
Pathophysiology
As the uterus grows and the lower segment thins in the third trimester, the inelastic placenta cannot stretch, leading to placental separation. This disruption of maternal vessels causes painless hemorrhage. The bleeding is typically maternal in origin, though fetal distress can occur if significant separation leads to fetal blood loss.
Clinical Manifestations
The hallmark is sudden, painless, bright red vaginal bleeding occurring after 28 weeks gestation. Patients often report a sentinel bleed that resolves spontaneously before recurring. Digital vaginal examination is strictly contraindicated as it can provoke catastrophic hemorrhage.
Diagnosis
The transvaginal ultrasound is the diagnostic modality of choice due to superior accuracy over transabdominal imaging. A placenta edge within <2 cm of the internal os is generally considered diagnostic of previa. If the placenta is >2 cm from the os, it is classified as a low-lying placenta.
Treatment
Management focuses on maternal stabilization and fetal lung maturity using betamethasone. If the patient is hemodynamically unstable, immediate delivery is required regardless of gestational age. Avoid all vaginal exams until previa is ruled out. Stable patients are managed with pelvic rest and scheduled C-section at 36-37 weeks.
Prognosis
Patients are at significantly increased risk for placenta accreta, increta, or percreta, especially with a history of prior C-sections. Close monitoring for postpartum hemorrhage is essential due to the poor contractility of the lower uterine segment.
Differential Diagnosis
Abruptio placentae: painful bleeding with uterine tenderness
Vasa previa: fetal distress and bradycardia following membrane rupture
Cervical polyps: bleeding usually post-coital, visible on speculum exam
Cervical cancer: friable lesion visible on exam
Bloody show: associated with labor contractions and mucus plug