Placenta Previa

Placenta previa occurs when the placenta implants over or near the internal cervical os — and USMLE Step 1's highest-yield fact on this topic is that digital cervical examination is absolutely contraindicated. Touching the placental edge precipitates catastrophic hemorrhage; always use ultrasound. As the lower uterine segment grows and the cervix effaces, the placenta separates from the wall, causing painless bleeding — no uterine contraction, no retroplacental clot. That painlessness is the single most important distinguishing feature from abruption that the exam will test. The classic vignette is a third-trimester patient with sudden, painless bright-red vaginal bleeding, often without a clear precipitant.

The exam tests previa from two main angles: recognizing the presentation (and not confusing it with abruption) and knowing the management rules — especially what you absolutely cannot do. Students consistently miss the digital cervical exam contraindication, which is one of the highest-yield management facts on USMLE Step 1. Touching the cervix in previa can disrupt the placental edge and trigger massive hemorrhage. This is not a relative contraindication — it's absolute. Use ultrasound instead to confirm the diagnosis.

The tricky part is that students blur previa and abruption. Abruption = painful, dark blood, firm uterus, often with hypertension or trauma. Previa = painless, bright red, soft uterus. Risk factors are also distinct — previa is tied to prior uterine surgery (especially C-sections) and multiparity because scarred or distorted endometrium favors abnormal low implantation. If you see a vignette with a grand multipara or prior cesarean and painless third-trimester bleeding, that's previa until proven otherwise.