Plasenta Previa

01Pathophysiology

Placenta previa occurs when the placenta implants in the lower uterine segment, partially or completely covering the internal cervical os. Under normal circumstances, the placenta implants in the upper uterine body (fundus), well away from the os. When implantation occurs low, the placenta becomes vulnerable to mechanical disruption as the lower uterine segment thins and elongates during the late second and third trimesters.

The lower uterine segment undergoes progressive effacement and dilation as pregnancy advances, and this stretching creates a shearing force between the anchored placental villi and the changing uterine wall. Because the placenta cannot stretch with the myometrium, the attachment tears, exposing the maternal spiral arteries at the implantation site. This is the mechanism behind the painless bleeding: the hemorrhage originates from disrupted maternal vessels at the placental-decidual interface, not from uterine contractions or abruption. The uterus itself is not irritated or contracting, which is why palpation reveals a soft, non-tender uterus, a finding that directly contrasts with placental abruption.

The painless nature of the bleeding is the single most important clinical feature. The vignette will almost always describe a patient in the third trimester with sudden, bright red vaginal bleeding and no abdominal pain, no uterine tenderness, and no contractions. If the stem adds pain or a rigid uterus, the test-writer is steering you toward abruption instead.

Prior uterine scarring (from cesarean sections, curettage, or myomectomy) damages the endometrium, forcing the blastocyst to implant in less optimal, lower regions. This explains why prior cesarean delivery is the highest-yield risk factor tested. Each additional cesarean section compounds the risk.

02Classification and Clinical Manifestation

A key concept tested on exams is placental migration. This does not mean the placenta physically moves. As the lower uterine segment grows, the relative position of the placental edge shifts upward and away from the os. Up to 90% of previas identified on second-trimester ultrasound will resolve by the third trimester, which is why a diagnosis at 20 weeks does not automatically commit the patient to a cesarean. Only previas that persist beyond 32 to 35 weeks are considered clinically relevant.

Clinical Manifestation

03Diagnostic Workup

When a pregnant patient presents with painless vaginal bleeding in the third trimester, the first step is transabdominal ultrasound. This is widely available, noninvasive, and will identify the location of the placenta relative to the cervical os. It is the best initial test because it can be performed rapidly at the bedside.

If the transabdominal ultrasound confirms or suggests a low-lying placenta but the exact relationship to the os is unclear, transvaginal ultrasound is the next step and the most accurate confirmatory test. Many students mistakenly believe that transvaginal ultrasound is contraindicated in previa, but this is incorrect. The probe is placed in the vaginal fornix, not through the cervix, and does not contact the placenta. It is both safe and superior for precise localization.

The cardinal rule: never perform a digital cervical examination until placenta previa has been excluded by imaging. A finger introduced through the cervix can directly disrupt the overlying placenta and trigger uncontrollable hemorrhage. This is one of the most commonly tested "do not do" principles in obstetric vignettes. If a question stem describes a physician reaching for a digital exam in a patient with undiagnosed third-trimester bleeding, that is the wrong answer.

Baseline labs including a complete blood count, blood type with antibody screen, and coagulation studies should be drawn simultaneously. These do not diagnose previa but are essential for managing hemorrhage and preparing for potential operative delivery.

04Management and Treatment

Expectant management is the cornerstone for the stable preterm patient. The goal is to prolong the pregnancy to reduce neonatal morbidity from prematurity while keeping the mother safe. This includes strict pelvic rest, avoidance of any vaginal manipulation, and close surveillance with serial ultrasound to track placental position and fetal growth.

Antenatal corticosteroids (betamethasone 12 mg IM, two doses 24 hours apart, or dexamethasone 6 mg IM every 12 hours for four doses) should be administered between 23 and 34 weeks to accelerate fetal lung maturity. If the patient has not delivered by 7 days after the initial course, a rescue course can be considered if the gestational age remains below 34 weeks.

For the patient with active hemorrhage, the decision hinges on severity. Mild, self-limited episodes can be managed conservatively with observation, but brisk or recurrent hemorrhage that threatens maternal hemodynamic stability mandates immediate cesarean delivery. There is no role for attempting vaginal delivery in a patient with a complete placenta previa regardless of clinical circumstances.

A commonly tested nuance involves tocolysis. While some clinicians use magnesium sulfate to suppress contractions in preterm previa with concurrent uterine activity, this practice is controversial. The rationale is that contractions worsen bleeding by further disrupting the placental-uterine interface. However, tocolysis should never delay delivery when the mother or fetus is in danger.

Placenta accreta spectrum must always be considered in a patient with placenta previa and a history of prior cesarean delivery. The combination of a previa overlying a prior uterine scar raises the probability of abnormal placental invasion (accreta, increta, or percreta). If suspected, MRI can help delineate the depth of invasion, and delivery should be planned at a tertiary center with multidisciplinary surgical support.

05Differential Diagnosis and Distractors

The most frequently tested comparison is previa versus abruption. The vignette will hinge on two details: presence or absence of pain, and uterine tone. If both are absent (painless, soft uterus), the answer is previa. If both are present (painful, rigid uterus), the answer is abruption. A third-trimester bleeding vignette that mentions cocaine or hypertension is pushing you toward abruption, while one that mentions prior cesarean or smoking without pain is pushing toward previa.

Vasa previa is a more subtle distractor. The classic scenario involves rupture of membranes immediately followed by vaginal bleeding and fetal heart rate abnormalities (often sinusoidal pattern). The key teaching point is that in vasa previa, the blood is fetal in origin, so even a small amount can be lethal to the fetus. An Apt test (alkali denaturation test) distinguishes fetal hemoglobin from maternal hemoglobin in the vaginal blood.

06Traps and High-Yield Pearls

The single most common trap is the student who selects digital cervical examination as the initial step for evaluating third-trimester vaginal bleeding. The exam will present a patient with bright red, painless bleeding and offer "bimanual exam" or "cervical dilation check" as an answer choice. This is always wrong until ultrasound has excluded previa. Choosing this answer demonstrates a failure to recognize the potentially fatal consequence of digitally disrupting a previa.

A second common error is assuming that a placenta previa identified at the 20-week anatomy scan requires immediate cesarean planning. Most early previas resolve through apparent migration. The correct management is follow-up ultrasound at 32 weeks. Only persistent previa warrants delivery planning.

Students also confuse the safety of transvaginal ultrasound with digital examination. Transvaginal ultrasound is safe and is, in fact, the gold standard for diagnosis. The probe does not pass through the cervical canal and does not contact the placenta. Never choose "avoid transvaginal ultrasound" as a correct answer when evaluating suspected previa.

Finally, when the vignette pairs placenta previa with multiple prior cesarean deliveries, the test-writer is often testing your knowledge of placenta accreta spectrum. The correct next step in that scenario is to evaluate for abnormal placental invasion (ultrasound findings such as placental lacunae, loss of the retroplacental clear zone, or bladder wall irregularity) and to plan delivery at a facility equipped for potential cesarean hysterectomy.

The core competency being tested across all placenta previa questions is the ability to distinguish painless from painful third-trimester bleeding, to sequence the diagnostic workup correctly (ultrasound first, never digital exam), and to recognize when expectant management transitions to emergent cesarean delivery.