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Summary Table

Feature	Detail
Risk Factors	Prior cesarean delivery, prior placenta previa, multiparity, advanced maternal age (>35), multiple gestation, smoking, cocaine use, assisted reproductive technology
Etiology	Abnormal implantation of the placenta over or near the internal cervical os
Patient Presentation	Painless, bright red vaginal bleeding in the late second or third trimester, often sudden onset without provocation
Classic Physical Exam	Soft, non-tender uterus; normal fetal heart tracing (unless hemorrhage is severe); high-lying presenting part or malpresentation (breech/transverse) on Leopold maneuvers
Key Diagnostic Results	Transabdominal ultrasound showing placental tissue covering or adjacent to the internal os; confirmed with transvaginal ultrasound for precise localization
Management	Hemodynamically stable + preterm: expectant management with pelvic rest, serial ultrasound, antenatal corticosteroids; active hemorrhage or term gestation: cesarean delivery
Keywords	"Painless vaginal bleeding," "third trimester," "soft non-tender uterus," "placenta over the os," "no digital exam"

1. Pathophysiology

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.

2. Classification and Clinical Manifestation

Classification	Relationship to Internal Os	Clinical Implication
Complete previa	Placenta entirely covers the internal os	Vaginal delivery is absolutely contraindicated; cesarean delivery required regardless of gestational age at presentation
Partial previa	Placenta partially covers the internal os	Cesarean delivery still required in nearly all cases
Marginal previa	Placental edge reaches but does not cover the os	May allow trial of labor if the edge is >2 cm from the os on follow-up imaging
Low-lying placenta	Placental edge is within 2 cm of the os but does not reach it	Often resolves with uterine growth ("placental migration"); follow-up ultrasound at 32 weeks recommended

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

Feature	Placenta Previa
Timing of bleeding	Late 2nd trimester or 3rd trimester (commonly after 28 weeks)
Character of bleeding	Bright red, sudden onset
Pain	Absent
Uterine tone	Soft, relaxed, non-tender
Fetal status	Usually reassuring unless maternal hemorrhage is severe
Recurrence	Bleeding episodes tend to recur, each potentially more severe
Fetal lie	Frequently malpresentation (breech, transverse) because the placenta occupies the lower pole

3. Diagnostic Workup

Test	Role	Key Finding
Transabdominal ultrasound	Best initial test	Placental tissue identified in the lower uterine segment overlying or near the os
Transvaginal ultrasound	Most accurate test	Precise measurement of the distance between the placental edge and the internal os (accuracy within 2 mm)
Digital cervical exam	Absolutely contraindicated	Can provoke catastrophic hemorrhage by disrupting the placenta
Speculum exam	Performed cautiously	May be used to confirm vaginal source of bleeding, but only after ultrasound rules out previa
CBC, type and screen, coagulation panel	Supportive	Assess degree of hemorrhage, prepare for possible transfusion
Kleihauer-Betke test	If Rh-negative mother	Quantify fetomaternal hemorrhage to guide RhIg dosing

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.

4. Management and Treatment

Clinical Scenario	Management
Preterm (<37 weeks), stable, minimal bleeding	Admit, pelvic rest (no intercourse, no digital exams), IV access, type and crossmatch, serial ultrasound every 2 to 4 weeks, antenatal corticosteroids if <34 weeks
Preterm, active hemorrhage but hemodynamically stable	Admit, betamethasone 12 mg IM x2 doses 24 hours apart (if <34 weeks), tocolysis is controversial (magnesium sulfate may be used cautiously; avoid beta-agonists), continuous fetal monitoring
Preterm, life-threatening hemorrhage or fetal distress	Emergency cesarean delivery regardless of gestational age
Term (>36 to 37 weeks), complete previa	Scheduled cesarean delivery at 36 to 37 weeks for complete previa
Term, marginal/low-lying with edge >2 cm from os	Trial of labor may be considered with in-house obstetric and surgical teams ready
Rh-negative mother	Administer RhIg (300 mcg IM) after each bleeding episode; Kleihauer-Betke test to determine if additional dosing is required

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.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
Placental abruption	Both cause third-trimester vaginal bleeding	Abruption presents with painful bleeding, uterine tenderness, and a rigid/hypertonic uterus; previa is painless with a soft uterus
Vasa previa	Both involve bleeding from placental vessels near the os	Vasa previa bleeding is fetal blood (sinusoidal fetal heart rate pattern, rapid fetal deterioration), often triggered by rupture of membranes; previa bleeding is maternal blood with usually reassuring fetal status
Uterine rupture	Sudden vaginal bleeding in a patient with prior cesarean scar	Rupture presents with acute abdominal pain, loss of fetal station ("recession of the presenting part"), and often fetal bradycardia; uterine contour may be abnormal
Cervical pathology (polyp, cervicitis, cervical cancer)	Vaginal bleeding during pregnancy	Bleeding is typically lighter, may be postcoital, and ultrasound shows normal placental position; speculum exam reveals the cervical lesion
Bloody show / early labor	Vaginal bleeding near term	Mucoid, blood-tinged discharge associated with cervical dilation and regular contractions; placenta is normally positioned
Placenta accreta spectrum	Often coexists with previa, especially with prior cesarean	Accreta is not a separate bleeding presentation but rather a complication discovered during delivery (placenta fails to separate); ultrasound may show loss of the retroplacental clear space and placental lacunae

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.

6. Traps 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.

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