Placental questions are classic “read the bleeding pattern” vignettes—but the test writers know you’ve memorized the buzzwords. The real points come from distinguishing painful vs painless bleeding, linking findings to risk factors, and predicting next step and complications. Let’s run a Q-bank-style case and then do what top scorers do: make every answer choice teach you something.
The Vignette (Q-Bank Style)
A 32-year-old G3P2 at 34 weeks presents with sudden onset vaginal bleeding. She denies abdominal pain. Pregnancy has been uncomplicated. She has had two prior cesarean deliveries. Vitals are stable. Fetal heart tracing is reassuring. On exam, the uterus is nontender. Speculum exam shows bleeding from the cervical os. No digital cervical exam is performed. Transabdominal ultrasound suggests placenta covering the internal cervical os.
Question: What is the most likely diagnosis?
Correct Answer: Placenta previa
Why it’s correct
Placenta previa = placenta implanted in the lower uterine segment, over or near the internal cervical os.
Hallmarks
- Painless, bright red vaginal bleeding in the 2nd or 3rd trimester
- Soft, nontender uterus
- Often recurrent bleeding episodes
- Fetal status may be reassuring unless bleeding is severe
Risk factors (high yield)
- Prior cesarean delivery (big one)
- Prior uterine surgery/curettage
- Multiple gestation
- Multiparity
- Advanced maternal age
- Smoking
Key management rule
- Do NOT perform a digital cervical exam until placenta previa is excluded (can precipitate catastrophic hemorrhage).
- Diagnosis is by transvaginal ultrasound (more accurate than transabdominal and safe when done properly).
Typical management (Step-friendly)
| Scenario | Management |
|---|---|
| Stable mom + reassuring fetus + preterm | Expectant: pelvic rest, avoid intercourse, monitor, corticosteroids if indicated |
| Heavy bleeding, labor, or fetal/maternal instability | C-section (often emergent if unstable) |
| Rh-negative with bleeding | Rho(D) immune globulin |
Now, Why the Distractors Are Wrong (and What They’re Trying to Teach You)
Distractor 1: Placental abruption
Why students pick it: “3rd trimester bleeding” reflex.
Why it’s wrong here: Abruption is classically painful bleeding with a tender, hypertonic uterus.
Placental abruption = premature separation of the placenta from the uterine wall.
Classic features
- Painful vaginal bleeding (may be concealed—so bleeding can be minimal)
- Uterine tenderness, rigidity (“board-like uterus”)
- Frequent contractions / uterine hypertonicity
- Nonreassuring fetal heart tracing is common
Risk factors to memorize
- Hypertension (chronic or preeclampsia)
- Abdominal trauma (e.g., MVC)
- Cocaine use
- Prior abruption
- Smoking
- Sudden decompression of uterus (e.g., rapid loss of amniotic fluid)
Complications (very testable)
- DIC due to release of tissue factor (thromboplastin) from decidua/placenta
- Think: low fibrinogen, thrombocytopenia, prolonged PT/PTT
- Fetal distress or demise
Key differentiator
- Previa: painless + soft uterus
- Abruption: painful + tender/rigid uterus ± fetal distress
Distractor 2: Placenta accreta spectrum (accreta/increta/percreta)
Why students pick it: Prior C-sections are a huge clue—but for accreta, the bleeding problem usually shows up at delivery, not as painless antepartum bleeding.
Placenta accreta spectrum = abnormal adherence of placental villi to the uterine wall due to defective decidua basalis.
Depth definitions (high yield)
- Accreta: villi attach to myometrium (no decidua)
- Increta: villi invade into myometrium
- Percreta: villi penetrate through myometrium (can invade bladder)
Clues
- Major risk factor: placenta previa + prior C-section (the combo is very Step 2–favorite)
- Often suspected on prenatal ultrasound/MRI, but classically presents with:
- Failure of placental separation after delivery
- Massive postpartum hemorrhage
Management
- Planned C-section hysterectomy in many cases (especially increta/percreta), with blood products ready.
Why it’s wrong in this vignette
- This patient’s presentation is antepartum painless bleeding with ultrasound showing placenta over the os → previa.
- Accreta is a complication strongly associated with previa, especially with prior C-sections, but it’s not the primary diagnosis driving this specific bleeding pattern.
Test-writer move: They give you prior C-sections to make you think accreta. Don’t ignore the bleeding pattern + placental location.
Distractor 3: Vasa previa
Why students pick it: Another “painless bleeding” entity.
Vasa previa = fetal vessels traverse the membranes over the cervical os (often from velamentous cord insertion).
Classic presentation
- Painless vaginal bleeding after membrane rupture
- Fetal distress (bradycardia) because the blood is fetal, not maternal
Key clues
- Bleeding occurs with/after ROM
- Fetal heart rate drops quickly
- Apt test (alkali denaturation) can differentiate fetal hemoglobin from maternal
Why it’s wrong here
- No mention of rupture of membranes or fetal distress.
- Ultrasound suggests placenta covering the os (previa), not exposed fetal vessels.
Distractor 4: Uterine rupture
Why students pick it: Prior C-sections again.
Uterine rupture = full-thickness disruption of uterine wall, usually during labor.
Classic presentation
- Sudden severe abdominal pain
- Vaginal bleeding
- Loss of fetal station
- Nonreassuring fetal heart tracing
- Occurs during TOLAC/VBAC or obstructed labor
Why it’s wrong here
- She’s not in labor, has a nontender uterus, and fetal tracing is reassuring.
Distractor 5: Cervicitis/cervical cancer/cervical polyp
Why students pick it: Bleeding seen from the cervical os.
How to separate
- These cause postcoital bleeding or spotting, not typically brisk third-trimester hemorrhage.
- Often accompanied by:
- Cervicitis: mucopurulent discharge, friable cervix
- Cancer: abnormal Pap history, irregular bleeding, weight loss (sometimes)
- Polyp: benign bleeding/spotting
Why it’s wrong here
- The ultrasound localization + classic painless late-pregnancy bleeding pattern points to placenta previa.
High-Yield Summary Table: Previa vs Abruption vs Accreta
| Feature | Placenta previa | Placental abruption | Placenta accreta spectrum |
|---|---|---|---|
| Bleeding | Painless, bright red | Painful, may be concealed | Often postpartum hemorrhage (after delivery) |
| Uterus | Soft, nontender | Tender, hypertonic/rigid | Usually not a pain/tenderness diagnosis |
| Fetal tracing | Often reassuring | Often nonreassuring | Variable |
| Risk factors | Prior C-section, multiparity, smoking | HTN, cocaine, trauma, smoking | Prior C-section + placenta previa (highest risk combo) |
| Diagnosis | Transvaginal US | Clinical ± US (US can miss) | Prenatal US/MRI; confirmed at delivery |
| Big “don’t miss” | No digital exam | DIC risk | Massive hemorrhage, may require hysterectomy |
“Next Step” Pearls They Love to Ask
If placenta previa is suspected
- Transvaginal ultrasound (safe, best test)
- Avoid digital cervical exam
- Stabilize bleeding: IV access, CBC, type & cross
- Rh-negative: give Rho(D) immune globulin
If abruption is suspected
- Treat as an obstetric emergency if unstable
- Continuous fetal monitoring
- Check coagulation: fibrinogen is especially helpful (often low)
If accreta is suspected antenatally
- Plan delivery at a tertiary center
- Blood products ready
- Often C-section hysterectomy without attempting placental removal
Rapid-Fire Self-Check (1-liners)
- Painless third-trimester bleeding → think placenta previa (confirm with TVUS; no digital exam).
- Painful bleeding + tender rigid uterus → think abruption (watch for DIC).
- Prior C-section + previa → worry about accreta (massive postpartum hemorrhage; hysterectomy often needed).
- Bleeding after ROM + fetal bradycardia → think vasa previa (fetal blood loss).