Bit · OB/Gyn
OB Bleeding — Ectopic vs Molar vs Abruption vs Previa
Four causes of bleeding in pregnancy that NBME loves. The pivot is gestational age, painful vs painless, and ultrasound findings.
Mechanism
Bleeding in pregnancy is split first by trimester:
- First trimester — ectopic pregnancy, molar pregnancy, threatened/inevitable/missed/complete abortion.
- Third trimester — placental abruption (painful), placenta previa (painless), vasa previa (painless rupture of fetal vessels).
Differentiator Table
| Condition | Timing | Pain? | Risk factors | Ultrasound | Distinguishing |
|---|---|---|---|---|---|
| Ectopic pregnancy | First trimester (often 6–8 wks) | YES — unilateral pelvic pain | PID/STDs, prior ectopic, IUD, prior tubal surgery, smoking | No intrauterine gestational sac despite β-hCG > discriminatory zone (~1500–2000); may see adnexal mass | β-hCG plateau or slow rise instead of doubling |
| Complete molar pregnancy | First trimester | Variable | Extremes of maternal age | 'Snowstorm' or 'cluster of grapes'; NO fetal parts; uterus large for dates | β-hCG very high (often > 100,000); hyperemesis, theca-lutein cysts, pre-eclampsia < 20 wks |
| Partial molar | First trimester | Variable | Fetal parts + cystic placenta | Triploid (69,XXX/XXY); β-hCG less elevated than complete | |
| Placental abruption | Third trimester | YES — painful continuous bleeding; tetanic uterus | HTN/preeclampsia, cocaine, trauma, smoking, prior abruption | May show retroplacental hematoma (often missed) | Dark/concealed bleed possible; fetal distress common |
| Placenta previa | Third trimester | NO — painless bright red bleeding | Prior C-section, multiparity, advanced maternal age, smoking | Placenta covering or near internal cervical os | Diagnose on US — never digital cervical exam |
| Vasa previa | Third trimester at ROM | NO — painless bleeding at rupture of membranes | Velamentous cord insertion, low-lying placenta | Fetal vessels overlying internal os | Fetal blood loss; rapid fetal demise unless emergency C-section |
The Pivot
Three questions decide it:
- First or third trimester?
- Painful or painless? First trimester: painful + unilateral → ectopic. First trimester: no pain or non-specific + very high β-hCG → molar. Third trimester: painful + rigid uterus → abruption. Third trimester: painless → previa.
- Ultrasound: no intrauterine sac + adnexal mass → ectopic. Snowstorm → complete mole. Placenta covering os → previa.
NBME-Style Stem
A 32-year-old G3P2 woman at 32 weeks gestation presents with sudden onset of painless, bright red vaginal bleeding. She is hemodynamically stable. Examination shows a soft, non-tender uterus. Fetal heart rate is reassuring. Which of the following is the most appropriate next diagnostic step?
Concept Anchor
OB bleeding is decided by when (trimester), how (painful vs painless), and what the ultrasound shows. First trimester pain = ectopic; first trimester snowstorm = mole; third trimester rigid painful = abruption; third trimester painless = previa.