What are the causes of vaginal bleeding during pregnancy?

Causes of Vaginal Bleeding During Pregnancy

Vaginal bleeding during pregnancy has distinct causes that vary by trimester, with approximately 25% of pregnant women experiencing first trimester bleeding, ranging from benign implantation bleeding to life-threatening conditions like ectopic pregnancy. 1

First Trimester Causes

The first trimester presents the widest range of etiologies:

Common Causes

• Normal intrauterine pregnancy - Light spotting from implantation as the fertilized egg embeds into the endometrium and cervical hypervascularization occurs 1
• Subchorionic hematoma - Blood collection between the uterine wall and gestational sac in an otherwise viable pregnancy 1
• Threatened abortion - Bleeding in a potentially viable pregnancy 2
• Early pregnancy loss - Including embryonic demise, anembryonic pregnancy, and incomplete abortion 1, 2
• Ectopic pregnancy - Accounts for 1-2% of pregnancies but represents 6% of all maternal deaths, making it the most critical diagnosis to exclude 2
• Gestational trophoblastic disease - Abnormal placental tissue development 1

Critical point: Approximately half of women who bleed in the first trimester will ultimately miscarry, but the presence of fetal heart sounds is reassuring 3. Pain and heavy bleeding significantly increase the risk of pregnancy loss 2.

Second and Third Trimester Causes

Beyond 20 weeks, the differential shifts dramatically toward placental and cervical pathology:

Physiologic Causes

• "Bloody show" - Normal vaginal bleeding as the cervix prepares for labor, typically preceding labor onset 4
• Miscarriage - Can occur in the first half of pregnancy 4

Pathologic Causes (Life-Threatening)

• Placenta previa - The most common serious diagnosis, affecting approximately 1 in 200 pregnancies at delivery 4. Classically presents as painless bleeding 5
• Placental abruption - Affects approximately 1% of pregnancies, though ultrasound detects at most 50% of cases. Central abruption carries worse perinatal outcomes than marginal separation. Typically presents with abdominal pain and bleeding 4, 5
• Vasa previa - Rare (1 in 2,500 to 1 in 5,000 deliveries) but potentially catastrophic 4
• Uterine rupture - Rare but occurs most commonly in patients with prior cesarean delivery or uterine surgery 4
• Placenta accreta spectrum - Abnormal placental attachment 4

Cervical and Vaginal Causes (Any Trimester)

• Preterm labor with cervical change 4
• Cervicitis or cervical polyp 4
• Vaginal laceration 4
• Cervical varix - Rare vascular abnormality 6
• Incompetent cervix - Particularly in second trimester 7

Critical Clinical Distinctions

Important caveat: Approximately half of women with vaginal bleeding in pregnancy have no identifiable cause, yet this bleeding still increases risk for adverse outcomes 4.

Timing-Based Approach

• First trimester: Focus on distinguishing viable intrauterine pregnancy from ectopic pregnancy and early pregnancy loss
• Second/third trimester: Digital pelvic examination must be avoided until placenta previa, low-lying placenta, and vasa previa are excluded 4

Red Flags Requiring Urgent Evaluation

• Heavy bleeding with pain (increased risk of pregnancy loss or abruption)
• Hemodynamic instability
• Adnexal mass or signs of intraperitoneal bleeding (ectopic pregnancy until proven otherwise) 3
• Bleeding after 20 weeks (potential placental emergency)

The evidence consistently emphasizes that while many causes are benign, the potential for life-threatening maternal and fetal complications necessitates systematic evaluation with ultrasound as the primary diagnostic modality across all trimesters 1, 4.