What causes breast milk leakage?

Causes of Breast Milk Leakage

Breast milk leakage is primarily caused by normal physiologic mechanisms related to milk production and let-down reflex, affecting 50-80% of lactating women at some point during their breastfeeding journey. 1

Physiologic Causes

Normal Let-Down Reflex

• Milk ejection reflex (let-down) - The primary mechanism causing milk leakage
  • Triggered by physical stimuli (baby suckling, breast pump)
  • Triggered by psychological stimuli (thinking about baby, hearing baby cry)
  • Can occur in response to sexual arousal
  • Can be triggered by warm shower water on breasts

Hormonal Factors

• Oxytocin release - Causes milk ejection by contracting myoepithelial cells around milk ducts
• High prolactin levels - Increases milk production volume
• Postpartum hormonal fluctuations - Can cause unpredictable leaking in early weeks

Timing-Related Factors

• Fullness between feedings - More common when breasts are engorged
• Missed or delayed feedings - Increases likelihood of leakage
• Early postpartum period - Leaking is most common before milk supply regulates (typically first 6-12 weeks)

Pathologic Causes

Oversupply Issues

• Hyperlactation - Excessive milk production leading to frequent leaking
• Forceful let-down - Can cause leaking from opposite breast during feeding

Medical Conditions

• Galactorrhea - Abnormal milk production not associated with childbirth or nursing
  • Can be caused by medications affecting dopamine (antipsychotics, some antidepressants)
  • Can be caused by pituitary disorders (prolactinomas)
  • Can be caused by hypothyroidism
  • Cabergoline, a dopamine receptor agonist, can be used to treat pathologic galactorrhea by inhibiting prolactin secretion 2

Anatomical Factors

• Duct ectasia - Dilated milk ducts that can leak between feedings 1
• Intraductal papillomas - Can cause pathologic nipple discharge, though typically serous or bloody 1

Behavioral and Environmental Factors

Stress and Emotional Factors

• Stress - Can trigger let-down and leaking in established lactation 3
• Dysphoric Milk Ejection Reflex (D-MER) - Condition where milk let-down is accompanied by negative emotions; affects approximately 14.2% of lactating women 3
• Thoughts about baby - Can trigger let-down even when separated from infant

Physical Triggers

• Physical pressure on breasts - From tight clothing, hugging, or lying on stomach
• Sexual activity - Stimulation or arousal can trigger let-down

Management Considerations

• Most leaking resolves as lactation becomes established and regulated (typically by 3-4 months)
• For physiologic leaking, breast pads and pressure on nipples can help manage symptoms 4
• For pathologic discharge (especially unilateral, spontaneous, or bloody), diagnostic imaging is recommended to rule out underlying conditions 1, 5
• Persistent, unexplained leaking unrelated to lactation may require evaluation for galactorrhea and potential underlying causes

Clinical Pearls

• Leaking is extremely common, with 66% of breastfeeding mothers still experiencing it at 6 months postpartum 4
• The unpredictability of leaking can undermine maternal confidence and affect breastfeeding experience 4
• Physiologic leaking (bilateral, multiple ducts, white/green/yellow) is benign and requires no medical intervention 1
• Pathologic discharge (spontaneous, single duct, unilateral, serous/bloody) requires further evaluation due to 3-29% risk of underlying malignancy 1