New parents often feel an unexpected urge for a bowel movement while expressing milk. This common physiological connection is a direct result of the hormonal cascade initiated by milk expression. The reflex that releases milk is linked to other involuntary muscle responses, causing this effect.
The Physiological Connection
The direct link between pumping and the sudden need to defecate is the hormone oxytocin. Oxytocin is best known for its role in labor and the milk ejection reflex. When the nipple is stimulated, nerve signals prompt the release of this hormone from the pituitary gland. This release is immediate and pulsatile. While the primary effect is contracting the myoepithelial cells to push milk down the ducts, oxytocin acts systemically throughout the body. This hormone has receptors on various smooth muscle tissues beyond the breast and uterus, which explains the surprising digestive response.
How Oxytocin Influences Digestion
The urge to defecate is a direct result of oxytocin’s influence on the smooth muscle lining the gastrointestinal tract. Oxytocin receptors are present on the smooth muscle cells of the stomach and intestines. When the hormone is released into the bloodstream during pumping, it binds to these receptors and stimulates muscle contraction. These contractions increase the motility of the gastrointestinal tract, which is the movement of food and waste products through the digestive system. This enhanced movement is known as peristalsis, a wave-like muscular action that pushes contents along the intestines. By accelerating this process, the hormone can effectively bring on the urge for a bowel movement.
Related Postpartum Physical Changes
The systemic effects of oxytocin extend beyond the digestive tract, influencing other postpartum physical changes. The hormone’s most well-known action, outside of lactation, is contracting the uterus. During pumping or nursing, the oxytocin surge that initiates milk flow also causes the uterus to contract, a sensation often felt as mild cramping, sometimes called “afterpains.” These contractions help the uterus shrink back to its pre-pregnancy size and reduce postpartum bleeding. The hormonal release is also associated with other common systemic responses. Many individuals report feeling an intense thirst or a wave of hunger while pumping or nursing. Oxytocin is also known to have a calming, anti-stress effect, sometimes causing a feeling of relaxation.