Anatomy & Physiology: Milk production

Milk production is initiated in the breasts in the post-partum period due to prolactin production and decreased estrogen and progesterone after delivery of the placenta. The onset of lactogenesis (the production of breast milk) has been shown to be delayed by stressful events around delivery. Women who underwent an urgent Cesarean section or had a long duration of labor before vaginal deliveries were more likely to have a delayed onset of breast fullness in the first days after delivery ( Dewey, 2001 ).

Dewey and coworkers studied lactation outcomes in the first week of life, including the delayed onset of lactation (onset of breast fullness > 72 hours after delivery), infant weight loss > 10% at day 3 of life, and breastfeeding problems at age 7 days. The main risk factor for excess infant weight loss by day 3 of life was delayed onset of lactation. Risk factors for delayed onset of lactation were:

• Stage II labor > 1 hr,
• prepregnant maternal BMI > 27 kg/m²,
• breastfeeding problems at day 3,and
• being primiparous.

• flat or inverted nipples at day 7,
• stage II labor > 1 hour,
• birthweight < 3601 gms,
• prepregnant maternal BMI > 27 kg/m², and
• non breast milk fluids given in the first 48 hours of life ( Dewey, 2003 ).

By day 3 or 4 post-partum, stimulation of the breast by suckling is required to continue milk production. Mothers produce milk between feedings due to elevated baseline levels of prolactin. They produce more milk during feedings due to the prolactin surge caused by suckling ( Lawrence, 2005 p79-86, Daly (part 2), 1995 ). Women with a prepregnant BMI > 26 kg/m² were found to have a lower prolactin response to suckling at both 48 hours and 7 days post-partum. This decreased prolactin response may cause decreased milk supply in overweight women in the first week of life. Infants of overweight women may need closer monitoring of weight gain and the mothers may need more breastfeeding support ( Rasmussen, 2004 ).

During continued lactation, milk production is based on infant demand. The average mother's ability to produce milk is much greater than the average infant's appetite ( Daly, 1995 ). The rate of milk production varies over the day. If a breast is not emptied at the end of the feeding, that breast produces less milk prior to the next feeding than it would have if the breast had been emptied completely ( Daly (part 2), 1995 , Wilde, 1998 , Peaker, 1998 ). Suckling or any nipple or breast manipulation stimulates sensory nerves in the areola and nipple. These nerves stimulate the pituitary gland to release oxytocin along with prolactin. A conditioned milk ejection can occur when a woman hears her baby cry. This is due to a conditioned release of oxytocin without the release of prolactin ( Lawrence, 2005 p79-86).