How does lactation begin?

How Lactation Begins

Lactation begins when progesterone levels rapidly decline after delivery of the placenta, removing the inhibition on milk production that was present during pregnancy, allowing prolactin to stimulate milk synthesis. 1, 2

Hormonal Preparation During Pregnancy

• Mammary gland development begins as early as the sixth week of gestation and continues throughout life, with significant development occurring during puberty under the influence of estrogens 3
• During pregnancy, the mammary gland undergoes extensive development in preparation for lactation:
  • Estrogen stimulates mammary duct growth 4
  • Progesterone combined with estrogen is required for lobuloalveolar development 1
  • Prolactin levels increase 10-20 times over normal amounts by the end of pregnancy 5
  • Placental lactogen can substitute for pituitary prolactin during pregnancy 1
• The breast is so effectively prepared during pregnancy that lactation could potentially begin even if pregnancy were discontinued at 16 weeks 3

Initiation of Lactation (Lactogenesis)

• Despite high prolactin levels during pregnancy, milk production is inhibited by elevated progesterone 5, 2
• After birth and delivery of the placenta, progesterone levels rapidly decline, removing this inhibition 1, 2
• The minimal hormonal requirements for lactation to begin are:
  • Prolactin (the principal lactogenic hormone)
  • Insulin
  • Hydrocortisone (glucocorticoids) 5
• Initial milk production begins with colostrum (high-density, low-volume feed) for approximately the first 30 hours after birth 3
• Transitional milk follows colostrum from days 7-14 postpartum 6
• Mature milk is established around 2-3 weeks postpartum 6

Maintenance of Lactation

• Infant suckling is crucial for maintaining lactation through two primary mechanisms:
  1. Stimulation of oxytocin release from the posterior pituitary, triggering the milk-ejection reflex through contraction of myoepithelial cells 1
  2. Stimulation of prolactin release from the anterior pituitary to maintain milk production 5, 1
• The volume of milk produced is governed by the efficiency of infant suckling and milk removal rather than maternal hormone levels 3
• Mothers who allow infants to feed ad libitum typically experience large volumes of milk 24-48 hours after birth 3

Hormonal Actions in the Mammary Gland

• Prolactin has several key functions in lactation:
  • Stabilizes and promotes transcription of casein mRNA
  • May stimulate synthesis of alpha-lactalbumin (regulatory protein of lactose synthetase)
  • Increases lipoprotein lipase activity in the mammary gland 5
• Glucocorticoids (cortisol) support milk synthesis and regulate stress response, energy regulation, and have anti-inflammatory effects 6
• Insulin regulates metabolism by promoting glucose absorption for milk production 6

Factors That Can Affect Lactation

• Medications can impact prolactin levels and lactation:
  • Domperidone and metoclopramide can increase milk supply by increasing prolactin levels 7
  • Bromocriptine suppresses lactation by inhibiting prolactin secretion and should be avoided in women who wish to breastfeed 8
  • Clonidine may reduce prolactin secretion and potentially reduce milk production in early postpartum period 8
• Frequent and intensive suckling is essential for maintaining normal postpartum lactation 1

Composition Changes During Lactation

• Colostrum contains high concentrations of immunoglobulins and proteins that help establish the infant's immune system 6
• As breastfeeding continues, immunoglobulin and protein concentrations decrease as milk transitions from colostrum to transitional milk 6
• Mature milk exhibits less compositional variability but still changes throughout the day and during feedings:
  • Fore milk (beginning of feeding) has lower fat content
  • Hind milk (end of feeding) contains 2-3 times more fat 6
• Milk composition varies with time of day - day and evening samples are more energy-dense than night or morning samples 6

Human milk provides not just nutrition but also bioactive components that support infant development and health, making it uniquely suited to the needs of human infants 3.