Hormonal influences From the eighteenth week of
pregnancy (the second and third
trimesters), a woman's body produces
hormones that stimulate the growth of the
milk duct system in the
breasts: •
Progesterone influences the growth in size of
alveoli and lobes; high levels of progesterone inhibit lactation before birth. Progesterone levels drop after birth; this triggers the onset of copious milk production. •
Estrogen stimulates the milk duct system to grow and differentiate. Like progesterone, high levels of estrogen also inhibit lactation. Estrogen levels also drop at delivery and remain low for the first several months of breastfeeding. Other hormones—notably insulin, thyroxine, and cortisol—are also involved, but their roles are not yet well understood. Although biochemical markers indicate that Secretory Activation begins about 30–40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in the breast") until 50–73 hours (2–3 days) after birth.
Colostrum is the first milk a breastfed baby receives. It contains higher amounts of white blood cells and
antibodies than mature milk, and is especially high in
immunoglobulin A (IgA), which coats the lining of the baby's immature intestines, and helps to prevent pathogens from invading the baby's system. Secretory IgA also helps prevent food allergies. Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk. Research also suggests that draining the breasts more fully also increases the rate of milk production. Thus the milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk from the breast. Low supply can often be traced to: • not feeding or
pumping often enough • inability of the infant to transfer milk effectively caused by, among other things: • jaw or mouth structure deficits • poor latching technique • premature birth • drowsiness in the baby, due to illness, medication or recovery from medical procedures • rare maternal endocrine disorders • hypoplastic breast tissue • inadequate calorie intake or malnutrition of the mother
Milk ejection reflex This is the mechanism by which milk is transported from the breast alveoli to the
nipple. Suckling by the baby stimulates the
paraventricular nuclei and
supraoptic nucleus in the
hypothalamus, which signals to the posterior
pituitary gland to produce
oxytocin. Oxytocin stimulates contraction of the
myoepithelial cells surrounding the alveoli, which already hold milk. The increased pressure causes milk to flow through the duct system and be released through the nipple. This response can be
conditioned e.g. to the cry of the baby. Milk ejection is initiated in the mother's breast by the act of suckling by the baby. The milk ejection reflex (also called let-down reflex) is not always consistent, especially at first. Once a woman is conditioned to nursing, let-down can be triggered by a variety of stimuli, including the sound of any baby. Even thinking about breastfeeding can stimulate this reflex, causing unwanted leakage, or both breasts may give out milk when an infant is feeding from one breast. However, this and other problems often settle after two weeks of feeding.
Stress or anxiety can cause difficulties with breastfeeding. The release of the hormone
oxytocin leads to the
milk ejection or
let-down reflex. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently. Some feel a slight tingling, others feel immense amounts of pressure or slight pain/discomfort, and still others do not feel anything different. A minority of mothers experience a
dysphoric milk ejection reflex immediately before let-down, causing anxiety, anger or nausea, amongst other negative sensations, for up to a few minutes per feed. A poor milk ejection reflex can be due to sore or cracked nipples, separation from the infant, a history of breast
surgery, or tissue damage from prior
breast trauma. If a mother has trouble breastfeeding, different methods of assisting the milk ejection reflex may help. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.
Milk ejection reflex mechanism This is the mechanism by which milk is transported from the breast alveoli to the nipple. Suckling by the baby innervates slowly adapting and rapidly-adapting
mechanoreceptors that are densely packed around the
areolar region. The electrical impulse follows the
spinothalamic tract, which begins by innervation of fourth
intercostal nerves. The electrical impulse then ascends the
posterolateral tract for one or two vertebral levels and synapses with second-order neurons, called tract cells, in the posterior dorsal horn. The tract cells then decussate via the
anterior white commissure to the anterolateral corner and ascend to the
supraoptic nucleus and
paraventricular nucleus in the
hypothalamus, where they synapse with oxytocinergic third-order neurons. The somas of these neurons are located in the hypothalamus, but their axon and axon terminals are located in the
infundibulum and
pars nervosa of the
posterior pituitary, respectively. The oxytocin is produced in the neuron's soma in the supraoptic and paraventricular nuclei, and is then transported down the infundibulum via the
hypothalamo-neurohypophyseal tract with the help of the carrier protein,
neurophysin I, to the pars nervosa of the posterior pituitary, and then stored in
Herring bodies, where they are stored until the synapse between second- and third-order neurons. Following the electrical impulse, oxytocin is released into the bloodstream. Through the bloodstream, oxytocin makes its way to
myoepithelial cells, which lie between the extracellular matrix and luminal epithelial cells that also make up the alveoli in breast tissue. When oxytocin binds to the myoepithelial cells, the cells contract. The increased intra-alveolar pressure forces milk into the lactiferous sinuses, into the lactiferous ducts (a study found that lactiferous sinuses may not exist. If this is true then milk simply enters the lactiferous ducts), and then out the nipple.
Afterpains A surge of oxytocin also causes the uterus to contract. During breastfeeding, mothers may feel these contractions as
afterpains. These may range from period-like cramps to strong labour-like contractions and can be more severe with second and subsequent babies. ==Without pregnancy, induced lactation, relactation==