The physiology of breastfeeding

The physiology of breastfeeding
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The Mammary gland is a secondary endocrine-dependent sex organ (located at the level of the 3rd and 4th ribs under the pectoralis major muscle).

Mammary gland structure

  • The mammary gland consists of 15-20 lobes (another source: 7-10 lobes);
  • Each lobe consists of 30-80 lobules;
  • Each lobe has 150-200 alveoli (alveoli is a hollow sac that is filled with secretion/milk);
  • The walls of the alveoli are formed by a layer of glandular cells called lactocytes;
  • Inside the lactocytes, there are secretory tubules through which milk protein is transported into the lumen of the alveoli (for the first 3-4 days after delivery there are large gaps between the alveolar cells. Then secretory cells increase in size, swell, and dense contacts are formed between them. This is how stable milky secretion begins);
  • The alveoli open into the mammary ducts (mammary ducts are ducts formed by the fusion of lobular ducts emanating from each lobe of the mammary gland. The walls of the ducts are formed by 1 layer of lactocytes, but their secretory activity is lower than inside the alveoli. Visually, they are similar to broccoli sprouts. The main milky ducts branch closer to the nipple. The network of ducts is not always arranged radially and symmetrically);
  • There may be a continuation of the glandular tissue towards the axilla (called Spence’s Tail. It is at risk for lactostasis and can be easily squeezed by underwear);
  • An extra lobule may be present (An additional lobule is a part of the breast that cannot get milk out because it is closer to the axilla).

Stages of mammary gland development


  • An increase in the size and weight of the breast;
  • growth of the breast under the influence of the hormones estrogen and progesterone.


  • Glandular tissue predominates over connective and fatty tissue;
  • initiation of milk synthesis occurs.


  • from day 9 postpartum until the child is 2 1/2 years old;
  • direct supply-and-demand breastfeeding;
  • Decrease in breast size between 6 and 9 months.


  • starts after the last breastfeeding and lasts for 40 days;
  • no breast filling even after a long interval between feedings (24 hours);
  • milk composition changes (immunoglobulins and sodium content increases);
  • involution can be reversed before lactopoiesis, as the glandular tissue does not disappear, but only decreases.

Fatty tissue in the mammary gland

  • Is the largest in cell count;
  • Is more abundant outside of lactation;
  • Dies off during pregnancy (as alveolar cells enlarge, proliferate, and divide. After completion of GW, alveolar tissue apoptosis occurs, but the number of alveolar cells still remains greater than before pregnancy);
  • Does NOT degenerate into alveolar (one cell type cannot become the other);
  • The ratio of glandular to adipose tissue is 2 to 1 (according to old data – the same).


  • Has smooth muscle longitudinal ring fibers, dense somatic and autonomic innervation;
  • Is connected to the brain and hypothalamus (and through it to the pituitary gland) via spinal tracts;
  • Can have from 4 to 20 outlets (according to old data 15-20 outlets);
  • Its size and shape are individualized and do not affect breast function;
  • Can be retracted in 3% of women and is bilateral in 87% of cases (of the 3% of women, 4% will have a true retracted nipple with denser and less distensible tissue).


  • The average diameter in a lactating woman is 4 to 6 cm;
  • darkens during pregnancy due to the secretion of a protective layer (and also to be visible to the newborn baby, as the baby’s vision is unfocused for the first time. The baby sees a blurred image, picking out from it a dark, amniotic fluid-smelling spot – the areola and nipple of the mother);
  • can be painful when applied correctly (if the areola is completely white, without a protective layer);
  • clearly shows the trace of the child’s mouth after sucking (it is necessary to pay attention to this trace);
  • has tubercles – Montgomery’s glands.

Montgomery’s glands are bumps around the nipple that secrete an antiseptic, moisturizing lubricant.

  • Regular flushing of this lubricant from the breasts can cause breast soreness;
  • It is hypothesized that the odor emitted by these glands is similar to amniotic fluid, in other words, amniotic fluid (the baby’s intrauterine environment).

Parturition is a milestone for hormones

Hormones that affect lactation (breastfeeding):

💡 Progesterone is an ovarian hormone that is produced by the corpus luteum and placenta during pregnancy.

  • preserves pregnancy;
  • weakens the uterus’ readiness to contract;
  • activates the growth of the mammary glands;
  • inhibits estrogen-stimulated cell division;
  • protects the alveoli and milky ducts from overgrowth under the influence of estrogen;
  • promotes lobule formation.

▪️ Estrogen is an ovarian hormone that is secreted by the placenta.

  • responsible for puberty;
  • is responsible for the development of the female reproductive organs;
  • promotes the development of the mammary glands;
  • causes an increase in uterine contractility and sensitivity to oxytocin;
  • stimulates the formation of additional ducts and the lengthening of existing ducts;
  • causes hypoplasia (if deficient).

▪️ Insulin

  • acts on MJ cells indirectly through other hormonal influences.

▪️ Cortisol is a hormone of the adrenal cortex.

  • promotes prolactin formation in the MJ;
  • stimulates prolactin-stimulated epithelial cell growth.

▪️ Prolactin

  • promotes the growth of the MJ;
  • is produced in the anterior lobe of the pituitary gland (as soon as progesterone and estrogen levels drop dramatically (immediately after childbirth), the anterior lobe of the pituitary gland begins to produce prolactin);
  • causes and maintains milk secretion (responsible for the quantity of milk);
  • helps to realize the instinct of motherhood;
  • starts to grow as soon as the placenta stops supplying progesterone and estrogens to the body (during pregnancy a small amount of prolactin is produced, after childbirth it reaches its maximum level – of 200-400 mg/l);
  • suckling of the child causes a sharp rise – this is an important mechanism in milk production (during the day there are from 7 to 20 emissions of prolactin);
  • Its level fluctuates under the influence of suckling the baby (if a woman does not breastfeed her baby, prolactin level reaches normal levels by day 7);
  • Prolactin levels are higher at night than during the day;
  • remains elevated while the woman is breastfeeding (higher in plasma with frequent feedings);
  • decreases with stress, smoking, alcohol consumption;
  • The amount of milk produced depends little on the prolactin level when mature lactation is established;
  • New prolactin receptors are formed in the breasts during the first breastfeeding (during prolonged breastfeeding, many receptors have time to form and fewer hormones respond);
  • Prolactin receptors are most active in the first 10-14 days after childbirth, in response to frequent stimulation of the breast (if in the first 3 days, a woman did not feed or decant, there may be problems with lactation in the future);
  • Prolactin inhibitor production starts if a woman stores milk in her breasts because at this moment epithelial cells are squeezed.

▪️ Dopamine is a hormone that reduces lactation.

▪️ Prolactoliberin is a hormone that increases lactation.

  • suppresses ovulation;
  • is produced irregularly throughout the day;
  • relaxant;
  • affects memory/sleepiness;
  • peak production between 3:00 and 8:00 a.m.

▪️ Oxytocin is the love hormone.

  • produced by the hypothalamus (posterior lobe of pituitary gland secretes oxytocin);
  • triggers the milk-release reflex (secreted in response to suckling – increases dramatically at 1 minute of suckling and decreases to baseline within 6 minutes);
  • enters through the blood into the tissues of the MJ (connects to receptors on the surface of myoepithelial cells located around the areola);
  • promotes contraction of the alveoli and milk secretion through the ducts from the breast;
  • helps the uterus contract (causes pain when the uterus contracts);
  • induces a feeling of calm;
  • causes a feeling of thirst;
  • causes tingling in the breasts before and after feeding;
  • secretes milk even at the mere thought of a baby;
  • multiplies in the blood from pleasant events in a woman’s life (massage/sex/nipple stimulation/interaction with the baby);
  • reduced, relative to the proper level, in the case of cesarean section surgery in a woman.

The Physiology of Breastfeeding

Latching On: The First Step

The journey of breastfeeding starts with the baby’s latch. Proper latching ensures efficient milk transfer and minimizes discomfort for the mother. During this process, the baby’s mouth covers a significant portion of the areola, forming a seal that allows them to draw milk from the breast’s milk ducts. This natural instinct is facilitated by the infant’s rooting reflex.

Milk Production: A Miraculous Process

Milk production is a highly orchestrated process orchestrated by the endocrine system. The hypothalamus signals the pituitary gland to release prolactin, which stimulates milk production in the mammary glands. As the baby feeds, the act of suckling triggers the release of oxytocin, causing the milk to flow through ducts to the nipple. It’s a beautifully synchronized dance between hormones and nature.

Colostrum: Liquid Gold

In the initial days after birth, the breast secretes colostrum, often referred to as “liquid gold.” This thick, yellowish substance is rich in antibodies and essential nutrients, providing the baby with a strong immune foundation. Colostrum also acts as a gentle laxative, aiding in the passage of meconium, the baby’s first stool.

Supply and Demand

Breast milk production operates on a supply and demand principle. The more frequently and effectively the baby feeds, the more milk the mother’s body produces. This remarkable adaptation ensures that the baby’s changing needs are met, whether they’re experiencing growth spurts or illness.

Bonding Through Breastfeeding

Breastfeeding isn’t just about nutrition; it’s a profound bonding experience. As the baby feeds, both mother and child release oxytocin, often referred to as the “love hormone.” This hormone fosters feelings of attachment, trust, and affection, strengthening the emotional connection between them.

Read more:

Types of Breastfeeding Positions You and Baby Will Love

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