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Kittens nursing White Cat Nursing Four Kittens HQ.jpg
Kittens nursing
A pig nursing her piglets. Pig lactation.jpg
A pig nursing her piglets.

Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all post-pregnancy female mammals, although it predates mammals. [1] In humans the process of feeding milk is also called breastfeeding or nursing. Newborn infants often produce some milk from their own breast tissue, known colloquially as witch's milk.


In most species, lactation is a sign that the individual has been pregnant at some point in her life, with the exception of humans and goats. [2] [3] Nearly every species of mammal has nipples; except for monotremes, egg-laying mammals, which instead release milk through ducts in the abdomen. In only one species of mammal, the Dayak fruit bat from Southeast Asia, is milk production a normal male function.

Galactopoiesis is the maintenance of milk production. This stage requires prolactin. Oxytocin is critical for the milk let-down reflex in response to suckling. Galactorrhea is milk production unrelated to nursing. It can occur in males and females of many mammal species as result of hormonal imbalances such as hyperprolactinaemia.


The chief function of a lactation is to provide nutrition and immune protection to the young after birth. Due to lactation, the mother-young pair can survive even if food is scarce or too hard for the young to attain, expanding the environmental conditions the species can withstand. The costly investment of energy and resources into milk is outweighed by the benefit to offspring survival. [4] In almost all mammals, lactation induces a period of infertility (in humans, lactational amenorrhea), which serves to provide the optimal birth spacing for survival of the offspring. [5]


Milk secretion from a human breast Lactation.jpg
Milk secretion from a human breast

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:

It is also possible to induce lactation without pregnancy. Protocols for inducing lactation are called the Goldfarb protocols. Using birth control pills to mimic the hormone levels of pregnancy, then discontinuing the birth control, followed by use of a double electric breast pump for 15 minute sessions at regular 2-3 hour intervals (100+ minutes total per day) helps to induce milk production.

Breastfeeding (Correct Latch-On Position) Blausen 0118 Breastfeeding CorrectLatch-On 02.png
Breastfeeding (Correct Latch-On Position)
Breastfeeding a newborn baby Breastfeeding02.jpg
Breastfeeding a newborn baby
Breastfeeding of an older child Zanzibar 31.JPG
Breastfeeding of an older child

Secretory differentiation

During the latter part of pregnancy, the woman's breasts enter into the Secretory Differentiation stage. This is when the breasts make colostrum (see below), a thick, sometimes yellowish fluid. At this stage, high levels of progesterone inhibit most milk production. It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production.

Secretory activation

At birth, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels. This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Secretory Activation.

When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later. The release of prolactin triggers the cells in the alveoli to make milk. Prolactin also transfers to the breast milk. Some research indicates that prolactin in milk is greater at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m. [7]

Other hormonesnotably insulin, thyroxine, and cortisolare 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. [8] Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk. [6]

Autocrine control - Galactopoiesis

The hormonal endocrine control system drives milk production during pregnancy and the first few days after the birth. When the milk supply is more firmly established, autocrine (or local) control system begins.

During this stage, the more that milk is removed from the breasts, the more the breast will produce milk. [9] [10] Research also suggests that draining the breasts more fully also increases the rate of milk production. [11] 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
  • 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 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 [12] and rapidly-adapting [13] 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-aveolar pressure forces milk into the lactiferous sinuses, into the lactiferous ducts (a study found that lactiferous sinuses may not exist. [14] If this is true then milk simply enters the lactiferous ducts), and then out the nipple.


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. [15] [16]

Without pregnancy, induced lactation, relactation

In humans, induced lactation and relactation have been observed frequently in some cultures, and demonstrated with varying success in adoptive mothers. It appears plausible that the possibility of lactation in women (or females of other species) who are not biological mothers does confer an evolutionary advantage, especially in groups with high maternal mortality and tight social bonds. [17] [18] The phenomenon has been also observed in most primates, in some lemurs, and in dwarf mongooses. [19] [20]

Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods. [21] Some couples may stimulate lactation outside of pregnancy for sexual purposes.

Rare accounts of male lactation (as distinct from galactorrhea) exist in historical medical and anthropological literature, although the phenomenon has not been confirmed by more recent literature. [22]

Domperidone is a drug that can induce lactation. [23] [24]


Charles Darwin recognized that mammary glands seemed to have developed specifically from cutaneous glands, and hypothesized that they evolved from glands in brood pouches of fish, where they would provide nourishment for eggs. [1] The latter aspect of his hypothesis has not been confirmed; however, more recently the same mechanism has been postulated for early synapsids. [25]

As all mammals lactate, lactation must have evolved before the last common ancestor of all mammals, which places it at a minimum in the Middle or Late Triassic when monotremes diverged from therians. [26] O. T. Oftedal has argued that therapsids evolved a proto-lacteal fluid in order to keep eggs moist, an adaption necessitated due to diapsids parchment shelled eggs which are more vulnerable to evaporation and dehydration than the mineralized eggs produced by some sauropsids. [25] [27] This protolacteal fluid became a complex, nutrient-rich milk which then allowed a decline in egg size by reducing the dependence on a large yolk in the egg.[20] The evolution of lactation is also believed to have resulted in the more complex dentition seen in mammals, as lactation would have allowed the prolonged development of the jaw before the eruption of teeth. [25]

During early evolution of lactation, the secretion of milk was through pilosebaceous glands on mammary patches, analogous to the areola, and hairs on this patch transported the nourishing fluids to the hatchlings as is seen in monotremes. [28] The development of the nipple occurred in the mammal lineages that diverged after monotremes, metatheria and eutheria. [28]

Occurrence outside Mammalia

Another well known example of nourishing young with secretions of glands is the crop milk of columbiform birds. Like in mammals, this also appears to be directed by prolactin. [29] Other birds such as flamingos and penguins utilize similar feeding techniques. [30]

The discus fish (Symphysodon) is known for (biparentally) feeding their offspring by epidermal mucus secretion. [31] [32] A closer examination reveals that, as in mammals and birds, the secretion of this nourishing fluid may be controlled by prolactin. [33] Similar behavior is seen in at least 30 species of cichlids. [31]

Lactation is also the hallmark of adenotrophic viviparity - a breeding mechanism developed by some insects, most notably tsetse flies. The single egg of the tsetse develops into a larva inside the uterus where it is fed by a milky substance secreted by a milk gland inside the uterus. [34] The cockroach species Diploptera punctata is also known to feed their offspring by milky secretions. [35]

Toxeus magnus , an ant-mimicking jumping spider species of Southeast Asia, also lactates. It nurses its offspring for about 38 days, although they are able to forage on their own after 21 days. Blocking nursing immediately after birth resulted in complete mortality of the offspring, whereas blocking it 20 days after birth resulted in increased foraging and reduced survival. This form of lactation may have evolved from production of trophic eggs. [36]

See also

Related Research Articles

Breast Region of the torso of a primate containing the mammary gland

The breast is one of two prominences located on the upper ventral region of the torso of primates. In females, it serves as the mammary gland, which produces and secretes milk to feed infants. Both females and males develop breasts from the same embryological tissues. At puberty, estrogens, in conjunction with growth hormone, cause breast development in female humans and to a much lesser extent in other primates. Breast development in other primate females generally only occurs with pregnancy.

Nipple Part of the breast

The nipple is a raised region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant. The milk can flow through the nipple passively or it can be ejected by smooth muscle contractions that occur along the ductal system. The nipple is surrounded by the areola, which is often a darker color than the surrounding skin. A nipple is often called a teat when referring to non-humans. Nipple or teat can also be used to describe the flexible mouthpiece of a baby bottle. In humans, nipples of both males and females can be stimulated as part of sexual arousal. In many cultures, human female nipples are sexualized, or "regarded as sex objects and evaluated in terms of their physical characteristics and sexiness."

Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals, usually females, to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing. It is secreted heavily in pulses in between these events. Prolactin plays an essential role in metabolism, regulation of the immune system and pancreatic development.

Mammary gland Exocrine gland in humans and other mammals

A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word mamma, "breast". The mammary glands are arranged in organs such as the breasts in primates, the udder in ruminants, and the dugs of other animals. Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals, lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur. With humans, male lactation can occur only under specific circumstances.

In zoology, male lactation is production of milk (lactation) from a male mammal's mammary glands. It is well-documented in the Dayak fruit bat and the Bismarck masked flying fox. The term male lactation is not used in human medicine. It has been used in popular literature, such as Louise Erdrich's The Antelope Wife, to describe the phenomenon of male galactorrhea, which is a well-documented condition in humans, unrelated to childbirth or nursing. Newborn babies of both sexes can occasionally produce milk; this is called neonatal milk and not considered male lactation.

Breast milk Milk produced by the mammary glands in the breast of a human female

Breast milk or mother's milk is milk produced by mammary glands, located in the breast of a human female. Breast milk is the primary source of nutrition for newborns, containing fat, protein, carbohydrates and variable minerals and vitamins. Breast milk also contains factors that are important for implications protecting the infant against infection and inflammation, whilst also contributing to healthy development of the immune system and gut microbiome.

Breast engorgement Medical condition

Breast engorgement occurs in the mammary glands due to expansion and pressure exerted by the synthesis and storage of breast milk. It is also a main factor in altering the ability of the infant to latch-on. Engorgement changes the shape and curvature of the nipple region by making the breast inflexible, flat, hard, and swollen. The nipples on an engorged breast are flat or inverted. Sometimes it may lead to striae on nipples, mainly a preceding symptom of septation mastitis.


A galactagogue, or galactogogue, is a substance that promotes lactation in humans and other animals. It may be synthetic, plant-derived, or endogenous. They may be used to treat low milk supply.

In mammalian species, pseudopregnancy is a physical state whereby all the signs and symptoms of pregnancy are exhibited, with the exception of the presence of a fetus, creating a false pregnancy. The corpus luteum is responsible for the development of maternal behavior and lactation, which are mediated by the continued production of progesterone by the corpus luteum through some or all of pregnancy. In most species, the corpus luteum is degraded in the absence of a pregnancy. However, in some species, the corpus luteum may persist in the absence of pregnancy and cause "pseudopregnancy", in which the female will exhibit clinical signs of pregnancy.

Lactiferous duct

Lactiferous ducts are those ducts that converge and form a branched system connecting the nipple to the lobules of the mammary gland. When lactogenesis occurs, under the influence of hormones, the milk is moved to the nipple by the action of smooth muscle contractions along the ductal system to the tip of the nipple. They are also referred to as galactophores, galactophorous ducts, mammary ducts, mamillary ducts or milk ducts.

Lactational amenorrhea Post-partum infertility due to breast feeding

Lactational amenorrhea, also called postpartum infertility, is the temporary postnatal infertility that occurs when a woman is amenorrheic and fully breastfeeding.

Erotic lactation Sexual activity involving the stimulation of womans breast

Erotic lactation is sexual arousal by breastfeeding on a woman's breast. Depending on the context, the practice can also be referred to as adult suckling, adult nursing, and adult breastfeeding. Practitioners sometimes refer to themselves as being in an adult nursing relationship (ANR). Two persons in an exclusive relationship can be called a nursing couple.

Breastfeeding difficulties Medical condition

Breastfeeding difficulties refers to problems that arise from breastfeeding, the feeding of an infant or young child with milk from a woman's breasts. Although babies have a sucking reflex that enables them to suck and swallow milk, and human breast milk is usually the best source of nourishment for human infants, there are circumstances under which breastfeeding can be problematic, or even in rare instances, contraindicated.

Breastfeeding Feeding of babies or young children with milk from a womans breast

Breastfeeding, also called nursing, is the process of feeding a mother's breast milk to her infant, either directly from the breast or by expressing the milk from the breast and bottle-feeding it to the infant. The World Health Organization (WHO) recommends that breastfeeding begin within the first hour of a baby's life and continue as often and as much as the baby wants. During the first few weeks of life babies may nurse roughly every two to three hours, and the duration of a feeding is usually ten to fifteen minutes on each breast. Older children feed less often. Mothers may pump milk so that it can be used later when breastfeeding is not possible. Breastfeeding has a number of benefits to both mother and baby, which infant formula lacks.

Lactation suppression refers to the act of suppressing lactation by medication or other non pharmaceutical means. The breasts may become painful when engorged with milk if breastfeeding is ceased abruptly, or if never started. This may occur if a woman never initiates breastfeeding, or if she is weaning from breastfeeding abruptly. Historically women who did not plan to breastfeed were given diethylstilbestrol and other medications after birth to suppress lactation. However, its use was discontinued, and there are no medications currently approved for lactation suppression in the US and the UK. Dopamine agonists are routinely prescribed to women following a stillbirth in the UK under the NHS.

Breast development, also known as mammogenesis, is a complex biological process in primates that takes place throughout a female's life. It occurs across several phases, including prenatal development, puberty, and pregnancy. At menopause, breast development ceases and the breasts atrophy. Breast development results in prominent and developed structures on the chest known as breasts in primates, which serve primarily as mammary glands. The process is mediated by an assortment of hormones, the most important of which include estrogen, progesterone, prolactin, and growth hormone.

Mammary alveolus

A mammary alveolus is a small cavity or sac found in the mammary gland. Mammary alveoli are the site of milk production and storage in the mammary gland. Mammary alveoli cluster into groups called mammary lobules, and each breast may contain 15 to 20 of these lobules. The lobules drain milk through the lactiferous ducts out of the nipples.

Hypothalamic–pituitary–prolactin axis

The hypothalamic–pituitary–prolactin axis, also known as the hypothalamic–pituitary–mammary axis or hypothalamic–pituitary–breast axis, is a hypothalamic–pituitary axis which includes the secretion of prolactin from the lactotrophs of the pituitary gland into the circulation and the subsequent action of prolactin on tissues such as, particularly, the mammary glands or breasts. It is involved in lobuloalveolar maturation of the mammary glands during pregnancy and the induction and maintenance of lactation following parturition. Hormones that control the secretion of prolactin from the pituitary gland include dopamine, estradiol, progesterone, thyrotropin-releasing hormone (TRH), and vasoactive intestinal peptide (VIP).

In breastfeeding women, low milk supply, also known as lactation insufficiency, insufficient milk syndrome, agalactia, agalactorrhea, hypogalactia or hypogalactorrhea, is the production of breast milk in daily volumes that do not fully meet the nutritional needs of her infant.

Breastfeeding and fertility are the effects of breastfeeding on the fertility of postpartum women. Hormones associated with lactation and breastfeeding can variably and unreliably inhibit processes involved with fertility. Because of the high variation of this process, breastfeeding is not recommended to be a method of contraception by medical providers. The physiological importance of this inhibition is so that women who are breastfeeding have a decreased likelihood of a subsequent pregnancy while they are still wanting to breastfeed.


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