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 sexually mature female mammals, although it may predate mammals. [1] The process of feeding milk in all female creatures is called nursing, and in humans it is also called breastfeeding . 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 female has been pregnant at some point in her life, although in humans and goats, it can happen without pregnancy. [2] [3] Nearly every species of mammal has teats; 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]
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 through combinations of birth control pills, galactagogues, and milk expression using a breast pump.
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.
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 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. [8] Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk. [6]
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:
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.
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-alveolar 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]
In humans, induced lactation and relactation have been observed frequently in some cultures, and demonstrated with varying success in adoptive mothers and wet nurses. [17] [18] 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. [19] [20] The phenomenon has been also observed in most primates, in some lemurs, and in dwarf mongooses. [21] [22]
Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods. [23] Several protocols for inducing lactation were developed by Dr. Jack Newman and Lenore Goldfarb and are commonly called the Newman-Goldfarb protocols. The "regular protocol" involves the use of birth control pills to mimic the hormone levels of pregnancy with domperidone to stimulate milk production, followed by discontinuing the birth control and the introducing use of a double electric breast pump to induce milk production. [24] Additional protocols exist to support an accelerated timeline and to support induced lactation in menopausal parents.
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. [25] Most recently a subject of transgender health care, multiple case reports have described transgender women successfully inducing lactation. [26] [27] Research has indicated that such breast milk is nutritionally comparable to both the milk of naturally lactating and induced lactating women. [28]
Domperidone is a drug that can induce lactation. [29] [30]
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. [31]
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. [32] O. T. Oftedal has argued that therapsids evolved a proto-lacteal fluid in order to keep eggs moist, an adaptation necessitated due to synapsids’ parchment shelled eggs which are more vulnerable to evaporation and dehydration than the mineralized eggs produced by some sauropsids. [31] [33] 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. [31]
Oftedal also proposed that the protolacteal fluid was initially secreted through pilosebaceous glands on mammary patches, analogous to the areola, and that hairs on this patch transported the fluid to the hatchlings as is seen in monotremes. This would have occurred in the mammal lineages that diverged after monotremes, metatheria and eutheria. In this scenario, some genes and signaling pathways involved in lactation evolved from ancient precursors which facilitated secretions from spiny structures, which themselves evolved from odontodes. [34]
Another well known example of nourishing young with secretions of glands is the crop milk of columbiform birds. As in mammals, this also appears to be directed by prolactin. [35] Other birds such as flamingos and penguins utilize similar feeding techniques. [36]
The discus fish (Symphysodon) is known for (biparentally) feeding their offspring by epidermal mucus secretion. [37] [38] A closer examination reveals that, as in mammals and birds, the secretion of this nourishing fluid may be controlled by prolactin. [39] Similar behavior is seen in at least 30 species of cichlids. [37]
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. [40] The cockroach species Diploptera punctata is also known to feed their offspring by milky secretions. [41]
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. [42]
The breast is one of two prominences located on the upper ventral region of the torso among humans and other primates. Both sexes develop breasts from the same embryological tissues. The relative size and development of the breasts is a major secondary sex distinction between males and females.
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 breastfeed an infant. The milk can flow through the nipple passively or it can be ejected by smooth muscle contractions that occur along with the ductal system. Male mammals also have nipples but without the same level of function, and often surrounded by body hair.
Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals 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.
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.
A prolactin cell is a cell in the anterior pituitary which produces prolactin in response to hormonal signals including dopamine, thyrotropin-releasing hormone and estrogen, which are stimulatory. Prolactin is responsible for actions needed for body homeostasis, the development of breasts, and for lactation. The inhibitory effects of dopamine override the stimulatory effects of TRH in non-pregnant, non-lactating sexually mature females. Depending on the sex of the individual, prolactin cells account for 20% - 50% of all cells in the anterior pituitary gland. The inhibitory effects of dopamine override the stimulatory effects of TRH in non-pregnant, non-lactating sexually mature females. Other regulators include oxytocin and progesterone.
Areolar glands, also known as glandulae areolares, Montgomery glands, and tubercula areolae, are 10-15 elevations found on the areola. They are usually arranged in a circle around the nipple, and can be particularly visible when the nipple is erect. Their role is to promote adequate breastfeeding of the infant.
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.
Witch's milk or neonatal milk is milk secreted from the breasts of some newborn human infants of either sex. Production of neonatal milk by infants usually resolves itself and does not require treatment unless it is caused by an underlying condition or medications. It is thought to be caused by the exposure to an elevated level of estrogen to infants during pregnancy or decreased exposure of estrogen to infants after birth. Its production also may be caused by certain medications. The composition of neonatal milk is similar to maternal milk for most of their components except for fats and one type of antibody.
A galactagogue, or galactogogue, also known as a lactation inducer or milk booster, is a substance that promotes lactation in humans and other animals. It may be synthetic, plant-derived, or endogenous. They may be used to induce lactation and to treat low milk supply.
Lactiferous ducts are 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, also called postpartum infertility, is the temporary postnatal infertility that occurs when a woman is amenorrheic and fully breastfeeding.
Erotic lactation is sexual arousal by sucking on a female or male 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, variously known as chestfeeding or nursing, is the process where breast milk is fed to a child. Breast milk may be from the breast, or may be pumped and fed to the infant. The World Health Organization (WHO) recommend that breastfeeding begin within the first hour of a baby's birth and continue as the baby wants. Health organizations, including the WHO, recommend breastfeeding exclusively for six months. This means that no other foods or drinks, other than vitamin D, are typically given. The WHO recommends exclusive breastfeeding for the first 6 months of life, followed by continued breastfeeding with appropriate complementary foods for up to 2 years and beyond. Of the 135 million babies born every year, only 42% are breastfed within the first hour of life, only 38% of mothers practice exclusive breastfeeding during the first six months, and 58% of mothers continue breastfeeding up to the age of two years and beyond.
Breast development, also known as mammogenesis, is a complex biological process in primates that takes place throughout a female's life.
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.
Delayed onset of lactation (DOL) describes the absence of copious milk secretion (onset of lactation) within the first 72 hours following childbirth. It affects around 20–40% of lactating women, the prevalence differs among distinct populations.
Nipple pain is a common symptom of pain at the nipple that occurs in women during breastfeeding after childbirth. The pain shows the highest intensity during the third to the seventh day postpartum and becomes most severe on the third day postpartum.
Establishment of breastfeeding refers to the initiation of providing breast milk of mother to baby. According to the World Health Organization(WHO), breastfeeding is the best way to provide nourishment, including essential nutrients, energy and antibodies, to infants and toddlers. The start of breastfeeding is supported by the milk production which depends on the development of internal and external breast structure and hormonal control on milk secretion. Besides milk supply, adopting the correct approach of breastfeeding helps build up the maternal bond, which in turn promotes breastfeeding. Not only does nursing strengthen the mother-child relationship, but it also improves the intelligence and immunity of breastfed children and diminishes breastfeeding mothers' risks to have ovarian and breast cancer.
Neohormones are a group of recently evolved hormones primarily associated to the success of mammalian development. These hormones are specific to mammals and are not found in other vertebrates—this is because neohormones are evolved to enhance specific mammalian functions. In males, neohormones play important roles in regulating testicular descent and preparing the sperm for internal fertilisation. In females, neohormones are essential for regulating early pregnancy, mammary gland development lactation, and viviparity. Neohormones superimpose their actions on the hypothalamic-pituitary-gonadal axis and are not associated with other core bodily functions.
Hormones during pregnancy are the result of an intricate interaction between hormones generated by different glands and organs. The primary hormones involved comprise human chorionic gonadotropin (hCG), progesterone, estrogen, human placental lactogen (hPL), and oxytocin. Hormones are synthesized in certain organs, including the ovaries, placenta, and pituitary gland. These hormones have essential functions in pregnancy test, maintaining the uterine lining, fetal development, preventing premature labor, and the initiation and support of labor.