Breastfeeding

• ANATOMY OR SYSTEM AFFECTED: Breasts, glands, reproductive system, digestive system, immune system
• DEFINITION: The preferred feeding method for infants, providing optimal nutrition for the infant (including immunologic protection), mother-infant bonding, and enhanced maternal health.

Process and Effects

The terms breastfeeding, nursing, and lactation all refer to the recommended method of infant feeding. The American Academy of Pediatrics and the World Health Organization recommend exclusive breastfeeding for the first six months of an infant's life with continued breastfeeding following the introduction of solid foods up to at least one year of age and beyond. Although there are a few exceptions, most mothers can breastfeed and thereby provide low-cost nutritional support for her infant.

Although it is often thought otherwise, the size of the mother’s breast has no relationship to successful lactation. In fact, the physiology of successful lactation is determined by the maturation of breast tissue, the initiation and maintenance of milk secretion, and the ejection or delivery of milk to the nipple. This physiology is dependent on hormonal control, and most women have the required anatomy for successful lactation unless they have had surgical alteration of the breast. For women who are unable or prefer not to exclusively breastfeed, commercial infant formulas are a suitable replacement for breast milk when prepared correctly.

Hormonal influence on breast development begins in adolescence. Increased estrogen causes the breast ducts to elongate and duct cells to grow. (The ducts are narrow tubular vessels that run from the segments of the breast into the tip of the nipple.) More fibrous and fatty tissue develops, and the nipple area matures. As adolescence progresses, regular menstrual cycle hormones cause further development of the alveoli, which are the milk-producing cells.

The elevated levels of estrogen present during pregnancy promote the growth and branching of milk ducts, while the increase in progesterone promotes the development of alveoli. Throughout pregnancy and especially during the first three months, many more milk ducts are formed. Clusters of milk-producing cells also begin to enlarge, while at the same time, placental hormones promote breast development.

Shortly before labor and delivery, the hormone prolactin is produced by the pituitary gland. Prolactin, which is necessary for starting lactation and sustaining milk production, reaches its peak at delivery. Another hormone, oxytocin, which is also produced by the pituitary, stimulates the breast to eject milk. This reaction is called the letdown reflex, which causes the milk-producing alveoli to contract and force milk to the front of the breast. Oxytocin serves an important function after delivery by causing the uterus to contract to its prepregnancy size. Initially, the letdown reflex occurs only when the infant suckles, but later, it may be initiated simply by the baby’s cry. An efficient letdown reflex is critical to successful breastfeeding. Emotional upset, fatigue, pain, nervousness, or embarrassment about lactation can interrupt this reflex; these psychological factors, rather than breast size or physiology, are predictive of successful lactation in most cases.

Breastfeeding is a natural response to childbirth, and the nutrient content of breast milk is ideal for the human infant. More than one hundred constituents of breast milk, both nutritive and nonnutritive, are known. Although the basic nutrient content is a solution of protein, sugar, and salts in which fat is suspended, those concentrations vary depending on the period of lactation and even within a given feeding.

Colostrum, often called first milk, is produced in the first few days after birth. It is lower in fat and calories (kilocalories) and higher in protein and certain minerals than mature breast milk. Colostrum is opaque and yellow because it contains a high concentration of vitamin A–like substances called carotenes. It also has a high concentration of antibodies and white blood cells, which pass on immunologic protection to the infant.

Within a few days after birth, the transition is made from colostrum to mature milk. There are two types of mature milk. Foremilk is released first as the infant begins to suckle. It has a watery, bluish appearance and is low in fat and rich in other nutrients. This milk accounts for about one-third of the baby’s intake. As the nursing session progresses, the draught reflex helps move the hindmilk, with its higher fat content, to the front of the breast. It is important that the nutrient content of breast milk be determined from a sample of both types of milk to make an adequate assessment of all nutrients present.

Breast milk best meets the infant’s needs and is the standard by which infant formulas are judged. Several nutrient characteristics make it the ideal infant food. Lactose, the carbohydrate content of breast milk, is the same simple sugar found in any milk, but the protein content of breast milk is uniquely tailored to meet infant needs. An infant’s immature kidneys are better able to maintain water balance because breast milk is lower in protein than cow’s milk. Most breast milk protein is alpha-lactalbumin, whereas cow’s milk protein is casein. Alpha-lactalbumin is easier to digest and provides two sulphur-containing amino acids that are the building blocks of proteins required for infant growth.

The fat (lipid) content of breast milk differs among women and may even vary day to day in milk from the same woman. The types of fatty acids that make up most of the fat component of milk may vary in response to maternal diet. Mothers fed a diet containing corn and cottonseed oil produce a milk with more polyunsaturated fatty acids, which are the predominant fatty acids in those oils. Breast milk is higher in the essential fatty acid called linoleic acid than cow’s milk, and it also contains omega-3 fatty acids. About 55 percent of human milk calories come from fat, compared to about 49 percent of calories found in infant formulas. In addition, enzymes in breast milk help digest fat in the infant’s stomach. This digested fat is more efficiently absorbed than the products that result from digesting cow’s milk or infant formula.

Breast milk contains more cholesterol than cow’s milk, which seems to stimulate development of the enzymes necessary for degrading cholesterol, perhaps offering protection against atherosclerosis in later life. Cholesterol is also needed for proper development of the central nervous system.

The vitamin and mineral content of breast milk from healthy mothers supplies all that is needed for growth and health except for vitamin D and fluoride, and these are easily supplemented. Breast milk and the infant’s intestinal bacteria also supply all the necessary vitamin K, but since no bacteria are present at birth, an injection of vitamin K should be given to prevent deficiencies.

Breast milk mineral content is balanced to promote growth while protecting the infant’s immature kidneys. Breast milk has a low sodium content, which helps the immature kidneys to maintain water balance. No type of milk is a good source of iron. Although breast milk contains relatively small amounts of iron, about 50 percent of this iron can be absorbed by the body, compared to only 4 percent from cow’s milk. This phenomenon is called bioavailability. Because of the high bioavailability of breast milk iron, the introduction of solids, which are given to replace depleted iron stores, can be delayed until six months of age in most breastfed infants; this delay may help to reduce the incidence of allergies in susceptible infants. There is also evidence that zinc is better absorbed from breast milk.

The vitamin content of milk can vary and is influenced by maternal vitamin status. The water-soluble vitamin content of breast milk (the B vitamins and vitamin C) will change more because of the maternal diet than the fat-soluble vitamin content (vitamins A, E, and K). If women have diets that are deficient in vitamins, their levels in breast milk will be lower. Yet, even malnourished mothers can breastfeed, although the quantity of milk is often less. As the maternal diet improves, the level of water-soluble vitamins in the milk increases. There is a level, however, above which additional diet supplements will not increase the vitamin content of breast milk.

There are many nonnutritive advantages to breastfeeding. A major advantage is the immunologic protection and resistance factors that it provides to the infant. Bifidus factors, found in both colostrum and mature milk, favor the growth of helpful bacteria in the infant’s digestive tract. These bacteria, in turn, offer protection against harmful organisms. Lactoferrin, another resistance factor, binds iron so that harmful bacteria cannot use it. Lysozyme, lipases, and lactoperoxidases also offer protection against harmful bacteria.

Immunoglobulins are present in large amounts in colostrum and in significant amounts in breast milk. These protein compounds act as antibodies against foreign substances in the body called antigens. Generally, the resistance passed to the infant is from environmental antigens to which the mother had been exposed. The concentration of antibodies in colostrum is highest in the first hour after birth. Secretory IgA is the major immunoglobulin that provides protection against gastrointestinal organisms. Breast milk also contains interferon, an antiviral substance that is produced by special white blood cells in milk. Protection against allergies is another advantage of breastfeeding. It is not known, however, whether less exposure to the antigens found in formula or some substance in the breast milk itself provides this protection. Normally, a mucous barrier in the intestine prevents the absorption of whole proteins, the root of an allergic reaction. In the newborn, this barrier is not fully developed to allow whole immunologic proteins to be absorbed. The possibility that whole food proteins will be absorbed as well is greater if cow’s milk or early solids are given, and this absorption increases the potential for allergic reactions.

Other possible benefits of breastfeeding are protection against the intestinal disorders Crohn’s disease and celiac sprue. The reasons for this protection are not clear. Other health benefits of breastfeeding in children include a reduced risk for infectious diseases, including diarrhea and respiratory disease, and a reduced risk of obesity and type 2 diabetes later in life. Furthermore, because the taste of breast milk varies with the mother's diet, some studies have suggested that children who were breastfed as infants are less picky eaters and more willing to try new foods than children who were exclusively formula-fed.

Breastfeeding also encourages infant bonding, a process in which the mother and baby form an attachment. It is a matter of controversy whether breastfeeding mothers bond more easily than bottle-feeding mothers. If a mother has early and prolonged skin-to-skin contact with her baby, however, the mother is more likely to breastfeed and to nurse her baby for more months.

Milk from mothers delivering preterm infants is higher in protein and nonprotein nitrogen, calcium, IgA, sodium, potassium, chloride, phosphorus, and magnesium. It also has a different fat composition and is lower in lactose than mature milk of mothers delivering after a normal term. These concentrations support the rapid growth of a preterm infant.

Maternal Benefits

Breastfeeding is not only good for the baby but also good for the mother. There is an association between reduced breast cancer rates and breastfeeding, although the reason is not known. In addition, the hormonal influences caused by suckling the infant help to contract the uterus, returning it to prepregnancy size and controlling postpartum blood loss. Breastfeeding also helps to reduce the mother’s weight. Calories required to make milk are drawn from the fat stores that were deposited during pregnancy. Nevertheless, breastfeeding should be viewed not as a quick weight loss program but as a healthy, natural weight loss process. Other maternal benefits of breastfeeding include a reduced risk of ovarian cancer, type 2 diabetes, and postpartum depression.

If a woman breastfeeds exclusively, which means that no supplements or solid foods are given, until the baby is six months of age, often she will not menstruate. Many women find this lack of menstrual periods convenient while not realizing the physiological benefit of restoring the iron stores that were depleted during pregnancy and delivery. An important advantage to breastfeeding in developing countries is that it can help to space pregnancies naturally, as exclusive breastfeeding in the first six months after birth is associated with a lower (1 to 2 percent) likelihood of pregnancy. Most infant malnutrition occurs when the second child is born, because breastfeeding is stopped for the first child. The first child is weaned to foods that often do not supply enough nutrients. By spacing pregnancies out, the first child has a chance to nurse longer.

Breastfeeding can be very convenient and does not require time to mix and prepare formula or sterilize bottles. However, exclusive breastfeeding can also be time intensive, particularly in the first three months after birth, and places a lot of responsibility solely on the mother. Fresh breast milk is always sterile and at the proper temperature. The money needed for the extra food required to produce breast milk is much less than that required to purchase commercial formula. This can be a major benefit for women with low incomes and is critically important for the health of those babies born in developing countries.

Complications and Disorders

Some special problems or circumstances can make breastfeeding difficult. The breasts may become engorged—so full of milk that they are hard and sore—making it difficult for the baby to latch onto the nipple. Gentle massaging of the breasts, especially with warm water or a heating pad, will allow release of the milk and reduce pain in the breast. Using a breast pump to encourage milk expression is also helpful to reduce breast engorgement. This situation is common during the first few weeks of nursing but will occasionally recur if a feeding is missed or a schedule changes.

Sometimes a duct will become plugged and form a hard lump. Massaging the lump and continuing to nurse will remedy the situation. If influenza-like symptoms accompany a plugged duct, the cause may be a breast infection known as mastitis. Since the infection is in the tissue around the milk-producing glands, the milk itself is safe. A short course of antibiotics may be needed to control the infection. The mother should also apply heat, get plenty of rest, maintain hydration, and keep emptying the breast at least every six hours. Stopping nursing would plug the duct further, making the infection worse.

There are a few instances in which a woman cannot or should not breastfeed her infant. Women who have had certain surgeries to their breasts, including mastectomy, breast augmentation, and breast reduction surgeries, may not be able to express breast milk. Babies with a rare genetic disorder called galactosemia cannot nurse, since they lack the enzyme to metabolize milk sugar. Phenylketonuria (PKU), another genetic disorder, requires close monitoring of the infant’s blood phenylalanine level, but the infant can often be totally or at least partially breastfed.

Breastfeeding is contraindicated for women with malaria, active tuberculosis, or a chronic disease that results in maternal malnutrition. In 2024, the American Academy of Pediatrics announced that breastfeeding was no longer contraindicated for women with the human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency syndrome (AIDS), as research showed that the risk of HIV transmission through breast milk was very low. HIV-positive mothers, however, must actively use antiretroviral therapy and maintain an undetectable viral load to be considered safe to breastfeed their infants. The presence of other conditions, from diabetes to the common cold, do not require a woman to discontinue breastfeeding.

Of concern to many mothers are reports of contaminants in breast milk. Drugs, environmental pollutants, certain viruses, caffeine, alcohol, and food allergens can be passed to the infant through breast milk. Drug transmission depends on the drug's administration method, which influences the speed with which it reaches the blood supply to the breast. Whether that drug can remain functional after it is subjected to acid in the baby’s digestive tract varies. Women who are taking prescription or over-the-counter medications should consult with their doctors to determine whether they are safe to take while breastfeeding. Large amounts of caffeine in breast milk can produce a wakeful, hyperactive infant, but this situation is corrected when the mother curtails her caffeine consumption; moderate caffeine intake has a negligible effect on breastfed infants. Large amounts of alcohol in breast milk can cause sleep disturbances in infants. Women should consult with their doctors about a safe level of alcohol consumption when breastfeeding. In general, breastfeeding is contraindicated for women suffering from alcoholism or other forms of substance abuse.

Nicotine also enters milk, but the impact of secondhand smoke may pose more of a health threat than the nicotine content of breast milk. Mothers who smoke are encouraged to quit; however, although nicotine may be present in breast milk, adverse effects to the breastfed infant have not yet been documented. The American Academy of Pediatrics encourages breastfeeding mothers to consider smoking cessation, but it does not indicate that mothers who smoke cannot breastfeed, as the benefits of breastfeeding seem to greatly outweigh the risks. Similarly, researchers continue to investigate the effects of a mother's marijuana use on breastfeeding, especially as evidence suggests marijuana has become more commonly used among pregnant and lactating women. One study published in 2018 found that THC, the principal mind-altering ingredient in marijuana, can pass into breast milk and potentially disrupt a child's brain development or cause other harm, though evidence remained inconclusive. By 2022, the US Substance Abuse and Mental Health Services Administration reported that marijuana was the most widely used drug during pregnancy, but noted this drug puts babies and mothers at risk and should be avoided while pregnant and breastfeeding.

Also of concern is the presence of contaminants that cannot be avoided, such as pesticide residues, industrial waste, or other environmental contaminants. Polychlorinated biphenyls (PCBs) and the pesticide DDT have received the most attention; however, both DDT and PCBs were banned in the United States in 1972 and 1979, respectively. Many countries worldwide have also banned the agricultural and commercial use of these substances. Nevertheless, PCBs may be present in products that were produced before the ban. Long-term exposure to contaminants promotes their accumulation in the mother’s body fat, and the production of breast milk is one way to rid the body of these contaminants. Concentrations present in breast milk vary. Ordinarily, these substances are in such small quantities that they pose no health risk. Women who have consumed large amounts of fish from PCB-contaminated waters or have had occupational exposure to this chemical, however, should consider having their breast milk tested. It is also possible for these substances to enter the infant’s food supply from other sources.

Perspective and Prospects

Although breastfeeding is the best method of infant feeding, many women choose not to breastfeed. Before the eighteenth century, human milk was the only source for infant feeding. If a mother did not breastfeed, another woman called a wet nurse fed her baby. At the end of the nineteenth century, formula feeding became popular when bottles were developed and water sanitation improved. In the United States, the percentage of infants who started out breastfeeding declined to only about 20 percent by 1970 but increased to more than 83.2 percent by 2015, according to the US Centers for Disease Control and Prevention (CDC). Of infants born in the United States in 2019, 55.8 percent were breastfed until they were at least six months of age, according to the CDC's 2022 Breastfeeding Report Card. The breastfeeding rate at the age of twelve months was 35.9 percent.

Breastfeeding used to be more prevalent among more-educated, higher-income mothers. Increased employment of women outside the home, however, dramatically altered trends in breastfeeding. Although mothers may opt to breastfeed in the hospital, many quit because they are returning to work and believe that it would be too difficult to continue. A working mother typically needs four to six weeks at home to establish successful breastfeeding. Mothers who are unable to maintain a regular breastfeeding schedule because of work or other obligations often use pumps to collect milk that can then be stored, thus allowing infants to benefit from nutritious breast milk even when their mothers are absent and enabling women to return to their normal schedules. Many health insurance plans will cover the cost of breast pumps and related supplies.

Formula use increased in developing countries throughout the mid-twentieth century. Because formula is very expensive, it is often diluted with more water than the packaging indicates and, therefore, does not provide enough nutritional support to the infant and can disrupt the infant's electrolyte balance. The quality of water is often poor, which causes the infant to be exposed to disease-causing organisms. In addition, formula-fed infants do not receive the immunologic protection of breast milk. The result is a higher infant mortality rate.

One difficulty that some breastfeeding mothers face is backlash to breastfeeding in public spaces. Although all US states and several European countries have enacted laws that allow women to breastfeed in any public location, many other countries have no laws that address breastfeeding in public. Many breastfeeding mothers report being asked to leave restaurants and stores or to cover up when nursing their infants in public. Critics argue that breastfeeding in public is inappropriate, while breastfeeding advocates argue that discouraging women from nursing their hungry babies while in public amounts to child abuse. Others have argued that many people's discomfort with public breastfeeding stems from a widespread cultural fetishization of women's breasts that ignores the most basic biological purpose of breasts—lactation and nursing.

By the early 2020s, all US states had enacted laws that allow women to breastfeed in any public or private location, while thirty-one states also had laws that exempt breastfeeding women from public indecency laws and thirty states had laws related to breastfeeding or pumping in the workplace. Several states, including California, Illinois, Minnesota, and Missouri, as well as Puerto Rico, have implemented breastfeeding awareness campaigns to increase the public's understanding of the health benefits of breastfeeding. The 2010 Patient Protection and Affordable Care Act (ACA), a US federal law, included a provision that amended the 1938 Fair Labor Standards Act to require that employers give nursing employees a reasonable amount of break time to express milk for up to one year after the birth of their child; this break time is not required to be paid. The ACA also requires employers to provide a place other than a bathroom for nursing employees to express milk; this requirement is waived for employers with fewer than fifty employees if it causes undue hardship. In recognition of the many health benefits of breastfeeding, the ACA also requires new private health insurance plans to provide coverage of women's preventative health services, including breastfeeding support, breast pumps, and lactation consultations.

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