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Breastfeeding is the practise of a human mother feeding a baby (and sometimes a young toddler, see below) with milk produced from her mammary glands[?], usually directly from the nipples. Babies have a sucking instinct allowing them to extract the milk. (Alternatively a baby can be fed special industrial milk, "infant formula".)
Throughout pregnancy a woman's body produces hormones which stimulate the growth of the milk duct system in the breasts: progesterone, estrogen, follicle stimulating hormone[?] (FSH), luteinizing hormone (LH), prolactin, oxytocin, and human placental lactogen[?] (HPL). By the fifth or sixth month of pregnancy, the breasts are sufficiently developed to produce milk. Near the time of birth, the breasts may begin to secrete a thick, yellowish fluid called colostrum[?] (or "beestings"), which is the first milk the infant receives. It contains important antibodies from the mother's body providing immunological protection. Colostrum has no fat and little sugar - these substances appear three to four days after birth when the suckling action of the infant further stimulates the breast to produce mature breast milk. Thereafter the breast produces milk on a basis of supply and demand in response to how often a child feeds and how much milk he or she consumes. The production, secretion and ejection of milk is called lactation.
The exact integrated properties of breast milk are unknown, but the nutrient content after this period is relatively consistent and draws its ingredients from the mother's food supply. If that supply is found lacking, content is obtained from the mother's bodily stores. The exact composition of breast milk varies from day to day, depending on food consumption, meaning that the ratio of water to fat fluctuates. Foremilk[?], the milk released at the beginning of a feeding, is low in fat and high in carbohydrates relative to the hindmilk[?] which is released as the milk supplies are drawn down. The breast can never be truly "emptied" since milk production is a continuous biologic process.
Breast milk consumption has been linked to a decreased risk for several infant conditions including Sudden Infant Death Syndrome (SIDS). The sucking technique required of the infant encourages the proper development of both the teeth and other speech organs.
Numerous health benefits of breastfeeding have been medically documented.. According to the American Academy of Pediatrics[?]' policy statement on breastfeeding and the use of human milk, "Extensive research, especially in recent years, documents diverse and compelling advantages to infants, mothers, families, and society from breastfeeding and the use of human milk for infant feeding. These include health, nutritional, immunologic, developmental, psychological, social, economic, and environmental benefits." (http://www.aap.org/policy/re9729)
Breastfeeding has also been shown to be beneficial to the mother. The act of breastfeeding releases hormones which have been found to both relax the mother and cause her to experience nurturing feelings toward her infant. Breastfeeding as soon as possible after giving birth increases levels of oxytocin which encourages a more rapid contraction of the uterus and in turn decreases postpartum bleeding. Breastfeeding can also allow the mother to return to her pre-pregnant weight as the fat stores accumulated during pregnancy are utilized in milk production. Frequent and exclusive breastfeeding delays the return of menstruation and fertility (this is known as lactational amenorrhea) allowing for improved iron stores and the possibility of natural child spacing. Breastfeeding mothers experience improved bone re-mineralization postpartum, and a reduced risk for both ovarian and pre-menopausal breast cancer.
The American Academy of Pediatrics (AAP) provides the following recommendation, "Exclusive breastfeeding is ideal nutrition and sufficient to support optimal growth and development for approximately the first 6 months after birth. [...] It is recommended that breastfeeding continue for at least 12 months, and thereafter for as long as mutually desired." (http://www.aap.org/policy/re9729)
The World Health Organization (WHO) states that "a recent review of evidence has shown that, on a population basis, exclusive breastfeeding for 6 months is the optimal way of feeding infants. Thereafter infants should receive complementary foods with continued breastfeeding up to 2 years of age or beyond."  (http://www.who.int/child-adolescent-health/NUTRITION/infant_exclusive.htm)
There are certain women who, for one reason or another, may be unable to breastfeed. These reasons include damage to the mammary glands[?] through previous surgery, breast infection or breast abscess, cancer (especially of the breast), previous radiation treatment[?], or inadequate milk supply (which is unknown among healthy, adequately nourished women). Many women with previous surgeries, abscesses and cancer can breastfeed successfully.
Breastfeeding to the infant can be harmful if the mother:
Excessive caffeine consumption by the mother can cause irritability, sleeplessness, nervousness and increased feeding in the breastfed infant. Moderate use (one to two cups per day) usually produces no effect. Breastfeeding mothers are advised to avoid or restrict caffeine intake.
Breastfeeding mothers must use caution if they regularly consume nicotine through tobacco smoking. In addition to reducing the milk supply, heavy use of cigarettes (more than 20 per day) has been shown to cause vomiting, diarrhea, rapid heart rate, and restlessness in infants. In general, though, nicotine in breast milk is not easily absorbed into the infant's intestinal tract[?] and is quickly metabolized. Research is ongoing to determine whether the benefits of breastfeeding out-weigh the potential harm of nicotine in breast milk.
Heavy alcohol consumption is known to be harmful to the infant, but there is no consensus on how much alcohol may be safely consumed. It is generally agreed that small amounts of alcohol may be occasionally consumed by a breastfeeding mother. Levels of alcohol in breast milk peak 30 to 90 minutes after one drink of moderate alcoholic content. Considering the known dangers of alcohol exposure to the developing fetus, many medical professionals believe it is prefereable to err on the side of caution with alcohol exposure to a baby and have breastfeeding women restrict their alcoholic intake.
The recreational use of marijuana in conjunction with breastfeeding is a controversial issue. The AAP Committee on Drugs lists marijuana in their table of Drugs of Abuse for Which Adverse Effects on the Infant During Breastfeeding Have Been Reported yet they reference only one study in the literature and this study reports no effect.  (http://www.aap.org/policy/0063) There is a lack of research on the effects of marijuana on the breastfed infant.
In the few cases where it is not possible for a mother to breastfeed, breast milk may be expressed and fed to an infant by bottle, milk may be offered from another breastfeeding woman, or if necessary, infant formula can be offered. While consider inferior to breast-feeding, infant formula has in recent times been marketed in developing countries as being a superior feeding-solution
Although not widely known in developed countries, women who have never been pregnant are able to lactate and therefore breastfeed as well. If their nipples are stimulated in a breastfeeding manner for a while (such as a breast pump or an actual baby suckling), eventually the breasts will begin to produce milk which can be used to feed a baby. For this reason, adoptive mothers, usually initially in conjuction with some form of supplementation, are able to breastfeed their infants. (There is also anecdotal evidence for male lactation[?]  (http://freebirth.com/milkmen.htm)).