Breastfeeding, also known as nursing, is the feeding of babies and young children with milk from a woman's breast. Health professionals recommend 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. Deaths of an estimated 820,000 children under the age of five could be prevented globally every year with increased breastfeeding. Breastfeeding decreases the risk of respiratory tract infections and diarrhea, both in developing and developed countries. Other benefits include lower risks of asthma, food allergies, type 1 diabetes, and leukemia. Breastfeeding may also improve cognitive development and decrease the risk of obesity in adulthood. Mothers may feel pressure to breastfeed, but in the developed world children generally grow up normally when bottle fed. Benefits for the mother include less blood loss following delivery, better uterus shrinkage, and decreased postpartum depression. Breastfeeding delays the return of menstruation and fertility, a phenomenon known as lactational amenorrhea. Long term benefits for the mother include decreased risk of breast cancer, cardiovascular disease, and rheumatoid arthritis. Breastfeeding is less expensive than infant formula. Health organizations, including the World Health Organization (WHO), recommend breastfeeding exclusively for six months. This means that no other foods or drinks other than possibly vitamin D are typically given. After the introduction of foods at six months of age, recommendations include continued breastfeeding until one to two years of age or more. Globally about 38% of infants are only breastfed during their first six months of life. In the United States in 2015, 83% of women begin breastfeeding and 58% were still breastfeeding at 6 months, although only 25% exclusively. Medical conditions that do not allow breastfeeding are rare. Mothers who take certain recreational drugs and medications should not breastfeed. Smoking, limited amounts of alcohol, or coffee are not reasons to avoid breastfeeding. Changes early in pregnancy prepare the breast for lactation. Before pregnancy the breast is largely composed of adipose (fat) tissue but under the influence of the hormones estrogen, progesterone, prolactin, and other hormones, the breasts prepare for production of milk for the baby. There is an increase in blood flow to the breasts. Pigmentation of the nipples and areola also increases. Size increases as well, but breast size is not related to the amount of milk that the mother will be able to produce after the baby is born. By the second trimester of pregnancy colostrum, a thick yellowish fluid, begins to be produced in the alveoli and continues to be produced for the first few days after birth until the milk 'comes in', around 30 to 40 hours after delivery. There is no evidence to support increased fluid intake for breastfeeding mothers to increase their milk production.Oxytocin contracts the smooth muscle of the uterus during birth and following delivery, called the postpartum period, while breastfeeding. Oxytocin also contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly produced milk into the duct system. Oxytocin is necessary for the milk ejection reflex, or let-down, in response to suckling, to occur. Not all of breast milk's properties are understood, but its nutrient content is relatively consistent. Breast milk is made from nutrients in the mother's bloodstream and bodily stores. It has an optimal balance of fat, sugar, water, and protein that is needed for a baby's growth and development. Breastfeeding triggers biochemical reactions which allows for the enzymes, hormones, growth factors and immunologic substances to effectively defend against infectious diseases for the infant. The breast milk also has long-chain polyunsaturated fatty acids which help with normal retinal and neural development. The composition of breast milk changes depending on how long the baby nurses at each session, as well as on the child's age. The first type, produced during the first days after childbirth, is called colostrum. Colostrum is easy to digest although it is more concentrated than mature milk. It has a laxative effect that helps the infant to pass early stools, aiding in the excretion of excess bilirubin, which helps to prevent jaundice. It also helps to seal the infants gastrointestional tract from foreign substances, which may sensitize the baby to foods that the mother has eaten. Although the baby has received some antibodies through the placenta, colostrum contains a substance which is new to the newborn, secretory immunoglobulin A (IgA). IgA works to attack germs in the mucous membranes of the throat, lungs, and intestines, which are most likely to come under attack from germs. Breasts begin producing mature milk around the third or fourth day after birth.Early in a nursing session, the breasts produce foremilk, a thinner milk containing many proteins and vitamins. If the baby keeps nursing, then hindmilk is produced. Hindmilk has a creamier color and texture because it contains more fat.