During pregnancy, a rise in hormones such as progesterone and prolactin stimulates the growth of breast tissue, and the breasts increase by an average of one to two cup sizes. This outward growth is one indicator that the breasts are undergoing the inward growth necessary to support lactation, including the transition of mammary glands (modified sweat glands) into secretory cells, extensive branching of the milk ducts, and enlargement of the Montgomery glands (sebaceous glands in the areola surrounding the nipple).^[4]

By 16 to 20 weeks of pregnancy, the breasts are fully ready to make milk. This marks the first stage of lactation: lactogenesis I. The end result is the formation of colostrum (“first milk”), the secretion and production of which is held in check by high levels of circulating progesterone.^[18] Colostrum is rich in immunological components (especially the antibody immunoglobulin A) and developmental factors (like epidermal growth factor). It serves as the infant’s food for the first few days of life.^[19] It also has a laxative effect in infants, helping the newborn expel meconium (the first stool).

After childbirth and expulsion of the placenta, progesterone drops and prolactin rises, cueing the second stage of milk production: lactogenesis II. This stage is known for copious milk production and begins by the fourth day postpartum with the production of around 500 mL of breast milk (called transitional milk). If lactogenesis II does not occur by four days postpartum, it is considered “delayed”. Approximately 20% to 30% of lactating parents have delayed lactogenesis II, and although the causes vary (e.g., obesity, primiparous parent/first-time parent, stress during delivery), the experience alone is associated with cessation of breastfeeding by four weeks postpartum.^[20] Appropriate clinical support of these parents is necessary.

The milk produced during lactogenesis II is different from colostrum. It contains less sodium and chloride and more lactose as the closing of tight junctions in the mammary epithelial cells alters what passes into the milk.^[18] While the milk production is still under endocrine control, frequent breastfeeding is important if the parent intends to continue breastfeeding. At around ten days postpartum, there is a switch from endocrine control to autocrine (or local) control of milk production. This ushers in the maintenance stage: lactogenesis III.

During lactogenesis III, the primary driver for increased milk supply is increased demand via milk removal. The breasts will make milk indefinitely as long as the milk is removed and there are no underlying lactation issues. (This explains how wet nurses maintained their occupation in lieu of repeated personal pregnancies.) Consequently, the role of systemic hormones in milk production and infant feeding is much smaller but not absent. For example, the hormone oxytocin is released from the posterior pituitary gland and is involved in the milk letdown reflex, and the hormone prolactin is released from the anterior pituitary gland and is involved in milk production. Stimulation of the nerve endings in the nipple by infant suckling or breast pumps is what triggers the release of these two hormones. ^[4]

By the end of the first month, the milk is considered fully mature and offers the infant approximately 0.9 to 1.2 grams of protein/dL, 3.2 to 3.6 grams of fat/dL, and 6.7 to 7.8 grams of lactose/dL.^[21] The exact macronutrient ratio does vary, especially when comparing milk from mothers of preterm infants to milk from mothers of term infants, and it also changes with time.^[21] When the mother begins to wean her infant and the local signals for milk production decrease, involution begins. Involution is the process by which the milk-producing cells are removed through apoptosis (programmed cell death) and replaced with fat cells. It may take up to 12 months for all lactating cells to be replaced.^[19]

Human milk is recommended as the optimal source of nutrition for infants by multiple medical organizations.^[22] Human milk contains far more than a combination of macronutrients and micronutrients; it contains living cells, growth factors, and immunoreactive substances, most of which can withstand an infant’s digestive enzymes.^[23][21] Breastfed infants have a reduced risk of gastrointestinal and respiratory diseases,^[24] as well as a lower prevalence of dental malocclusions (crooked teeth).^[25] Breastfeeding may even reduce the infant’s risk of developing chronic conditions like type 1 diabetes^[26] and obesity^[27], although data on these long-term outcomes are subject to certain limitations (e.g., confounding). In addition, the skin-to-skin contact accompanying breastfeeding may contribute positively to parent-infant bonding and infant emotional resilience.^[28]

Lactation also benefits the lactating parent, including a reduced risk of type 2 diabetes,^[29] cardiovascular disease,^[30] and breast, ovarian, and endometrial cancers.^[31][32]

During pregnancy, a rise in hormones such as progesterone and prolactin stimulates the growth of breast tissue, and the breasts increase by an average of one to two cup sizes. This outward growth is one indicator that the breasts are undergoing the inward growth necessary to support lactation, including the transition of mammary glands (modified sweat glands) into secretory cells, extensive branching of the milk ducts, and enlargement of the Montgomery glands (sebaceous glands in the areola surrounding the nipple).^[59]

By 16 to 20 weeks of pregnancy, the breasts are fully ready to make milk. This marks the first stage of lactation: lactogenesis I. The end result is the formation of colostrum (“first milk”), the secretion and production of which is held in check by high levels of circulating progesterone.^[18] Colostrum is rich in immunological components (especially the antibody immunoglobulin A) and developmental factors (like epidermal growth factor). It serves as the infant’s food for the first few days of life.^[19] It also has a laxative effect in infants, helping the newborn expel meconium (the first stool).

After childbirth and expulsion of the placenta, progesterone drops and prolactin rises, cueing the second stage of milk production: lactogenesis II. This stage is known for copious milk production and begins by the fourth day postpartum with the production of around 500 mL of breast milk (called transitional milk). If lactogenesis II does not occur by four days postpartum, it is considered “delayed”. Approximately 20% to 30% of lactating parents have delayed lactogenesis II, and although the causes vary (e.g., obesity, primiparous parent/first-time parent, stress during delivery), the experience alone is associated with cessation of breastfeeding by four weeks postpartum.^[20] Appropriate clinical support of these parents is necessary.

The milk produced during lactogenesis II is different from colostrum. It contains less sodium and chloride and more lactose as the closing of tight junctions in the mammary epithelial cells alters what passes into the milk.^[18] While the milk production is still under endocrine control, frequent breastfeeding is important if the parent intends to continue breastfeeding. At around ten days postpartum, there is a switch from endocrine control to autocrine (or local) control of milk production. This ushers in the maintenance stage: lactogenesis III.

During lactogenesis III, the primary driver for increased milk supply is increased demand via milk removal. The breasts will make milk indefinitely as long as the milk is removed and there are no underlying lactation issues. (This explains how wet nurses maintained their occupation in lieu of repeated personal pregnancies.) Consequently, the role of systemic hormones in milk production and infant feeding is much smaller but not absent. For example, the hormone oxytocin is released from the posterior pituitary gland and is involved in the milk letdown reflex, and the hormone prolactin is released from the anterior pituitary gland and is involved in milk production. Stimulation of the nerve endings in the nipple by infant suckling or breast pumps is what triggers the release of these two hormones. ^[59]

By the end of the first month, the milk is considered fully mature and offers the infant approximately 0.9 to 1.2 grams of protein/dL, 3.2 to 3.6 grams of fat/dL, and 6.7 to 7.8 grams of lactose/dL.^[21] The exact macronutrient ratio does vary, especially when comparing milk from mothers of preterm infants to milk from mothers of term infants, and it also changes with time.^[21] When the mother begins to wean her infant and the local signals for milk production decrease, involution begins. Involution is the process by which the milk-producing cells are removed through apoptosis (programmed cell death) and replaced with fat cells. It may take up to 12 months for all lactating cells to be replaced.^[19]

Providing food for one growing infant increases the lactating parent’s energy demands by approximately 30% compared to nonpregnant, nonlactating levels.^[33] The American Centers for Disease Control and Prevention (CDC) recommends that breastfeeding mothers consume an additional 450 to 500 calories per baby per day.

Keep in mind that this recommendation, despite being a good general rule, overestimates or underestimates the true metabolic requirements in about one-third of lactating parents.^[34] Determining the exact caloric requirements for each lactating parent simply isn’t feasible because it would necessitate careful calculations of the amount of milk produced, the energy density of the milk, and the energetic efficiency of milk synthesis, all of which are generally out of reach for the average parent. Following general recommendations and tweaking them as needed based on personal assessments alongside professional guidance is an acceptable method to determine caloric intake for most lactating parents.

Yes, to an extent. Although eating enough calories is important for milk production, “enough” does not necessarily equate to greater than the lactating parent’s daily caloric expenditure. A lactating parent can lose weight while still fully nourishing their baby with breast milk.^[3] In fact, creating a caloric deficit of 500 kcals/day alongside regular exercise is recommended for lactating parents who are overweight,^[35] but severe energy restriction is not advisable. Eating fewer than 1,500 kcal per day may significantly decrease milk supply.^[36][37]

Unlike the list of foods to avoid during pregnancy, the only food lactating parents are advised to avoid is fish with high concentrations of mercury (e.g., shark, swordfish, tilefish),^[38] unless the infant has a metabolic disease requiring special dietary restrictions, or the parent notices certain trends between their dietary intake and their infant's behaviors/digestion/temperament.

Avoiding high-mercury fish does not imply avoiding fish all together. The FDA recommends 8 to 12 ounces per week of fish and/or shellfish low in mercury (e.g., anchovies, salmon, sardines, hake, clams) for lactating parents.

Breastfeeding parents can drink coffee, tea, and other caffeine-containing beverages in moderation. Caffeine does appear in breastmilk soon after the parent has consumed it, and babies don’t metabolize caffeine as quickly as adults; caffeine accumulation in the infant can lead to symptoms such as wakefulness, restlessness, and irritability.^[39] Although there’s not enough high-quality research to support an evidence-based recommendation, a general limit of 300–500 mg of caffeine daily from all sources (including sodas and supplements like guarana, as well as coffee and tea) has been suggested for the lactating parent, with the caveat that this limit should be lowered for people who are breastfeeding preterm infants and newborns.^[40] Finally, it’s worth noting that high caffeine consumption may reduce iron in breastmilk. One study of Costa Rican mothers found that those who consumed more than 450 mg/day of caffeine had less iron in their breastmilk at 1 month postpartum than those who consumed less than 450 mg/day, so if iron deficiency is a concern for the infant, lactating parents may wish to discuss moderating caffeine intake with their healthcare provider (keeping in mind that even in the absence of caffeine intake, breastmilk is low in iron; full-term infants’ iron stores supply the majority of their iron needs during the first 6 months of life).^[40][41]

Certain foods and dietary patterns may alter the taste, color, and fatty acid profile of human milk without modifying the overall concentration of macronutrients and micronutrients. For example, parental consumption of strong spices (e.g., garlic and ginger) and certain vegetables may flavor human milk.^[42] Additionally, there is some evidence that fish consumption increases the concentration of omega-3 fatty acids in human milk, which are vital to infant brain and eye development, but whether this translates to improvements in outcomes for healthy full-term infants is less clear.^[43][44][45]

Yes, but lactating parents following a vegetarian or vegan diet should carefully monitor their diet to ensure adequate intake of essential macronutrients and micronutrients. It is not uncommon for vegetarians or vegans to underconsume complete proteins and omega−3 fatty acids, and many also experience deficiencies in iron, calcium, zinc, and/or vitamin B12. Because these nutrients are important for infant growth and development, lactating parents who are following a vegetarian or vegan diet should consider supplementation for any deficiencies that are present.^[46] Working with a healthcare provider to construct a balanced, plant-based diet is also recommended.

A strict ketogenic diet is usually not recommended for lactating parents. There are a few case reports of nondiabetic, lactating women on a ketogenic diet who developed severe and dangerous ketoacidosis.^[47][48][49]

There are limited data on the consumption of artificial (nonnutritive) sweeteners while lactating. In one study, breast milk samples taken from 20 lactating volunteers (19 of whom consumed 0–2 diet beverages per day, and one who consumed 7 per day) revealed that 65% of milk samples contained saccharin, sucralose, and acesulfame-potassium. Aspartame was not detected.^[50]

The implications of these findings are unknown. There is speculation that they may affect an infant’s gut microbiota and/or adversely affect an infant’s future cardiometabolic health, but there are no studies in humans to support this claim.^[51]

Maybe! If consuming a balanced, nutritious diet on a regular basis is a challenge, a multivitamin may be worth considering, but it certainly isn’t a requirement. Ultimately, it is a personal decision. Some data suggest that multivitamin use in women with no underlying nutritional deficiencies does offer potential benefits with few safety concerns. In one trial, women who took a postnatal supplement containing docosahexaenoic acid (DHA), 12 vitamins, five minerals, and lutein expressed breast milk with higher levels of DHA and other selected nutrients than those who took a placebo.^[52]

Yes, if breastfeeding is postponed until the alcohol has cleared from the lactating parent’s system. Alcohol passes freely into breast milk, but it does not get “trapped” in breast milk. Instead, the alcohol content of breast milk closely matches the level present in the lactating parent’s bloodstream, meaning that as it leaves the blood, it leaves the breast milk.

The time at which breast milk becomes alcohol free depends, in part, on how much alcohol the parent has consumed and their body weight. Generally, parents should abstain from breastfeeding for at least two hours after a single serving of alcohol. This negates the need to “pump and dump”, i.e., express milk and throw it away.^[53][54]

Parents should ensure their infant is being safely cared for if they decide to become intoxicated.

The primary driver of breast milk production after 10 days postpartum is the removal of milk from the breast tissue, meaning that the more milk is removed, the more the body will produce. A number of complex mechanisms orchestrate this localized ebb and flow of milk, but one key player is a whey protein in breast milk called feedback inhibitor of lactation (FIL). If the milk is not removed, the build-up of FIL in the breast triggers a negative feedback loop, inhibiting additional milk production. Therefore, it’s important to use feeding techniques that favor regular milk removal, such as the following:

• Ensure proper latch.
• Feed frequently and on demand.
• Empty breasts “fully” (i.e., until breasts feel softer and/or hand-expressing more milk is difficult) at each feeding.
• Offer both breasts each time.
• Alternate breasts if the baby is drowsy or is not suckling well. Otherwise, offer the first breast until the baby slows down and stops, then offer the second breast as well.
• Avoid giving bottles of formula or cereal in the first 6 months of life unless advised to do so by your healthcare provider, because the baby will drink less milk.
• Eat and drink enough to support lactation.^[15][55]

People with breastfeeding difficulties should work with a lactation consultant to address maternal or infant factors that may be affecting feeding.

Breastfeeding is contraindicated for parents who are HIV-positive, who have human T-cell lymphotropic virus type I or type II, who have or may have Ebola, and who are using illicit street drugs.^[56] It is also contraindicated for infants with galactosemia, a rare genetic disorder that makes the infant unable to correctly metabolize the sugars in milk, which can be detected by newborn screening programs. Infants with galactosemia must be fed with lactose-free formula exclusively.^[57]

The following should temporarily suspend breastfeeding — and should not feed their infants expressed milk — until cleared by a healthcare provider:

• Lactating parents with untreated brucellosis
• Lactating parents with an active herpes simplex virus (HSV) infection with sores on the breast
• Lactating parents undergoing diagnostic imaging with radiopharmaceuticals
• Lactating parents taking medications that could be harmful to the breastfeeding infant (see above).^[56]

To maintain milk supply, parents who intend to continue breastfeeding once they are cleared by a healthcare provider should continue lactation but should not feed the expressed milk to the baby (i.e., “pump and dump”).

Lactating parents with an active chickenpox infection and/or untreated active tuberculosis can feed their infants expressed breastmilk, but should not feed them directly from the breast.^[58]