Your body needs a specific combination of hormones, extra calories, fluids, and key nutrients to produce breast milk. The process starts with hormonal shifts during pregnancy and ramps up after delivery, but sustaining milk production day after day depends on regular breast emptying, adequate energy intake, and staying well hydrated.
Two Hormones Drive the Process
Milk production hinges on two hormones working in sequence: prolactin and oxytocin. Prolactin is the hormone that actually makes milk. During pregnancy, high levels of estrogen and progesterone from the placenta keep prolactin in check. Once you deliver the placenta, those pregnancy hormones drop sharply and prolactin takes charge, signaling tiny grape-like clusters in your breast tissue called alveoli to begin producing milk.
Oxytocin handles delivery. When your baby suckles (or you use a breast pump), nerve signals travel from the nipple to your brain, triggering the release of both prolactin and oxytocin. Oxytocin causes small muscles around the alveoli to contract, squeezing milk through the ducts and out of the nipple. This is the “let-down” reflex, and it typically kicks in about 30 seconds after suckling begins. So each feeding session does double duty: it pushes out the current milk and tells your body to make more.
How Your Body Regulates Supply
Breast milk production works on a supply-and-demand system, and the mechanism is surprisingly local. Your milk contains a small protein called the feedback inhibitor of lactation, or FIL. When milk sits in the breast for a long time without being removed, FIL accumulates and signals the milk-making cells to slow down. When you empty the breast frequently, FIL is cleared and production speeds up.
This is why frequent feeding or pumping is the single most important factor in building and maintaining supply. It’s also why each breast can regulate independently. If one side gets emptied more often, that side will produce more milk. The system is reversible and concentration-dependent, meaning it responds in real time to how much milk is being removed throughout the day.
How Milk Changes in the First Two Weeks
Your body doesn’t jump straight to full milk production. The process unfolds in three phases. During late pregnancy and the first couple of days after birth, your breasts produce colostrum, a thick, concentrated first milk rich in antibodies. Between about day 2 and day 14, transitional milk gradually replaces colostrum. During this stage, your breasts may feel noticeably fuller and warmer, and the milk shifts to a thinner, bluish-white color. By 10 to 15 days after birth, you’re producing mature milk, which will remain the standard composition for as long as you continue breastfeeding.
Extra Calories and Where They Go
Making milk is metabolically expensive. The CDC recommends an additional 330 to 400 calories per day for breastfeeding mothers, compared to what you were eating before pregnancy. That’s roughly equivalent to a substantial snack or a small extra meal.
Your resting metabolic rate does increase slightly during lactation, meaning your body burns a bit more energy even at rest. But the increase is modest and proportional to body weight. The bigger energy demand comes from the milk itself. Breast milk contains fat, protein, sugar, and dozens of other compounds, all of which your body has to synthesize from the food you eat or pull from your own reserves. If calorie intake falls too low, your body can still produce milk for a while by tapping into stored fat, but this isn’t sustainable long-term and can leave you depleted.
Hydration Makes a Real Difference
Breast milk is roughly 87% water, so your fluid needs go up substantially. The Academy of Nutrition and Dietetics recommends about 16 cups of total water per day for nursing mothers. That sounds like a lot, but it includes water from food and other beverages, not just glasses of plain water. The extra fluid compensates for the water that goes directly into milk production. A practical approach: drink a glass of water each time you sit down to nurse, and keep a water bottle within reach throughout the day.
Nutrients Your Body Pulls From Its Own Stores
Your body prioritizes milk production, sometimes at your expense. Calcium is the clearest example. During breastfeeding, your body pulls calcium from your bones to ensure the milk has enough for your baby’s developing skeleton. This happens regardless of how much calcium you consume. According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases, getting more than the recommended amount of calcium from food or supplements does not prevent this bone loss. The good news: these decreases in bone density are temporary and a normal part of lactation. Bone density typically rebounds after weaning.
Other nutrients in breast milk are more directly tied to what you eat day to day. Vitamins that dissolve in water and fat, like B vitamins and vitamin A, fluctuate based on your dietary intake. If your diet is low in these nutrients, the concentration in your milk can drop. Eating a varied diet with plenty of fruits, vegetables, whole grains, and protein sources covers most of these bases without special planning.
Vitamin D Requires Special Attention
One nutrient that diet alone rarely covers is vitamin D. Breast milk contains very little vitamin D regardless of what a mother eats, which is why the American Academy of Pediatrics recommends that breastfed and partially breastfed infants receive 400 IU of vitamin D daily, starting in the first few days of life. This is typically given as infant drops. Some families prefer the option of the mother taking a high-dose supplement instead, which can raise milk vitamin D levels enough to cover the baby’s needs, though this approach warrants a conversation with a healthcare provider about the right dosage.
Putting It All Together
Breast milk production isn’t driven by any single factor. It’s an interplay of hormonal signals that begin at delivery, a local feedback loop that responds to how often milk is removed, and a steady supply of energy and nutrients from your diet. The hormonal machinery handles itself automatically. What you can directly influence is the demand side (frequent feeding or pumping), your calorie intake (an extra 330 to 400 calories daily), your fluid intake (about 16 cups a day), and the overall quality of your diet. Your body is remarkably good at making milk even under imperfect conditions, but giving it the raw materials it needs makes the process more sustainable for both you and your baby.