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Baby at Dahlful breastfeeding center in manhattan new yorkLactating Breasts

Your breasts go through an amazing transformation when you are pregnant. They go from being primarily fatty tissue to milk making machines. In fact, the lactating breast has about twice as much milk making tissue as fatty tissue. These changes are usually complete by week 22 of your pregnancy. 12 weeks before you deliver, your breasts grow, get heavy, and start making colostrum.

Colostrum is produced in tiny amounts (20 to 40 ml per day) in the first few days after your baby is born. It is very rich in nutrients, which makes it ideal for your baby’s developing digestive system.

By Day 3, your breasts begin making transitional milk. Compared with colostrum, transitional milk is lower in protein and immunoglobulins, and higher in calories, fat, and lactose.

Initially, your milk volume increases regardless of whether or not you are nursing. By Day 3 or 4, your breasts produce around 300 to 400 ml per day. By Day 5, most breasts produce about 500 to 800 ml per day of milk. However, while milk production increases, the amount actually secreted by your breasts varies widely based on several factors. Your stress and pain from nursing can actually decrease your overall milk supply and create postpartum depression.

By Day 10 to 14, your breasts produce mature breast milk. Breast milk is often described as foremilk and hindmilk, but this delineation is misleading. Breasts only make one type of milk, but the fatty component sticks to the walls of the alveoli, while the liquid component travels down the ducts and mixes with any leftover milk from the previous feeding. When your baby begins a feeding, he first encounters the watery foremilk, which has a higher concentration of lactose and less fat. As he continues to nurse, the letdown reflex is triggered, and the fattier hindmilk is squeezed out of your breasts.

Therefore, the composition and amount of milk your baby gets from your breast depends on the how much milk you produce, how long your baby nurses on each breast, and how long you wait between feedings. For example, if you produce a large amount of milk, your baby may fill up foremilk before getting to the hindmilk. This can result in increased gas, colic, and lower weight gain.

Read on for more information about how your lactating breasts work.