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July 21, 2024

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The Science of Breast Milk

Your body is an amazing mechanism with many wonders ­— its ability to produce breast milk to nourish a newborn is just one of them. Learn what makes it so special.

Nothing gives you more satisfaction than the sight of your newborn dozing off with a full belly after a good breastfeeding session. There’s that feeling of knowing your body can provide such nourishment for someone so tiny. But, what exactly is breast milk? What makes it so good for Baby? You’ve heard over and over again that “breast is best,” but what exactly makes it that way? Let’s take it step by step.


Your breast milk begins developing while you’re pregnant, and continues evolving from the moment you deliver your baby. Your milk will change from colostrum to transitional milk to mature milk. Experts reveal the makeup of each stage and when those changes in your milk occur.


You may hear doctors, nurses and lactation consultants tell you how important it is for newborns to get the colostrum — that’s the first stage of your breast milk.

Pregnant women first produce colostrum around 16 weeks of pregnancy. According to international certified lactation consultant Jerilyn Boles with Saint Thomas Midtown, it’s high in protein, has less sugar and much less fat than mature breast milk. It primarily protects Baby’s digestive tract to prevent adherence of pathogens and has a laxative effect.

According to Holly Pendegraft, R.N. and certified breastfeeding specialist with The Breast Choice Lactation Services, colostrum typically lasts up to six days after delivery and is made up of vitamins, immunoglobulins and minerals.

“The immunoglobulins protect Baby from bacteria and viruses in the early days,” Pendegraft says.

Interesting fact: Baby doesn’t have to drink a lot of colostrum to benefit from all the good matter.

Colostrum is dense, almost gel-like, and is generally yellow in color, although it can be different colors. No need for alarm if your colostrum is clear or another shade of yellow.


As your milk “comes in,” it thins to transitional milk as the colostrum diminishes and mature milk develops. This is the second stage of your breast milk.
Interesting fact: Boles says the delivery of the placenta triggers this stage.

Transitional milk’s composition varies from mother to mother, but it’s mostly made of water, which is also a temperature-regulating mechanism for your newborn. Boles says this is because 25 percent of Baby’s heat loss is from evaporation of water from his lungs and skin.

“Transitional milk contains more calories than colostrum to help ‘pack on the pounds’ in the first few weeks of life,” says Pendegraft.

Interesting fact: Boles says your diet doesn’t usually change the fat content of your breast milk. However, the type of fats you eat can alter the type of fat in your milk!


The final stage is mature milk, which develops around seven to 10 days post partum until two weeks post partum. It’s maintenance is by supply and demand. “As long as baby nurses, your body should continue to supply enough milk for him,” says Boles.

Mature milk is thin and resembles skim milk. When you store a bottle of breast milk, you can see the separation of the two phases of mature milk — foremilk and hindmilk. The thin creamy layer— hindmilk — at the top is the high-fat milk.

The composition of your breast milk varies with the stage of lactation, maternal nutrition, individual variation and, interestingly enough, the time of day. But, the changes don’t stop there.

“At the beginning of the feed, the foremilk is mostly water used to hydrate Baby,” says Boles. “It then changes to hindmilk, which is higher in fat for Baby’s brain and body needs,” she adds.

Ten percent of mature milk is also made up of carbohydrates, fats and proteins to help with your infant’s growth, development and energy levels.

“More specifically, mature milk contains enzymes, amino acids, antibodies, sugars and hormones,” adds Pendegraft.

Boles says the fat content of the milk changes while you breastfeed, too, and increases at the end of the feeding.

Interesting fact: The fat content of your breast milk rises from early morning to midday, and the volume increases from two to five times as much.

Your breast milk can also change flavor, color and thickness. Pendegraft says it has to do with the nutrient components from the foods you eat.

“Nutrients for milk production come from the mom’s bloodstream, so whatever nutrients she has eaten is what the milk for the next feeding is made of,” adds Pendegraft. She says the flavor also varies. Some flavors, such as vanilla and garlic, can reproduce a stronger taste in breast milk, while other foods or flavors may change the color of the milk but not the flavor.


Did you know your breast milk goes through yet another change when Baby’s sick?

“Research has found that babies backwash when breastfeeding and through that, the mom’s body picks up on the pathogens in the saliva and produces milk to fight the pathogens with illness-specific antibodies along with the immunological makeup of the milk changing,” says Pendegraft.

Keep that in mind when Baby picks up a cold or other virus. Breastfeed as much as possible to allow him to get better faster.


Don’t pump and dump for no reason! If your body produces more than enough milk for your Baby, store it for him. You may need it on days where he’s fed via bottle by someone else. When in doubt, freeze it.

The Centers for Disease Control and Prevention (CDC) says breast milk kept at room temperature lasts up to four hours from time of expression, but only one to two hours if you thaw it from a frozen state.

Store freshly expressed/pumped breast milk in the refrigerator for up to four days; or freeze for six to 12 months. However, the CDC strongly advises against refreezing breast milk once it has been thawed.

Interesting fact: Freezing breast milk destroys some of the leukocytes or white blood cells of the milk, adds Boles.

Some moms bake brownies or cookies using their excess breast milk. However, Pendegraft says that if you heat breast milk higher than 106ºF, it begins to knock off nutrients. She also says if you have excess milk, instead of baking it into something, you can opt to donate it to your local hospital through a milk bank like Mothers’ Milk Bank of Tennessee.

Now that you know the science of breast milk, it’s time to nurse Baby again. Go on, keep that milk coming in!

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