Your body produces colostrum from roughly midway through pregnancy until about two to five days after birth, when it gradually transitions to mature milk. That means colostrum is present for several months before delivery, but the window your newborn actually receives it is short, typically the first two to four days of life.
When Colostrum Production Starts
Your breasts begin making colostrum during the second trimester, usually around weeks 16 to 20 of pregnancy. This is the first phase of milk production, driven by rising levels of prolactin and other hormones. Some people notice small amounts of thick, yellowish fluid leaking from their nipples in the final weeks of pregnancy, while others never see any leaking at all. Both are completely normal. Whether or not you can express colostrum before birth has no bearing on how much you’ll produce afterward.
The First Days After Birth
Colostrum is the only milk your body produces for roughly the first two to four days postpartum. The volumes are tiny but precisely matched to a newborn’s stomach. Your baby needs an average of about 1 to 2 ounces (37 to 56 mL) during the entire first 24 hours, and roughly twice that on the second day. That works out to just a few teaspoons per feeding, which often surprises new parents but is exactly the right amount.
Colostrum is thick, concentrated, and often golden or deep yellow. It’s packed with immune compounds, especially a protective antibody called secretory IgA, which is present at about four times the concentration found in mature breast milk (roughly 5.4 g/L versus 1.3 g/L). Protein content is also nearly double that of later milk, around 14 to 15 g/L compared to 7 to 8 g/L in mature milk. Fat, on the other hand, is lower in colostrum (20 to 30 g/L) than in mature milk (around 40 g/L). This composition makes colostrum less about calories and more about coating your baby’s digestive tract with a layer of immune protection.
The Shift to Transitional and Mature Milk
The transition away from colostrum is triggered by the sharp drop in progesterone that follows delivery of the placenta. As long as prolactin and cortisol levels are adequate, this hormonal shift activates the second phase of milk production. You’ll feel this as your milk “coming in,” a noticeable increase in breast fullness, and the milk itself becoming thinner, whiter, and higher in volume.
For most people, this shift happens between 48 and 72 hours postpartum. The milk you produce over the next week or two is called transitional milk, a blend that gradually moves from the high-protein, high-antibody profile of colostrum toward the fattier, higher-calorie composition of mature milk. By about two weeks postpartum, your milk is considered fully mature.
Why the Transition Sometimes Takes Longer
Several factors can delay when your milk transitions past the colostrum stage, pushing it beyond the typical 72-hour mark. Research has identified the most common risk factors:
- Unscheduled cesarean delivery can delay the hormonal cascade because the body’s labor signals are interrupted.
- Prolonged second stage of labor during vaginal delivery is also associated with a later transition.
- Higher body weight before or during pregnancy correlates with delayed onset of mature milk.
- Insulin-dependent diabetes affects the hormonal balance needed to trigger the shift.
- Infrequent nursing in the first days is one of the strongest modifiable risk factors. Babies who are fed formula exclusively before milk comes in are more likely to experience a delayed transition, because breast stimulation from suckling directly supports the hormonal signaling that moves production forward.
If your milk hasn’t noticeably increased by about 72 hours, frequent nursing or pumping is the single most effective thing you can do to encourage the transition. A delayed shift doesn’t mean your supply will be permanently low. It simply means your baby may rely on colostrum for a day or two longer than average.
Colostrum for Preterm Babies
Mothers who deliver prematurely still produce colostrum, and it’s especially valuable. Preterm colostrum tends to be higher in fat (about 2.8 g/100 mL versus 2.1 g/100 mL in term milk during the first week) and slightly higher in energy density. Protein starts high and decreases over the first four months, dropping from roughly 2.2 g/100 mL down to 1.5 g/100 mL. These differences reflect the body’s response to a baby who needs more concentrated nutrition in smaller volumes, since premature infants can only tolerate tiny feedings.
Even when a preterm baby can’t yet nurse directly, expressing colostrum by hand or pump within the first hours after birth helps establish supply and provides drops of colostrum that can be given by syringe or swab to the baby’s mouth.
What This Means in Practice
The colostrum phase is brief but biologically intense. The World Health Organization recommends initiating breastfeeding within the first hour of birth, largely so the baby receives colostrum as early as possible. Those first feedings deliver a concentrated dose of antibodies that help protect against infection during the most vulnerable period of a newborn’s life.
If you’re pregnant and wondering whether you’re producing colostrum yet, you almost certainly are by the third trimester, even if you can’t see it. And if you’re postpartum and waiting for your milk to come in, the small amounts of colostrum you’re producing are doing exactly what your baby needs right now. The volumes feel impossibly small, but they’re designed to match a stomach that’s roughly the size of a cherry on day one and a walnut by day three.