Colostrum is the "early milk" your breasts start producing in your third trimester of pregnancy, and then the first milk your newborn receives. Different from your "mature milk" that comes in at around Day 3 postpartum, colostrum is a thick, yellowish substance that is more dense and yields a much lower volume.
But don't let colostrum's short life span fool you into thinking it's not important. It packs a higher amount of protein than mature milk, meaning a little goes a long way in terms of filling your newborn's tiny tummy. Colostrum is full of antibodies, so in addition to being nutritionally ideal for your baby it also boosts her immune system.
When your mature milk comes in it will be thinner, whiter and much higher in volume. However, the disease-fighting properties of human milk don't disappear with your colostrum. As long as your baby is getting breast milk, she's also being protected against many different types of bacteria and viruses.
In the first few days of life, your baby will only need teaspoon amounts of colostrum to fill her up. Because of colostrum's viscous nature—it's much thicker than mature milk—it can sometimes be hard to pump, since it often sticks to the pump parts and doesn't collect as easily as mature milk does. It's a good idea to breastfeed your newborn at least 8 to 12 times (or more) each 24-hour period so she'll get the full benefits of colostrum. Frequent feeding will also help stimulate the production of your mature milk supply.
Source: Heather Kelly is an International Board Certified Lactation Consultant (IBCLC)