In the third trimester, pregnant women often start producing colostrum, a thick, milky, yellow liquid that is the precursor to breast milk, explains La Leche League. Colostrum slowly changes into mature breast milk. The change becomes noticeable three to four days after giving birth if the mother chooses to breastfeed.
Colostrum is thicker than mature breast milk and appears in much smaller quantities. Regular breastfeeding after birth stimulates the transition to mature breast milk and increases the volume of milk produced. Mothers who breastfeed develop a much larger milk supply of thin, whitish breast milk in the two weeks after the baby's birth, La Leche League says.