The female body goes through many physical changes during pregnancy. Included in these are changes to her breasts and nipples, to enable her to produce milk for her baby after the birth. If you want to gain an understanding of how the baby physically 'milks the breast' to feed, you can read
breastfeeding.
The breasts
The nipples
The milk
The breasts
When girls reach puberty, their breasts develop and enlarge under the influence of the hormone oestrogen. When a woman becomes pregnant, her breasts start to undergo further changes from about 4 to 6 weeks after the last menstrual period (or 2 to 4 weeks after conception), with the body producing greatly increased levels of progesterone and oestrogen. A woman's body cannot generally create mature breast milk until she has experienced at least 16 to 24 weeks of a pregnancy (this length of time varying between individual women).
The breasts of non-pregnant women are made up of two major structures. The smallest part is the underdeveloped glandular tissue, which is designed to grow and mature during pregnancy to eventually produce milk. The largest part is the supporting tissue that surrounds the glandular tissue. This incorporates smooth muscle, connective tissue, blood vessels, nerves,
lymphlymph vessels and fat, covered by a layer of skin. Most of the breast tissue (that people associate with a woman having 'large' or small' breasts) comprises of the supporting tissues that surrounds the glandular tissue. This is why the 'size' of a woman's breasts has nothing to do with her ability to feed her baby.
The glandular tissue made up of thousands of tiny sacs (called 'alveoli') arranged in 15 to 20 bunches in each breast (called 'lobules'). These lobules start to grow and enlarge from about 4 to 6 weeks of pregnancy, often making the breasts feel somewhat lumpy. From about 16 to 24 weeks of the pregnancy, the alveoli sacs start to become lined with special milk-producing cells (called 'acinar' cells).