Complex sentence: During pregnancy, the breasts become enlarged and denser (firmer) because of the prolactin-caused organ hypertrophy, which begins the production of breast milk, increases the size of the nipples, and darkens the skin color of the nipple-areola complex.
Split into simpler sentences: During pregnancy, the breasts become enlarged and denser (firmer) because of the prolactin-caused organ hypertrophy. This begins the production of breast milk, increases the size of the nipples, and darkens the skin color of the nipple-areola complex.