During pregnancy, the hormone levels change drastically. Shortly after the uterine implantation of the fertilised egg, the body begins to produce the hormone human chorionic gonadotropin (hCG). This has a pregnancy-sustaining effect by stimulating the production of progesterone. Furthermore, the HCG level reaches its peak towards the end of the first trimester. In early pregnancy, an increase in oestrogens causes the breasts to enlarge and prepares them for breastfeeding in the coming months.