The hCG is critical for the maintenance of the corpus luteum which is now able to continue to produce oestrogen and progesterone. These two hormones are necessary for the continuation of pregnancy during the first three months of foetal life. After three months the corpus luteum's role is superseded by the placenta. The placenta is able to provide at least four hormones necessary for pregnancy to continue. The levels of this hormone increase through pregnancy and appear to be necessary for the development of the breasts.
Progesterone is critical for pregnancy to continue since it maintains the endometrium.
The levels of progesterone reach a peak after about 3-4 weeks following fertilisation and then decline before increasing again up to the time of parturition. The latter rises are important for the development of the breasts. There are several oestrogen hormones secreted by the placental. Like progesterone, oestrogen is necessary for the uterus to develop to accommodate the growing foetus and for the latter stages in the development of the breasts. The mechanism by which labour is initiated is very poorly understood in humans.