Progesterone is
a. An enzyme for digesting proteins
b. A hormone that initiates uterine contraction
c. An amino acids which causes alkaptonuria
d. Hormones concerned with maintaining pregnancy

Hint: Progesterone belongs to a category called progestogens, which are steroid hormones. During the second half of the menstrual cycle, it is principally secreted by the corpus luteum in the ovary. It plays important roles in the menstrual cycle.

Complete answer:
The remains of the ovarian follicle that surrounds the developing egg form a structure called the corpus luteum during the menstrual cycle, when an egg is released from the ovary at ovulation. This releases oestradiol and, to a lesser degree, progesterone. In the case that the released egg is fertilised, the progesterone prepares the body for pregnancy. The corpus luteum breaks down if the egg is not fertilised, progesterone development falls and a new menstrual cycle starts.

Progesterone promotes the development of blood vessels that supply the lining of the womb and promotes the endometrial glands to secrete nutrients that nourish the early embryo if the egg is fertilised. Progesterone then prepares the uterine tissue lining to allow implantation of the fertilised egg and helps to maintain the endometrium during pregnancy.

Progesterone is still released by the corpus luteum during the early stages of pregnancy and is important for promoting pregnancy and establishing the placenta. Progesterone plays an significant role in the development of the foetus during pregnancy, promoting the growth of the maternal breast, avoiding lactation, and strengthening the muscles of the pelvic wall in preparation for labour. The amount of progesterone in the body increases gradually during pregnancy before labour occurs and the baby is born.

Hence, the correct answer is option (D).

Note: Throughout pregnancy, the amount of progesterone in the body gradually increases before childbirth happens and the baby is born. While the corpus luteum in the ovaries is the main site of human progesterone development, the ovaries themselves, the adrenal glands and, during pregnancy, the placenta also contain progesterone in smaller amounts.