Final answer:
The structure responsible for the production of progesterone to maintain pregnancy until the placenta forms is the corpus luteum.
Step-by-step explanation:
The structure responsible for the production of progesterone to maintain the pregnancy until the placenta forms is the corpus luteum. The corpus luteum develops from an ovarian follicle after ovulation and secretes progesterone and estrogen during pregnancy. Progesterone plays a crucial role in inhibiting the production of certain hormones and maintaining the endometrium, protecting the pregnancy.