Final answer:
As parturition nears, the levels of progesterone decrease and the levels of estrogen increase.
Step-by-step explanation:
As parturition nears, the levels of progesterone decrease and the levels of estrogen increase. During the seventh month of pregnancy, progesterone levels plateau and then drop, while estrogen levels continue to rise. This increasing ratio of estrogen to progesterone makes the uterine smooth muscle more sensitive to contractions, as progesterone no longer inhibits them.