Which of the following statements is true about membranes? A.
Mucus membranes line cavities that have an external opening in the body,
B.
Serous membranes line internal cavities that have no external opening in the body,
C.
When a membrane is attached to an organ it is called a visceral membrane,
D. All of the above.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.