How does a scientist come up with the theory?

By reviewing extensive amounts of evidence that support a scientific explanation of phenomena. For example, Darwin's Theory of Evolution is supported because of the evidence and research of the finches in the galapagos islands that make sense in a scientific world that evolution does indeed exist.

Explanation:

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.