Ways cross pollination happens naturally

Cross-pollination occurs when one type of flower pollinates another type which results in a hybrid plant.

It is simply  the process of applying pollen from one flower to the pistils of another flower and occurs in nature with the help of insects and wind

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.