What happens to the thickness of the uterine lining when the level of progesterone reaches its highest level?

Becomes thick with blood vessels passing through

Explanation:

Progesterone levels are the highest in the middle of the  luteal phase i.e at the eighth or ninth day of menstural cycle. it is the hormone which is released after ovulation. It is produced by the corpus luteum (an area created by the collapsed follicle) which consists of ovulated egg. At this stage the uterus lining gets thick and also estrogen is at high level. If the conception does not takes place the level of progesterone falls as the corpus luteum starts to break.