Progesterone

Progesterone is a heat producing hormone which is dominant during your luteal phase 
(time between ovulation and you period). The corpus luteum (remainder of follicle after ovulation) 
continues to produce  progesterone throughout the early weeks of pregnancy until the placenta takes 
over around 6-8 weeks.
If pregnancy does not occur the progesterone production decreases as the corpus luteum diminishes 
and menstruation starts.