Infertility is defined as the inability of a sexually active couple who are not using birth control to become pregnant after one year of trying. An estimated 35 to 40 percent of infertility cases are due to female infertility and an equal number can be due to male infertility. Approximately 20% of infertility cases occur from an unknown cause. 
Luteal phase deficiency (LPD) has been associated with infertility. It is described as a condition in which endogenous progesterone is not sufficient to maintain functional secretory endometrium and allow normal embryo implantation and growth .Exogenous supplemental progesterone is common element in the treatment regimen in infertility, particularly those related to Assisted reproductive technologies (ART).
1. Endocrine Society Hormone Health Network: www.hormone.org/questions-and-answers/2012/infertility-and-women
2. Practice Committee of the ASRM. Fertil Steril 2012