Luteal phase defect is a condition that is associated with recurrent miscarriage and possibly infertility. The ovulatory cycle is divided into two phases. The part of the cycle before ovulation is called the follicular phase. During this time, the follicle (the fluid-filled sac in the ovary that contains the egg) develops in preparation for the release of the egg. The developing follicle produces a type of estrogen (estradiol) that stimulates the growth or thickening of the uterine lining (endometrium). This production of estrogen is also responsible for increasing the production of cervical mucus and changing its characteristics to make it more favorable for sperm penetration. When the egg is released (ovulation), the cells remaining in the follicle undergo changes that allow them to produce another hormone called progesterone. This process is called “luteinization” and is triggered by the release of a hormone called luteinizing hormone. After this surge in luteinizing hormone levels, the follicle turns into a corpus luteum. This event is the beginning of the luteal phase, which makes up the second half of a woman’s cycle. Progesterone produced by the corpus luteum causes changes in the endometrium that make it more favorable for the attachment of the embryo (implantation). If progesterone production is lower than normal, the endometrium may not develop enough to implant an embryo. This situation is called a luteal phase defect. The developing endometrium depends on adequate progesterone production by the ovary. Although many doctors focus on the level of progesterone in the blood, it is actually more important that progesterone production is sufficient for the appropriate number of days. The absolute serum level of the hormone is not that important. Therefore, simply measuring the serum level can be misleading. It is more accurate to assess the effect of progesterone on the endometrium over time. This is done by examining a piece of uterine lining tissue under a microscope, a procedure called an endometrial biopsy. This biopsy is most accurate to perform near the end of the luteal phase, which is the most accurate time to assess the luteal phase. Another acceptable way to estimate the luteal phase is to count the days from ovulation to the start of a woman’s next period. A normal luteal phase should be at least 12 days. The most common treatment for a short luteal phase is to give the woman an extra amount of progesterone. Progesterone supplementation can effectively prevent pregnancy loss when given to women with a luteal phase defect. Progesterone supplementation is usually started three days after ovulation. Therefore, it is important to accurately document the day of ovulation, as starting progesterone too early can increase the risk of a tubal pregnancy.The peak in luteinizing hormone levels usually precedes ovulation by 18-30 hours, progesterone supplementation begins four days after the initial detection of the luteinizing hormone peak. References: 1. Introduction of Female Reproductive Processes and Reproductive Diseases. Zhang J, Li J, Yan J. 2. Adolescent Development of Biological Rhythms in Female Rats: Estradiol Dependence and Effects of Combined Contraceptives. Grant AD, Wilbrecht L, Kriegsfeld LJ.