Luteal Phase

ovulated because of a slight drop in their basal body temperature at the time of ovulation and because of changes in the cervical mucus that support sperm transport. Some women will experience pain at this time. Ovulation pain is also called mittelschmerz pain and, while it is not dangerous, it can be uncomfortable for some women.

Luteal Phase

The luteal phase begins after the fertilization of the egg or, if the egg is not fertilized, after the egg is released from the ovary. After the egg is released, the follicle becomes a corpus luteum. This is a temporary endocrine structure responsible for making progesterone, estradiol, and inhibin A. The corpus luteum is made entirely from granulosa cells that remain behind after the follicle has ruptured and after the egg cell as entered the fallopian tube.

Surrounding blood vessels provide a blood supply to the corpus luteum so that these cells can produce hormones that ultimately support the egg cell after fertilization. Granulosa cells increase in size and make up about 80% of the corpus luteum. At this time, they are known as granulosa lutein cells. They secrete very little protein and instead secrete estradiol, of which the base form is androstendione.

The theca interna cells turn into theca lutein cells and contribute to the outer aspect of the corpus luteum. These are much smaller cells than the granulosa cells and are responsible for steroid synthesis. Overall, the corpus luteum is rather large and increases even further in size during pregnancy. The corpus luteum and pregnancy is called a corpus luteum graviditatis, which can be up to a third of the size of the total ovary. The corpus luteum remains functional unless the egg is fertilized, at which time it becomes a structure called the corpus albicans.

When it comes to the endometrium, the luteal phase is also called the secretory phase or progestational phase. This is the phase in which the endometrium becomes the most receptive to the implantation of the fertilized egg. The endometrial layer is thick and rich with vasculature as would be necessary if an embryo were to implant and require the circulation necessary to attach to the embryo and start building the connections that will ultimately become the uterine/placental boundary.

During this phase, the estrogen made by the corpus luteum is responsible for the inhibition of FSH in the pituitary gland. This is evolutionarily appropriate because it means that there is no chance of multiple ovulations and there is a genetic benefit to having just a single fertilized egg that implants into the uterus to become the embryo and, eventually, the fetus.

The corpus luteum reaches peak size and peak development on the ninth day after ovulation. If there is no more stimulation from luteinizing hormone, the two cell types that make up the corpus luteum (the theca cells and the granulosa cells) will undergo apoptosis and die off. This