When NIH scientists did their research in the early 1970s on measuring and identifying hCG, they were not sure what the role of hCG was in normal pregnancy, what normal levels of hCG should be, or how they would rise and fall during the course of the pregnancy.
Much research has been done, however, in the last 30 years, to answer these questions. Here is what happens during the menstrual cycle and pregnancy as it relates to hCG.
There are three phases of the menstrual cycle:
Follicular phase: the pituitary gland increases release of follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH stimulates growth of follicles in the ovary and prompts the release of estrogen from the ovary. Estrogen, in turn, causes the thickening of the endometrium. One follicle will mature and release an ovum (egg). LH is secreted mid-cycle, and stimulates the release of that ovum from the follicle in the ovary.
Ovulatory phase: The ovum is released from the follicle and makes its way down the fallopian tubes toward the uterus, perhaps to be fertilized along the way. There are increased levels of LH during this phase. An ovulation prediction kit will detect a surge in LH about 1-2 days before ovulation.
Luteal phase: this covers the time from ovulation through the next menstrual period. Following ovulation the follicle that released the ovum closes and becomes the corpus luteum. LH sustains the corpus luteum until, in the event of pregnancy, it is supported by hCG. Increased production of estrogen and progesterone follow. Both hormones prepare the endometrium for the implantation of a fertilized egg.
