There are four main hormones which are active in the ovulation cycle. I say main hormones because in reality the process is infinitely more complex and intricate! These four hormones are FSH, LH, Estrogen, and Progesterone. Two of them come from the brain (well, the pituitary gland), and two of them from the ovaries. I want to talk in simple terms about the basic roles that each one of these performs in the ovulation cycle. This will be posted as a short series of four posts on Instagram which will introduce these hormones. The combination of these four posts is posted as one full blog here on my website.
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FSH: FSH is Follicle Stimulating Hormone. The name is perhaps quite self-explanatory! Early in the cycle, the hypothalamus in the brain releases something called GnRH (Gonadatropin Releasing Hormone) which in turn signals the pituitary gland to release FSH. The launch of FSH is what tells the ovaries to begin recruiting immature eggs for maturation, at the end of which process there will occur one (and only one!) ovulatory event in which one egg (and very occasionally two, in the same event) is released from an ovary. But we’re getting ahead of ourselves…simple put, the launch of FSH is what initiates the opening of the fertile time in a healthy cycle.
Estrogen: After FSH launches and stimulates the ovaries to begin maturing eggs, those maturing eggs then release Estrogen. Estrogen in turn tells the uterus to begin building up the endometrial lining to prepare for the potential of a baby implanting. Estrogen also tells the cervix to begin secreting estrogenic cervical mucus, the primary biomarker of fertility that we chart. Sperm cannot survive unless there is cervical mucus to sustain it, and so the observation of cervical mucus at the vulva, when wiping or walking, is what signals to a woman that she is now in the fertile time of her cycle, and in a healthy cycle ovulation should only be a few days away.
LH: The more mature the eggs become in the ovaries, the more Estrogen is released. This rise in Estrogen causes a negative feedback loop which suppressed FSH production. When Estrogen reaches its peak levels and FSH is suppressed, then the pituitary gland receives the signal to release a second hormone, LH (Luteinizing Hormone). LH triggers the follicle within which the dominant egg was maturing to rupture, and the egg is released and swept up into the Fallopian tubes. Following this LH then transforms that remaining follicle in the ovary into the corpus luteum (i.e. LH luteinizes the follicle). The corpus luteum is a temporary endocrine gland which releases the fourth and final hormone that we’re looking at.
Progesterone: After LH triggers ovulation and transforms the remaining follicle into the corpus luteum (the temport endocrine gland), the corpus luteum then releases the hormone Progesterone. Progesterone is the pro-gestation hormone, because it is what tells the uterus to hold onto the endometrial lining that it built up under the influence of Estrogen for up to 18 days (9-18 in a healthy cycle). This span of days where the endometrial lining is remaining is necessary to allow for the implantation of a baby should conception have occurred at the time of ovulation. It takes a few days for a fertilized egg to move through the Fallopian tubes and into the uterine cavity, so if Progesterone levels are healthy (aka if the corpus luteum is healthy, because a healthy ovulatory event occurred) then the endometrium is ready and waiting for implantation. If implantation occurs, HCG from the baby signals the corpus luteum to stick around beyond its usual lifespan and to keep producing Progesterone. If implantation does not occur, the corpus luteum has a natural expiration date which does not exceed 18 days (and is generally pretty consistent from cycle to cycle in the same woman), and at that point progesterone levels drop. When these levels drop, there’s nothing to tell the endometrium to remain, and so the endometrial lining is then shed as menstrual flow. At menstruation a new cycle begins, and all hormonal activity is generally quite low, until FSH again launches, reinitiating this entire beautiful pattern all over again.