How Hormones Drive Your Cycle 
At the start of your menstrual cycle, your estrogen levels are low.
These low estrogen levels stimulate your pituitary gland to produce Follicle Stimulating Hormone (FSH), which stimulates a number of follicles in your ovaries to start developing.
The developing follicles produce additional estrogen, which acts to thicken your uterine wall (preparing your uterus to support an embryo). The follicles are also responsible for the timing, type and quantity of cervical mucus your body is producing, a liquid pathway awaiting the introduction of sperm.
As you approach the half-way mark of your menstrual cycle, heightened estrogen levels lead to the production of 'sperm friendly' mucus and signals the beginning of your fertile days. Fertile mucus supports sperm survival and aids in sperm transport. In the absence of fertile mucus, sperm usually only survive for a few hours.
As your estrogen levels rise, your cervix opens to permit sperm to enter your uterus. Typically, only one of your ovary's follicles becomes dominant, preparing your ovary to release one of your eggs, while the other follicles regress. Sperm survival coupled with the life span of the dominant follicle's egg combine typically to provide up to a 6 day window of fertility during which you can become pregnant.
When your estrogen level peaks, your pituitary gland is stimulated to produce a surge in Luteinizing Hormone (LH). The egg will be released from the ovary's follicle 24-36 hours after this LH surge.
The egg travels into one of your fallopian tubes to await fertilization, and can survive for up to 24 hours after ovulation. Having released its egg, the empty follicle forms the corpus luteum, which secretes progesterone.
Progesterone reverses the effects of estrogen in your body, causing your cervix to close and non-sperm friendly mucus to be produced.
If your egg has been successfully fertilized by a sperm, your estrogen and progesterone levels will remain high. Your fertilized egg implants itself into the uterine wall and produces human Chorionic Gonadotrophin (hCG), a hormone which is necessary to maintain your pregnancy. It is this hormone, hCG, that is detected by pregnancy tests.
If the egg is not fertilized, the corpus luteum degenerates, your estrogen and progesterone levels lower and the blood that has thickened your uterine wall is shed. You experience this as menstruation. Your pituitary gland detects the falling estrogen levels and produces FSH, thus beginning a new cycle.
Learn more about your menstrual cycle.
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the Clearblue® Easy Fertility Monitor
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