Female fertility is a biological process regulated by female hormones. The brain uses hormones to send signals to the body to trigger events. The body returns feedback to the brain to help regulate these events. This process repeats every cycle in women of childbearing age.
An Introduction to Reproductive Hormones
There are several hormones involved in regulating female fertility. The following introduces some of the major ones and briefly describes each.
- Gonadotropin-Releasing Hormone (GnRH):
This hormone is produced by the hypothalamus gland in the brain. It is released in short pulses to stimulate the pituitary gland. If the pulses are spaces far apart, the pituitary gland releases follicle-stimulating hormone (FSH). Closely spaced pulses cause the pituitary gland to generate lutenizing hormone (LH).
- Follicle-Stimulating Hormone (FSH):
FSH is produced by the pituitary gland. It causes a follicle in the ovary to mature. Each ovarian follicle contains an ovum (or egg). As the follicle matures, it produces hormones to automatically reduce FSH production.
- Lutenizing Hormone (LH):
Lutenizing hormonal is responsible for triggering ovulation. It is also produced by the pituitary gland. A peak in LH levels coincides with ovulation. The now mature follicle releases its egg. Home ovulation predictor kits detect the LH surge to identify ovulation.
Estrogen is a hormone present throughout the cycle in varying levels. The maturing follicle produces estrogen. This causes the hypothalamus to send signals to the pituitary gland to increase production of lutenizing hormone. Estrogen also stimulates the female reproductive organs to prepare for possible fertilization. This produces observable characteristics which can be monitored to determine fertility.
Progesterone is produced by the corpus luteum after ovulation. The corpus luteum is what remains of the ovarian follicle after releasing the egg. Progesterone signals for the uterus to prepare for a possible implantation by a fertilized egg. It also sends a signal to the pituitary gland to scale back LH production. The presence of this hormone also causes a subtle elevated shift in core body temperature which can be charted.
- Human Chorionic Gonadotropin (HCG):
HCG is produced if the fertilized egg successfully implants in the uterine wall. It prevents the corpus luteum from degenerating so that progesterone and estrogen levels will remain high. HCG is the hormone detected by home pregnancy tests.
The Sequence of Hormonal Events
The following figure offers an overview of hormonal events related to female fertility. These steps are numbered in the diagram and described in more detail below.
- The hypothalamus produces GnRH early in the cycle. This is used to communicate with the pituitary gland and start production of FSH.
- In response to GnRH, the pituitary gland begins production of FSH. This hormone stimulates the ovaries and causes an ovarian follicle to begin to mature.
- As the follicle matures, FSH levels decrease. The maturing follicle produces estrogen which feeds back to the hypothalamus. The GnRH production changes frequency and causes the pituitary gland to begin producing lutenizing hormone.
- A surge in LH levels causes the mature follicle to release its egg. This event is known as ovulation. The egg proceeds down the fallopian tube awaiting possible fertilization.
- The portion of the follicle that remains after ovulation is the corpus luteum. It produces progesterone which feeds back to the brain to reduce LH levels.
- Progesterone also causes the uterus to prepare for possible implantation.
- Unless pregnancy results and HCG is produced, both progesterone and estrogen levels fall as the corpus luteum degenerates. This causes the body to prepare for menstruation and the start of another cycle if pregnancy does not result.
Perceivable Hormonal Symptoms
A number of hormonal events work together throughout the female reproductive cycle. This culminates with ovulation where a released egg has the opportunity to become fertilized by a male sperm. Fertility awareness tries to capitalize on changes in observable symptoms associated with these hormones. This makes it possible to detect ovulation and plan accordingly. Two of the most commonly tracked symptoms, basal body temperature and cervical mucus, are explained below.
As described, progesterone helps alert the uterus and brain that the follicle released an ovum. This hormone also has another observable side effect. The higher progesterone levels cause the body temperature to rise slightly by tenths of a degree. If the core body temperature, or basal body temperature, is charted, this increase can be detected and used to identify ovulation.
Cervical mucus is one symptom commonly tracked by the fertility awareness method. Estrogen levels increase during the cycle as ovulation nears. This hormone causes the cervical mucus to change characteristics. The mucus serves an important role as it help facilitate the transport of male sperm. By allowing sperm to survive longer, the fertile window increases beyond the 24-hour window of ovulation. Sperm can survive internally a day or more and await an opportunity to fertilize the female ovum. When charting fertility, these changes can be monitored to detect ovulation as it approaches.
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