Hormones of the Menstrual Cycle
The menstrual cycle is a complex interaction of the six key hormones (chemical messengers) that affect changes in the reproductive organs (uterus, ovaries, breasts, and oviducts). It is divided into three distinct phases: the follicular, ovulation, and the luteal phases.
The length of a menstrual cycle – counted from the beginning of one menstrual period to the beginning of the next – can be different for each woman or change from cycle to cycle for the same woman. You will see that many menstrual cycle diagrams represent a 28 day cycle, which is an average, but a healthy menstrual cycle can vary between 21-35 days.
The menstrual cycle begins when a woman begins bleeding, often called getting her period. The menstrual fluid contains blood, cervical mucus, vaginal secretions, and endometrial tissue, though most people just refer to it as blood. At this point, estrogen and progesterone are the lowest they will be throughout the cycle. Menstruation marks the beginning of the follicular phase, which ends at ovulation. The length of the follicular phase is variable and can be affected by diet, stress, or illness.
Triggered by a complex series of chemical signals, the pituitary gland in the brain begins to secrete two key hormones, Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH). The increase in FSH and LH cause about 10-20 of the follicles within the ovaries to begin to mature and they too release a hormone, called estrogen or estradiol. Levels of estrogen in the body steadily increase as the follicles continue to mature for about a week – this estrogen causes the lining of the uterus to begin to thicken. Usually only one of the maturing follicles becomes dominant, as it is most sensitive to estrogen, LH, and FSH. When the estrogen level reaches a certain threshold, it triggers the pituitary gland to release a large amount of LH.
Cervical fluid in the follicular phase is typically scant or tacky. For many women who are tracking cervical position, the cervix feels hard, closed, and low. As ovulation approaches, the cervical fluid may become more creamy and then watery and/or lubricative/slippery and stretchy. The cervix begins to soften as ovulation nears.
The spike in LH (seen as the peak in the green line on the graph below) cause the fully mature dominant follicle to burst through the wall of the ovary. The release of the mature ovum is called ovulation. Some women literally feel ovulation and experience ‘mittlesmertz,’ or a slight pain in one of their ovaries during ovulation.
The cervical mucous near ovulation is typically very stretchy, slippery, and clear (looks and feels like eggwhites) and/or watery. This fertile fluid helps the sperm move toward the egg and protects sperm from the acidity of the vagina, which would normally kill them. The cervix usually feels higher, softer, and more open.
The mature ovum released from the ovary at ovulation is swept by the fimbriae into the oviduct by rhythmic muscular movement of the oviduct, where it will live for about 24 hours if not fertilized by sperm. The remains of the dominant follicle in the ovary is called the corpus luteum and produces large amounts of progesterone during the luteal phase. The length of the luteal phase is not variable and typically lasts 14 days for most women, though a range of between 12-16 days is considered normal.
If conception or implantation does not occur, the corpus luteum in the ovary will shrivel about 14 days after ovulation and will cause a sharp decrease in both estrogen and progesterone, triggering the onset of menstruation and the beginning of a new menstrual cycle. The unfertilized ovum is about the size of a grain of sand; it is shed as part of the menstrual fluid.
Progesterone is high during the luteal phase; it is a heat inducing hormone and thus raises the basal body temperature (BBT) by several tenths of a degree for the duration of the cycle. The jump in basal body temperature that remains steady indicates a woman has ovulated.
The cervix usually feels low, firm, and closed during the luteal phase.
In order for conception to occur, semen must first survive the acidity of the vagina (fertile cervical fluid changes the vaginal pH to be more alkaline and sperm friendly) and then swim through the cervical canal into the body of the uterus and then into the oviduct to greet a woman’s egg. If an ovum is fertilized by sperm in the oviduct (called conception), the fertilized egg (blastocyst) migrates into the uterine lining, where it will implant approximately 7 to 14 days after ovulation. The corpus luteum will continue to provide high levels of progesterone to support pregnancy until the placenta takes over that job in approximately 12 weeks.
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