What is the menstrual cycle? The menstrual cycle starts on the first day of bleeding during the period and lasts until the first day of bleeding during the next cycle. The lining of the uterus is shed and then begins to thicken again in case pregnancy occurs. Ovulation generally occurs around the midpoint of the cycle (about 2 weeks before the start of the next cycle) in response to hormonal changes, and while the ovum (egg) is present, fertilization and pregnancy may occur. If there is no pregnancy, then the cycle starts over again. The length of the cycle can be calculated by using day 1 as the first day of bleeding and then counting days until bleeding starts again. Typical ranges can be from 24-38 days. Now that the cycle has been explained, we can move on to the important part – the hormones.
- FSH and LH – think of FSH as the director of a cycle. Follicle stimulating hormone (FSH) comes from the pituitary gland in the brain and is responsible for the growth of an egg during the menstrual cycle until ovulation is triggered by luteinizing hormone (LH). FSH is highest just before ovulation and then decreases following the release of an egg until it’s time for the next menstrual cycle. In menopausal women, levels are consistently very high because the pituitary gland wants to stimulate release of eggs from ovaries that are no longer responding.
- Progesterone – this hormone is released from the corpus luteum (a cyst formed on the ovary each month). It increases in the second half of the cycle in order to support a pregnancy should it occur, but if there’s no pregnancy, the corpus luteum goes away and the decreased progesterone triggers a menstrual cycle. High progesterone levels in the second half of the menstrual cycle are responsible for many of the PMS symptoms experienced by women including acne, mood change, headache, breast tenderness, and everyone’s favorite-bloating.
- Estrogen – estrogen is produced by the dominant follicle in the ovary and increases as the size of the follicle increases over the first half of the cycle. Increased estrogen thickens the lining of the uterus in preparation for possible implantation of the pregnancy. Once estrogen levels are high enough, then the LH surge triggers ovulation and the second half of the menstrual cycle begins. The decrease in estrogen in the latter part of the menstrual cycle stops the thickening of the uterine lining and works with the decreased progesterone levels to allow the menstrual cycle to start over if there hasn’t been a pregnancy. Persistently high estrogen levels, such as those caused by estrogen production in adipose tissue in women with PCOS, cause continued growth of the uterine lining. Continually elevated estrogen can ultimately result in unpredictable menstrual cycles as triggers for bleeding can be suppressed, and these cycles can often be very painful and heavy as several months worth of endometrial lining can be passed at the same time. Over a long enough time, this type of bleeding can even result in endometrial (uterine) cancer.
- Testosterone – testosterone, while generally thought of as a male hormone, is present in women, although in much smaller amounts. Testosterone is a driver for libido and plays a role in developing/maintaining muscle mass and bone density. There is a slight increase around the time of ovulation (in order to encourage procreation), but otherwise levels are pretty stable. If testosterone levels get too high, women can experience acne, facial/body hair growth, deepened voice, male pattern baldness, heart disease and increased clitoral size. While some of those effects are temporary, many of them may be permanent.