Women's Health Specialists of St. Louis
Obstetrics and Gynecology

Articles - The Normal Menstrual Cycle

The normal menstrual cycle is on average 28 days long. Normal range is 21-35 days. Periods that occur more frequently than 21 days or less frequently than 35 days are considered abnormal. When women have irregular periods it is often a sign that ovulation is not occurring. Ovulation is when an egg is released from the ovary. There are many different causes of the lack of ovulation or anovulation. For example, extreme stress, medications, chronic illness, and hormonal imbalances such as thyroid abnormalities can all cause anovualtion. Abnormal growths on the ovary can also interfere with normal ovulation. Abnormal growths on the ovary are most commonly referred to as cysts. This terminology is often confusing. A cyst is the term for a fluid-filled sac. It is normal for some types of cysts to be found on an ovary. For example, each month, several eggs begin to grow in the ovary. These eggs are surrounded by fluid-filled sacs or cysts. Though many eggs begin to grow only one is usually selected by the body to mature and the others then regress and die. As the selected egg matures it grows larger and the fluid-filled sac around it grows as well. The egg and sac together are also referred to as a follicle or follicular cyst. So cysts on the ovary can be normal depending on their size and contents.

Ovulation occurs when the follicular cyst breaks and the egg floats out on the fluid and is swept up by the fallopian tube. When a cyst breaks, the fluid can cause irritation to the surrounding tissues and bowel and cause pain. This is why some women can feel when they ovulate. There is even a word to describe the pain with ovulation. It is called Mittelschmerz and means middle pain in German. Once ovulation has occurred, the remaining cyst where the egg ovulated from then begins to secrete hormones to prepare the lining of the uterus for possible pregnancy. If a pregnancy does occur, then that same cyst will continue to provide the hormones to sustain the pregnancy until about 10 weeks of gestation when the placenta takes over the role.

Of course, some cysts on the ovary are abnormal. These are best evaluated by ultrasound. Cysts that are solid or have cystic and solid components are not usually follicles and may be the cause of anovulation. Unfortunately, it is not always easy to determine if a cyst is normal by a single ultrasound. Deciding whether a cyst is a normal follicle often requires looking at the ovary by ultrasound after a whole menstrual cycle has past to see if the cyst has changed or is gone. Sometimes a single egg is not chosen by the body to mature and be released. The multiple eggs that began to grow in the beginning of the cycle all become arrested in their development so that they are neither growing nor regressing. When multiple cysts are present in the ovary we use the term poly-cystic which means many cysts. Women with polycystic ovarian disorder often experience irregular periods as ovulation generally does not occur.

The take home message is that women with irregular menstrual cycles should be evaluated. The list of possible causes is long but may include stress, chronic illness, hormone imbalances or abnormal ovarian cysts. Evaluation will likely include a history and physical exam, laboratory studies, and an ultrasound.