Menstruation is characterized by shedding of the endometrial lining (uterine lining) associated with bleeding. It happens in monthly cycles all through a female’s reproductive life; the only exception is during pregnancy. Menstruation begins at menarche (during puberty) and permanently stops at menopause.
The regulation of menstrual cycle is done by the complex interaction of different hormones: follicle stimulating hormone (FSH), luteinizing hormone, and the female sex hormones progesterone and estrogen. Follicle stimulating hormone and luteinizing hormone, which are secreted by the pituitary gland, stimulate the ovaries to secrete progesterone and estrogen and promote ovulation. Progesterone and estrogen prepare the uterus and breasts for possible pregnancy if fertilization takes place. Let us discuss about the hormone levels during cycle.
Phases of the Menstrual Cycle and Changes in Different Hormones
Here are three phases of a menstrual cycle:
Follicular phase (before the egg is released)
Ovulatory phase (when the egg is released)
Luteal phase (after the egg is released)
Below is a chart to help you better understand, followed by detailed explanation.
Follicular Phase: Follicles Mature
This phase starts on the first day when menstrual bleeding occurs (day 1). However, the major event in this phase is the maturation and development of the follicles in your ovaries.
At the starting of the follicular phase, the uterine lining or endometrium is thick and engorged with nutrients and fluids designed to provide nourishment to a developing embryo. At this time the levels of both progesterone and estrogen are low, if fertilization of no egg has taken place. Due to these low levels of the female sex hormones the top layers of the uterine lining are shed in the form of menstrual bleeding.
Around this time, there is slight increase in the secretion of follicle-stimulating hormone by the pituitary gland. Then the growth of around 3-30 follicles is stimulated by this hormone. An egg is present in each follicle. In the later part of this phase, the level of the follicle stimulating hormone decreases and only one of the developing follicles continues to grow. This follicle is also referred to as the dominant follicle. This follicle soon starts to secrete estrogen and the other follicles start to break down. The increasing levels of estrogen start to prepare the uterus and also stimulate the surge of luteinizing hormone-suggesting that the hormone levels during cycle changes continuously.
On average, the duration of the follicular phase is around 13 or 14 days. Near menopause, it tends to get shorter. The follicular phase ends when there is a dramatic increase in the level of the luteinizing hormone (surge). This surge leads to ovulation or release of the egg from the ovary and marks the starting of the ovulatory phase.
Ovulatory Phase: Releasing an Egg
The surge in luteinizing hormone marks the beginning of this phase. Luteinizing hormone causes the dominant follicle to bulge out from the ovarian surface and rupture, thereby, releasing the egg. The increase in the levels of follicle stimulating hormone is to a lesser degree. The function of this increase is not understood clearly.
The duration of the ovualtory phase is usually 16 to 32 hours. It ends with the release of the egg from the ovary, about 10-12 hours after the luteinizing hormone surge. The fertilization of the egg can take place for up to around 12 hours after its release from the ovary.
The level of the luteinizing hormone can be measured in the urine and the surge of this hormone can be detected. This can be used to found when a female is most fertile. Fertilization is more likely to occur when sperm from the partner are present in the reproductive tract of the female before the release of the egg. The majority of the pregnancies happen when sexual intercourse occurs within three days before ovulation happens.
Egg release is random and does not appear to alternate between the two ovaries. In cases where one ovary is removed, an egg is released every month from the remaining ovary.
This phase starts after ovulation occurs. Its duration is around 14 days, unless fertilization happens, and it ends just before the start of a menstrual period. During this phase the ruptured ovarian follicle closes after the egg release and forms a structure referred to as the corpus luteum, which secretes increasing amounts of progesterone-another mention of the fact that hormone levels during cycle change.
The hormone progesterone secreted by the corpus luteum prepares the uterus for the implantation of an embryo. The progesterone cause thickening of the endometrium, filling it with fluids and various nutrients, which will be used to nourish the implanted embryo. Progesterone also thickens the cervical mucus, so that bacteria or sperm do not enter the uterus. The body temperature is also increased slightly by the progesterone during the luteal phase and it remains elevated till the beginning of a menstrual period. This elevation in temperature can be utilized to estimate whether the egg is released or not. The estrogen levels remain high during most of the luteal phase. Estrogen also stimulates the thickening of the endometrium.
The increase in progesterone and estrogen levels during luteal phase causes the widening or dilatation of the milk ducts. Due to this there may be swelling and tenderness of breasts.
If there is no fertilization of egg or there is no implantation of the fertilized egg, then there is degeneration of the corpus luteum after 14 days, it no longer produces progesterone and estrogen, the levels of progesterone and estrogen fall and a new menstrual cycle starts. The top layers of the uterine lining are broken and shed and menstrual bleeding happens.
If the implantation of the embryo occurs, the cells surrounding the developing embryo start producing a hormone referred to as human chorionic gonadotropin. The basis of pregnancy tests is measuring an increase in the levels of human chorionic gonadotropin level. The corpus luteum continually secretes progesterone, until the fetus can secrete its own hormones.
Hormone levels during cycle repeats in every menstrual cycle without pregnancy.