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The Phases of the Menstrual Cycle

menstrual phases

The period is just the first phase of the menstrual cycle. The menstrual cycle encompasses the ovarian cycle and uterine cycle, which interrelate and intersect. Hormones function as chemical signals that are sent through the brain, uterus, and ovaries to maintain the cycle.

A menstrual cycle begins with the first day of the period and concludes with the beginning of the subsequent period. Menstrual cycles typically last between 24-38 days, but the duration may vary each cycle or change over time. It’s completely normal and expected for cycle lengths to change between the beginning of puberty (menarche) and menopause (when menstruation stops in perpetuity). 

At varying stages of the menstrual cycle, some women observe changes in their hair, skin, bowel movements, chronic disease symptoms, mental health, migraines, or their sexual experiences.

Phase One: The first part of the cycle

Phase One in the Uterus: Menstruation

When: Menstruation occurs from the time bleeding begins to when it ends. 

What: Menstruation is when blood and the endometrium (the mucous membrane that lines the uterus) is shed from within the uterus through the cervix/vagina.

During menstruation, the body’s levels of estrogen and progesterone are low. 

The average period duration is about five or six days, but can last up to eight days. The duration and amount of bleeding of the period can vary from person to person as well as from cycle to cycle.

Phase One in the Ovaries: Follicular phase

When: The follicular phase lasts from the beginning of the period until the beginning of ovulation.

What: During the follicular phase, estrogen levels rise and the brain tells the ovaries to prepare the ovum to be released.

During this phase, the pituitary gland (a small, bean-shaped gland at the base of the brain) creates follicle stimulating hormone (FSH) that tells the ovaries to prepare the ovum for ovulation (releasing the egg from the ovaries.) Roughly midway through the follicular phase, as the period is ending, the largest follicle in the ovaries will become the dominant follicle and prepare to be released during ovulation. As the dominant follicle grows, it releases estrogen, which peaks right before ovulation occurs. The follicular phase can vary each cycle but typically lasts 10-22 days

Part Two of Phase One in the Uterus: Proliferative phase

When: The proliferative phase lasts from the end of the period until the beginning of ovulation.

What: The uterus builds up a thick inner lining during the proliferative phase.

The endometrium is the thinnest during the period, and thickens throughout this stage until ovulation begins, in order to create an environment where a prospective fertilized egg can be implanted and grow.

Interlude: Ovulation

When: Ovulation occurs approximately halfway through the menstrual cycle, but this can each vary. Additionally, ovulation separates the two stages of the ovarian cycle; the follicular phase and the luteal phase.

What: During ovulation, an ovum is released from the ovary into the fallopian tube.

As the dominant follicle grows larger, it also subsequently produces more estrogen, and peaks immediately prior to ovulation, declining promptly thereafter. Luteinizing hormone (LH) is caused when estrogen levels are high enough to trigger ovulation. Ovulation typically occurs about 13-15 days prior to the beginning of the next period. 

Phase Two: The second part of the cycle

Phase Two in the Ovaries: Luteal Phase

When: The luteal phase occurs from the start of ovulation until the very beginning of the next period.

What: The luteal phase occurs in the sac that contains the egg as well as produces the estrogen and progesterone.

In between ovulation and the beginning of menstruation, the body prepares itself for a potential pregnancy. The dominant follicle transforms into a corpus luteum (a follicle that houses a maturing egg) and starts to produce progesterone, in addition to estrogen. During this phase the progesterone is produced, peaks about midway through the phase, and then eventually drops. This phase is associated with regular premenstrual symptoms like; mood changes, headaches, hormonal acne, bloating, and tenderness of the breasts.

The progesterone will support an early pregnancy if there is a fertilized egg. If there is no fertilized egg, the corpus luteum will begin to disintegrate 9-11 days post ovulation. The resulting drop in estrogen and progesterone from the corpus luteum breaking down is what triggers the next menstruation cycle to begin. This phase typically lasts about two weeks, however a phase lasting between 9-16 days is also common.

Phase Two in the Uterus: Secretory Phase

When: The secretory phase begins during ovulation and ends once the next period starts.

What: The endometrium either releases chemicals that will assist an early pregnancy if there is a fertilized egg or break the lining down if there isn’t.

Throughout this phase, the uterine lining prepares to either assist a pregnancy or break down for menstruation. The elevated progesterone levels prevent the endometrium from thickening and prepare for a potentially fertilized egg. The secretory phase is named because of the fact that during this phase of menstruation, the endometrium is secreting (releasing) a multitude of chemical messengers. 

The most notable of these messengers are the prostaglandins, which are secreted by endometrial cells and cause changes to other cells nearby.

Prostaglandins are one kind of these messengers, furthermore two specific types of prostaglandins called “PGF2a” and “PGE2” cause the uterine muscle to cramp (contract) which assists in triggering the period. “PGF2a” and “PGE2” levels rise post ovulation and peak during menstruation. If the egg is fertilized, the production of prostaglandin is suppressed in order to ensure the cramps won’t affect an early pregnancy. If the egg isn’t fertilized, the corpus luteum ceases the production of estrogen and progesterone. The effects of the prostaglandins and the decrease in the hormone level causes the blood vessels to tighten and the endometrium tissue to break down.

It’s a good idea for women to track their periods so they can have more awareness of their menstrual cycle and body. If you have any concerns about your gynecological health and you’d like to schedule an appointment with Dr. Gregory Davis, a leading women’s health specialist, please contact Mangrove Women’s Health at (530) 345-0064, extension 281. 

Abstract:

      ●    Menstruation starts with the first day of the period and ends with the start of the next period.

      ●    Changes in the follicles in the ovaries and uterus result from hormone signals that pass between the brain and the ovaries.

      ●    The first part of the menstrual cycle prepares the ovary to release the ovum and strengthens the uterine lining.

      ●    The second part of the menstrual cycle prepares the body and the uterus to receive a fertilized egg or to begin the next menstrual cycle if there isn’t a fertilized egg.

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