Women are expected to have an average of about 400 menstrual cycles in their life time which occurs in the period between menarche and menopause. Menstrual cycle is characterised by the presence of menstrual bleeding following the failure of implantation or fertilisation of an oocyte. The cycle is initiated by changes in the steroids produced by the ovaries which are controlled by the pituitary glands and hypothalamus.
What is the normal length of menstrual cycle?
The length of menstrual cycle is determined by the rate and quality of follicular growth and development and varies in the early teenage years and 5 years before menopause. The normal average menstrual cycle length is 28 days. Cycle lengths between 21 – 35 days are also considered as normal. Very slim and obese women tend to have longer cycle lengths.
The normal duration of menstrual flow is between 2 – 6 days and the normal average volume of blood flow is between 30ml – 35 ml. Any bleeding greater than 80ml is considered as excessive.
Common terms associated with menstrual cycle
- Polymenorrhea: means having cycles with interval of 21 days or fewer resulting in more than one menstrual cycle in a month.
- Oligomenorrhea: means infrequent light menstrual periods with interval exceeding 35 days.
- Amenorrhea: means absence of menstrual period in a woman of reproductive age. Amenorrhea is normal during pregnancy, few months after delivery, lactating mothers, before puberty and after menaupause.
- Dysmenorrhea: means painful menstruation characterised by abdominal cramps.
- Hypomenorrhea: means abnormally light menstrual periods.
- Menorrhagia: refers to excessive flow of heavy and prolonged periods.
- Metrorrhagia: refers to bleeding at irregular times outside the expected interval of menstrual cycle.
- Menarche: refers to the first menstruation in women. The average age for menarche is 12 years.
- Menopause: refers to the period when a woman stops having menstrual periods. Average age for menaupause is 45 years.
What are the factors that affect normal menstrual cycle?
- Body mass index of the woman
- Psychological state of the woman
- Lactation (breastfeeding)
- Medications like contraceptives
- Any disease condition or infection that affects the hypothalamus, pituitary, ovaries or uterus.
How the body controls the normal menstrual cycle
The normal menstrual cycle is controlled by the interactions between the hypothalamus, pituitary, ovaries and endometrium through the effects of certain hormones and steroids produced from these parts of the body.
- Hypothalamus: Secretes Gonadotropin Releasing Hormone (GnRH) which acts on the pituitary to release it’s own hormones.
- Pituitary: Releases two hormones, Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) which all act on the ovaries to release its own androgens.
- Ovaries: Releases both estrogen and progesterone. Estrogen is vital for ovulation while progesterone acts on the endometrium to prepare it for implantation or menstruation if fertilisation does not occur.
- Endometrium: Supplies the layer of tissue (functionalis) which is being shed off during menstruation. It is this shedding off of the functionalis that results in the bleeding of the menstrual cycle.
Phases of the menstrual cycle
The interrelationship between these parameters is what brings about the phases of the menstrual cycle which is characterised by different activities that underly the entire menstrual cycle. There are three (3) phases of the menstrual cycle
- The Follicular or proliferative phase
- The ovulatory phase
- The luteal or secretory phase
The follicular phase lasts for a period between 7 – 21 days. During this phase, which is the beginning of the menstrual cycle, low circulating estrogen and progesterone levels trigger the hypothalamus and pituitary to secrete FSH and LH through a negative feedback mechanism. FSH levels increase and stimulates the maturation of follicles in the ovaries. These matured follicles cause further secretion of estrogen and progesterone levels for the next phase of the menstrual cycle.
During the ovulatory phase of the menstrual cycle, which lasts for between 2 – 6 days, high levels of FSH and estrogen levels are responsible for the formation of matured follicles which is now ready to be released as ovum. Meanwhile, at this stage also, matured follicles stimulate a surge of LH levels which begin to initiate the next phase while rising progesterone levels act on the endometrium resulting in endometrial growth. After ovulation, estrogen levels begin to decline as well as FSH and LH levels.
In the luteal phase, LH stimulates the ovarian follicles remaining after ovulation to transform into corpus luteum which is responsible for the release of high progesterone levels that results in a thickened and enlarged endometrium ready for the incoming ovum. If there is no fertilisation, the corpus luteum regresses and progesterone levels decline and the unfertilised ovum, together with the endometrium (functionalis) is shed off as menstruation. Luteal phase is fairly constant in duration of about 14 days after which the cycle commences again.
Wikipedia : “Disorders of the menstrual cycle ” (Accessed 16/01/2018).
A. K. Ghosh et al, (2008), “women’s health – menstruation ” Mayo Clinic Internal Medicine Review, 8th edition, (pg 1059).
S. Campbell, A. Monga (2006), ” The Normal Menstrual cycle ” Gynaecology by Ten Teachers, 18th edition, (pg 32-40).