Overview
The average length of a menstrual cycle ranges from 25-35 days. It is necessary for the production of eggs and preparation of the uterus for a pregnancy to occur. It occurs monthly from puberty to menopause.
It is divided into two types of cycle i.e. ovarian cycle and uterine cycle and each can be further divided into three phases. The ovarian cycle consists of the follicular phase, ovulation and the luteal phase while the uterine cycle is divided into menstrual proliferative and secretory phase. These cycles are controlled by the normal hormonal changes.
The ovarian cycle describes changes that occur in the follicles of the ovary and the uterine cycle describes the changes in the endometrial lining of the uterus.
Ovarian Cycle
The Ovarian Cycle consists of the Follicular Phase, Ovulation and Luteal Phase.
Follicular Phase
This is the first part of the ovarian cycle. Under the influence of FSH (follicle stimulating hormone ) the follicles are stimulated which lead to the formation of one dominant follicle.
The follicle that reaches maturity is called a tertiary, or Graafian, follicle, and it contains the ovum
Ovulation
This is the second phase of the cycle in which a mature egg is released from the follicle. Under the influence of rising Oestrogen levels, produced by the maturing follicle, a rapid surge of Luteinising Hormone (LH) is released by the anterior pituitary gland, which triggers Ovulation and causes the dominant mature follicle to release an oocyte. LH surge occurs 24-36 hours before Ovulation. This occurs between day 12-15 of the average 28-day cycle.
After the oocyte is released from the ovary it is taken up by the fallopian tube. The sperm meets the oocyte and an embryo starts developing in the fallopian tube. The developing embryo may take three to five days to reach the uterus and implant into the endometrium which may result in a pregnancy. If the egg is not fertilized within 12-24 hours of Ovulation, it will dissolve and be absorbed in the fallopian tube.
Luteal Phase
The follicle in the ovary that produced the egg becomes the corpus luteum, which produces the hormone Progesterone and Oestrogen to provide nutrition to the endometrium (lining). If the embryo implants into the endometrium it produces HCG. If there is no fertilization and implantation the ovary stops producing Progesterone and this fall in hormones causes the uterine lining to shed and a menstrual cycle starts.
Uterine Cycle
The Uterine Cycle consists of Menstruation, the Proliferative Phase, and the Secretory Phase.
Menstruation
Menstruation is the start of the Uterine Cycle symbolically called the weeping uterus as it has failed to achieve pregnancy.
Proliferative Phase
This is the second phase of the cycle when hormones cause the endometrial lining in the uterus to grow or proliferate. As the ovarian follicles start to mature Oestradiol is released; this causes the formation of a new layer of the endometrium. This coincides with the follicular phase of the ovarian cycle.
Secretory Phase
This corresponds with the Luteal Phase of the Ovarian Cycle. During the Secretory Phase, the follicle that released the egg (corpus luteum) produces the hormone Progesterone, which allows the endometrium to be receptive to implantation of the developing embryo and support early pregnancy.