– name given to all the changes occurring in a woman’s body each month in order to produce eggs and support a pregnancy if one should occur.
The cycle involves processes to
a) release a ripe egg or ovum from one of the two ovaries
b) prepare the body for an egg which has been fertilized by a sperm to grow into a fetus (name given to baby before it is born)
The fertilized egg implants into the lining of the womb, the endometrium, where it grows for nine months. But if the egg is not fertilized and a pregnancy is not started during the menstrual cycle, the lining of the womb is shed. This is the period or menstruation.
The menstrual cycle is controlled by hormones. Two main type of hormones involved are:
These hormones cause the development, growth and general ‘upkeep’ of the reproductive system; the womb, vagina, genitals and breasts. Estrogens cause the lining of the womb to grow and develop ready for a fertilized egg to grow there. Progesterone also causes the womb lining to develop and it affects other parts of the body. For example, it prepares the breasts to produce milk if a pregnancy starts. It also will causes the body temperature to rise slightly in the second part of the cycle.
Ovaries, Hormones and Womb
The menstrual cycle can be thought as three cycles, all happening at the same time.
1) Cycle of the ovary – an egg develops and is released each month.
2) Changing levels of hormones – which control the cycle.
3) Changes in the womb – when the lining build up and breaks down.
These three cycles all take about one month, the time from one period to the next.
Cycle of the ovary
From the start of one period to ovulation (Day 1 to Day 14) egg producing follicles develop in one of the two ovaries. The growing follicles produce estrogens. About the middle of the cycle (Day 14) when the developed follicle is ripe, it bursts and releases it egg. This call ovulation. The released egg then starts its journey down the fallopian tube. If the woman have intercourse and there are sperm in the fallopian tubes, a sperm might fertilize the egg.
After ovulation, the empty follicle turns into a corpus luteum or yellow body, so called because it look like a small yellow mass on the ovary. The corpus luteum produces estrogens and progesterone. About one week after ovulation, if the egg has not been fertilized, the corpus luteum stops producing its hormones and it shrinks in size.
Before ovulation, only estrogens are produced by the follicles in the ovary. The amount of the estrogens increases, reaching a peak just before ovulation. After ovulation, both estrogens and progesterone are made in the ovary so the amount of these hormones increases, reaching a high point about one week after ovulation. If the egg was not fertilized by a sperm and the corpus luteum begins to shrink, so the amount of estrogens and progesterone starts to drop gradually in a week before the period.
This cycle of hormones affects the cycle of the womb. During the menstrual cycle the lining of the womb changes so that, if necessary, it can provide a place for a fertilized egg to grow and develop into a fetus. In the very early days of a pregnancy the womb lining is the principal caretaker and provides all the blood and food supply for the growing baby. Later, the placenta develops to nourish the fetus.
Before ovulation, estrogens cause the womb lining to develop and grow. Tiny blood vessels and glands develop and the lining thickens. After the ovulation both progesterone and estrogens cause the lining to grow and thicken even more. In a menstrual cycle in which the egg is not fertilized the woman do not become pregnant, the fall in hormones in the last week or so of the cycle means that the thickened lining is not getting enough support from hormones and it can no longer survive. The lining begins to break down and is sloughed off. This is the period. The period signals both the end of one menstrual cycle and the beginning of the next.