During the reproductive cycle, your body releases an egg, prepares itself
for fertilization of the egg by sperm and creates an environment in
the uterus in which the fertilized egg could implant and form a developing
embryo (ultimately, a baby). If the egg is not fertilized, the lining
of the womb is discharged from the body - that's your period. It's
a pretty amazing thing going on inside us women. Let's see how it works.
Girls start to have their periods (to menstruate) around the age of 13,
usually about 2 years after breasts first start to develop, and continue
having periods until menopause, which occurs, on average, at about
the age of 51.
The length of the reproductive cycle can vary from a short cycle of only
21 days to a long cycle of 40 days. The length of the cycle is calculated
by counting the first day of bleeding as day 1 and then counting until
the very last day before the next bleed (period). Even though, the length
of the reproductive cycle varies among women, it is commonly described
as 28 days.
||There are three parts to
the reproductive cycle: the menstrual cycle, the hormonal cycle and the
ovarian cycle. In the diagram below, you can see the changes taking places
during each cycle. (skip down to diagram)
Menstruation takes place during the first 5 days of the cycle – that's
when blood, mucus, and tissues are discharged from the uterus; this commonly
lasts from day 1 to day 5. During this phase, if fertilization of the egg
hasn’t happened, the lining of the uterus, which is called the endometrium,
comes away from the uterus wall and the blood and tissues pass out through
the vagina. Most women bleed for between 3 and 5 days. The lining of the
endometrium will end up about 1 mm thick at the end of the period. As well
as the loss of the endometrial tissue, about 35 to 50 ml of blood is lost
from the broken endometrial blood vessels in a typical period. This blood
does not usually clot unless bleeding is very heavy.
On the 5th day, the endometrium starts regrowing. About 3 to 30 follicles
grow between days 8 and 10. Each follicle contains an egg, but by days
10 to 14 one follicle has overtaken the rest and has reached the correct
stage of maturity.
During days 6 to 14, the lining of the uterus is repaired and, as can be
seen on the diagram, builds
up to be thicker. This phase is called the proliferative phase. As the
selected follicle develops, it begins to produce estrogen on about the
7th day. Estrogen enters the circulation and influences the growth of the
endometrium. The lining of the uterus will now be about 3 mm thick and
is also more velvety again.
On about the 14th day of the menstrual cycle, the combined increases in
concentration of FSH and LH and estrogen induce ovulation – the bursting
of the mature follicle and the release of the immature ovum into the uterine
tube (also called fallopian tube). The other follicles over-ripen and break
Some women can feel a pain on one side of the abdomen around the time the
egg is released. This is known as ‘mittelschmerz’ – a
German word translating as ‘middle pain’. An egg is released
from the right or left ovary at random and takes about 5 days to travel
down the fallopian tube to the uterus.
After the follicle ruptures as it releases its egg, it closes and forms
a corpus luteum. The corpus luteum secretes more and more progesterone,
which acts on glands in the endometrium and causes them to secrete. The
purpose of this secretion is to feed the developing egg until a placenta
has formed. Even if the egg is not fertilized and pregnancy has not happened,
the secretion is still produced. The progesterone secreted by the corpus
luteum causes the temperature of the body to rise slightly until the start
of the next period.
If there is no fertilization, the corpus luteum lasts about 14 days and
then starts to break down, begins to curl in on itself, forming the corpus
albicans. This is when progesterone production rapidly drops and the estrogen
level decreases. This lack of hormones causes blood vessels in the endometrium
to go into spasm and they cut off the blood supply to the top layers of
the endometrium. Without oxygen and nutrients from the blood, the endometrial
cells begin to die, tissue breaks down and there is bleeding from the damaged
blood vessels. The broken tissue (or menstruum), secretions, blood and
one or more unfertilized eggs make their way toward the vagina.
And, the cycle starts again.
image above was reproduced and modified from: The Anatomy Coloring
Book, Wynn Kapit and Lawrence M. Elson
Follicle: the primary form of egg with one layer of cells
Primary Follicle: during this
stage, the primary egg completes its growth
Secondary Follicle: full-size
follicle with more cell layers surrounding it
Mature Follicle: final stage
of differentiation of the egg
Ovulation (Ovum): mature follicle
bursts and releases egg (ovum)
Corpus Luteum: the yellow
glandular mass that is formed by the mature follicle once it has
discharged its ovum. The corpus luteum secretes progesterone.
Corpus Albicans: an unfertilized
corpus luteum that has curled inward on itself
follicle-stimulating hormone – stimulates the development
of ovarian follicles (developing eggs in their sacs) and stimulates
the release of estrogen
LH: luteinizing hormone – acts
with follicle-stimulating hormone to stimulate sex hormone release.
Estrogen – helps to
produce an environment suitable for the fertilization, implantation
and nutrition of the early embryo, and is responsible for the development
of the female secondary sex characteristics.
Progesterone – is produced
in the corpus luteum to counteract estrogen. Promotes the development
of new parts of the uterine lining and the implantation of the
early. It also prevents further follicular development (no more
eggs developing at this time).
Menstruation: The cyclic discharge
through the vagina of blood and tissues from the nonpregnant uterus.
Proliferative: The phase of
the menstrual cycle when the follicles in the ovary grow and form
an egg; the lining of the uterus is repaired and thickens; there
is a surge of hormones, and ovulation occurs.
Secretory: In this phase hormone
secretions prepare to feed the developing embryo until a placenta
If fertilization has not occured, the corpus luteum starts to break down;
progesterone and estrogen levels decreases, and blood and tissue are discharged
information on this page was compiled and paraphrased from: myDr
(for a Healthy Australia); The Anatomy Coloring Book, Wynn Kapit
and Lawrence M. Elson.
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