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Female Hormones
The main feature of a woman's reproductive years
is the menstrual cycle. This process, in which an egg (oocyte)
ripens and is liberated every month ready for fertilization, is
controlled by an elaborate system involving the gonadotropins
hormones.
A women's monthly menstrual cycle occurs in three phases:- Follicular phase: Days 1 to
13.
- Ovulatory phase: Around day
14.
- Luteal phase: Days 15 to
28.
The cycle is controlled by the following
hormones: - Follicle stimulating hormone
(FSH)
- Luteinizing hormone (LH)
- Oestrogen
- Progesterone
Day one of the menstrual cycle is marked by the
first day of the menstrual bleeds. The first phase of the cycle is
called the follicular phase and lasts approximately 14 days. The
second phase, after ovulation has taken place, is called the luteal
phase (1).
The number of eggs that a woman can produce in her lifetime is
determined before birth. During her embryonic life, millions of
special germ cells are produced in the ovaries. Many of them
degenerate and the rest are ready to develop into mature eggs when
the time comes.
Three months before ovulation, up to 300 eggs are recruited for
growth and development. Each of the eggs is contained in a
fluid-filled sac or ‘follicle' that surrounds and nurtures it
during development (5). About two weeks before ovulation, the
secretion of FSH rises (3), stimulating the growth and development
of the follicles. The growing follicles secrete increasing amounts
of the hormone oestrogen (4), which causes the lining of the uterus
(or endometrium) to thicken (6), and promotes changes in the
cervical mucus that enable optimal penetration of the sperm.
Under the influence of FSH, one ‘dominant' follicle grows and
matures faster than the others that then degenerate (5). About 32
hours before ovulation, oestrogen secretion peaks, causing a surge
in the production of LH (3,4). This triggers the release of the
mature egg from the dominant follicle (5). What remains of the
follicle is subsequently transformed into the corpus luteum, which
secretes progesterone to prepare the endometrium for implantation
(5). Due to the progesterone, the body temperature rises (2).
If the egg is not fertilized by a sperm within 72 hours of its
release from the follicle, the corpus luteum will ultimately
degenerate and menstruation occurs (5). However, if fertilization
does take place and the resulting embryo continues its development,
hCG is produced. This causes the corpus luteum to secrete the two
hormones oestrogen and progesterone to ensure favourable conditions
for the implantation of the fertilized egg. In fact, the presence
of hCG in the serum or urine is the first indicator of early
pregnancy.
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