The sexual cycle of the woman
In order to understand why a woman cannot become (or remain) pregnant, we first need to know how her body functions hormonally and how her reproductive system is constructed and works, plus if there are any abnormalities present. This area of medicine is the domain of the gynaecologist. However, it is primarily due to reproductive medicine and its manipulation of hormones and their effects, that women who at one time would never have been able to conceive, are able to have children today.
Female hormones
The fertility of the female is controlled by the same hormones as those required for sperm production in men.
The hypothalamus regularly secretes (every 60-90 minutes) the hormone GnRH (gonadatrophin releasing hormone), which stimulates the release of LH and LSH from the hyperfyse, a small gland located just below the hypothalamus,
if the sex organs have signalled that this is necessary (low oestrogen level, see below).
The big difference between the male and female systems is fact that the hormonal process in women follows a pre-determined cycle pattern. Unlike a man, the woman does not need to constantly manufacture reproductive cells. She has all the eggs she will ever have right from birth. However it is necessary to allow one of them to ripen every month and be released, so that it can have the chance of becoming fertilized.
Supply of eggs
A woman is born with a lifetime supply of 300 000-400 000 egg cells, which are distributed over the two ovaries, in so called 'sleeping follicles' The egg cells are enclosed in feeding cells called granular cells.
The woman's body does not manufacture any new egg cells at any point in her lifetime. On the contrary, a natural degeneration process of egg cell results in a reduction of eggs to a mere 100 000-200 000 by the time of puberty. At the time of menopause, no viable eggs remain and the monthly cycle of egg-ripening and release stops.
During this monthly process, one egg is ripened and released from one of the ovaries. Actually several eggs begin to ripen, but only one reaches maturity.
Even before puberty and during your fertile years, egg cells naturally degenerate and die. This even happens during pregnancy and whilst taking a contraceptive pill. Contrary to what many people think, no more eggs are lost during fertility treatment than during a natural cycle. In fact, quite the opposite is true. The hormonal stimulation restricts the number of eggs which would otherwise be lost during the natural ripening process, so that more egg cells survive the cycle. |
The menstrual cycle
In theory, the menstrual cycle has a duration of 4 weeks. In reality however, the length of the cycle varies from woman to woman and indeed from cycle to cycle. It begins with the ripening of one egg cell and ends either in menstruation or pregnancy. After ovulation, the egg cell is pick up by the feathery finger like ends of the fallopian tubes where it can potentially be fertilized by a sperm. Tiny hairs on the inside of the fallopian tube fan the as yet unfertilized egg in the direction of the uterus, the lining of which has been prepared to receive a potential embryo, which might implant and develop into a baby.
If the egg isn't fertilized, or if implantation doesn't occur, menstruation takes place. The mucous membrane lining of the womb is shed and expelled via the vagina. In the chart below, the progression of the monthly cycle is shown. There are two phases; the follicular phase and the luteal phase.
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The Menstrual cycle illustrated |
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An Egg cell surrounded by Granular cells |
From Day 1 to ovulation
(follicular phase)
The menstrual cycle is set in motion as a result of low oestrogen levels. The diminished levels of this hormone in the woman's bloodstream triggers the hypothalamus to produce larger amounts of GnRH and thereby stimulate the hypofyse to produce FSH and LH. (See hormones and what they do).
Under influence of FSH (follicle stimulating hormone) several follicles in the ovaries begin to develop. Follicles are tiny fluid filled sacs in which the egg cell is located. Follicles produce and release oestrogen, which stimulates the lining of the womb to thicken in preparation for the potential implantation of an embryo. The rise in oestrogen from the follicles stimulates the hypothalamus to reduce the levels of GnRH. The levels FSH then drop, causing a halt in the development of the follicles. However, there is always one dominant follicle which is able to adjust to the lower levels of FSH and continues to grow. Under the influence of FSH and LH (luteinising or ripening hormone) the egg cell within the dominant follicle matures.
When oestrogen levels reach sufficient levels, the production of LH reaches its peak, causing ovulation to take place. 32-48 hours elapse between the LH peaking and the ovulation itself. LH peak occurs around day 12 of the cycle and ovulation around day 14, based on a normal cycle.
From ovulation to day 28
(luteal phase)
After ovulation of the ripe egg cell, the journey towards the uterus begins. During this time, the empty follicle in the ovary where ovulation occurred, begins to alter into a yellowish structure called the Corpus Luteum, which continues to produce oestrogen just like the follicle did, but additionally begins to produce progesterone. Together, these hormones stimulate the development and thickening of the lining within the uterus and suppress the production of FSH ans LH in the hypofyse.
periods If the egg cell does not become fertilized, the corpus luteum begins to generate about 10 days after ovulation. Oestrogen and progesterone levels sink considerably, causing the lining of the womb to begin breaking down. The woman menstruates and the production of FSH and LH begins again. The first day of menstruation is the first day of the new cycle.
In order to time a fertility treatment, the first day on which the woman gets up in the morning with blood loss is referred to as day 1.
Natural fertilization
At 0.2mm, the egg cell is the largest cell found in the body. Sperm cells on the other hand, are amongst the smallest. If a sperm cell finds its way to an egg cell at just the right moment and manages to penetrate it, we can speak of new life.
- After ovulation, the egg cell remains viable for about one day. Sperm remains viable for about two days after ejaculation.
- Fertilization occurs in the fallopian tube, during the eggs' journey towards the uterus. Millions of sperm swim up in the other direction from the vagina. Semen constitution changes drastically once it arrives in the vagina. The viscous liquid becomes much thinner in consistency and the vast majority of sperm flows back out through the vagina, or doesn't make it through the mucous plug at the neck of the womb. At most, a few hundred of the fittest sperm complete the journey all the way to the fallopian tubes. If there is an egg present in either of the tubes, the chances are it will be fertilized, by one single sperm. It can be compared to climbing Everest. Only the fittest and strongest make it to the summit.
- The embryo that is created divides many times and becomes a blastocyst (an embryo consisting of many cells, with a liquid hollow). Over a period of 5-6 days, the embryo is transported to the uterus.
Implantation
Within the uterus, a recognition process must take place between the embryo and the lining of the uterus. Once this happens, the embryo implants in the lining and hCG, or pregnancy hormone begins to be produced. The hCG ensures that the corpus luteum remains intact and that it continues to produce and release oestrogen and progesterone. During early pregnancy, delicate hormonal balances are crucial in the continuing delvelopment of the embryo.
Quite quickly, the main source of hCG is produced by the placenta, the part of the embryo which connects to the uterus and ensures a nutritional exchange between the mother and embryo. Presence of hCG can then be detected in the mothers' urine and a positive pregnancy test can be performed. The placenta produces oestrogen and progesterone as well. By the end of the first trimester, the continuing evaluation of the pregnancy is completely autonomous in its progression.