Getting pregnant
To understand the problem of infertility, you need to know what is normal. It may seem obvious to some people how their body works, particularly the parts to do with sexual activity and childbirth. But getting pregnant is more than just having sexual intercourse or the joining of a sperm with an egg.
FEMALE FERTILITY
Every month, an egg is released from one of a woman’s two ovaries, as part of her menstrual cycle. This is controlled by hormones formed in the brain, the pituitary gland (at the base of the brain), and the ovaries themselves. In most women, the menstrual cycle is regular and lasts about 28 days.
The main female hormones are: gonadotrophin-releasing hormone (GnRH), which is produced in the hypothalamus at the centre of the brain; follicle-stimulating hormone (FSH) and luteinising hormone (LH), which are produced in the pituitary; and oestrogen and progesterone, which are produced in the ovaries.
At the beginning of the menstrual cycle, the levels of oestrogen and progesterone are low and GnRH stimulates the secretion of FSH from the pituitary. FSH in turn stimulates an egg to grow within a follicle (a fluid sac) inside one of the ovaries.
In each menstrual cycle, around ten small egg follicles reach the size and stage of maturity at which they could respond to FSH. However, usually only one will become the major or dominant follicle in that cycle, the one destined to ovulate. All the others will die off.
As the dominant follicle grows, it produces increasing amounts of oestrogen, which enters the bloodstream. Oestrogen has a direct action on the lining of the uterus or womb (the endometrium), making it thicker in preparation for a possible pregnancy. After about 10–12 days, when oestrogen levels in the bloodstream reach a critical level, the pituitary releases a sudden surge of another hormone, LH, which acts on the follicle and the egg to prepare them for ovulation. LH reaches its peak in the bloodstream within a few hours and triggers ovulation within about 36 to 42 hours. At ovulation, the follicle bursts, releasing the ripe egg into the fallopian tube.
After ovulation, the follicle walls collapse, then re-form and mature into a small mass of yellow tissue, which begins to produce progesterone as well as oestrogen. Progesterone increases the supply of blood to the endometrium. The endometrium becomes more succulent and produces a sugar-rich compound, glycogen, which can be used by a developing embryo.
The egg can be fertilised for 24 hours after ovulation. If it meets sperm in the fallopian tube, then fertilisation is likely to occur. If the egg remains unfertilised, or if it fertilises but fails to implant in the uterus, the endometrium is shed as the menstrual period about 14 days after ovulation.
MALE FERTILITY
A man’s testes have two main functions: to make the major male sex hormone, testosterone, and to produce sperm. The production of sperm is mainly controlled by the hormone FSH and testosterone production is mainly regulated by LH, the same hormones that control egg production in a woman. In men, LH is sometimes called by another name, interstitial cell-stimulating hormone (ICSH). FSH acts on tissues in the testes called the seminiferous tubules, which are the main areas of sperm cell production. LH acts on other cell types in the testes that produce testosterone, the main hormone responsible for hair growth, muscular build and virility.
Sperm production is a more lengthy process than the production of an egg. However, in contrast to egg production, where the eggs have been present since birth, sperm production occurs from cells of very recent origin, not more than six months old. The sperm cells produced in the seminiferous tubules are immature and not capable of fertilisation. They mature as they pass through the epididymis (a narrow system of tiny tubes on the surface of the testes) into the vas deferens. Mature sperm, which are capable of fertilisation, can be found in the vas deferens (a thin cord-like structure above the testes). It takes between three and four months for sperm to develop from the immature stages within the seminiferous tubules to mature sperm within the vas deferens. During this time, sperm production may be affected by several factors, including any illness that causes a fever or exposure to substances that are toxic to sperm, such as some industrial chemicals or use of recreational drugs, such as those used for body building.
At ejaculation, powerful contractions occur in the urethra (the tube within the penis that carries urine and semen to the outside of the body), the seminal vesicles (which secrete seminal fluid) and the ejaculatory ducts. These force the sperm and seminal fluid out of the urethra in spurts. If the penis is in the vagina at the time, the force carries the sperm up to the woman’s cervix (the neck of her uterus). About 100 million sperm are ejaculated, and one million may swim into the cervical mucus. The sperm then swim through the mucus secretions in the cervix and uterus and out along the fallopian tubes, a journey that takes about 12 hours. Only about 500 to 1,000 sperm remain by the time
A man’s testes have two main functions: to make testosterone and to produce sperm. Sperm production is mainly controlled by the hormone FSH, and they reach the egg at the outer end of the fallopian tube which is where fertilisation normally occurs.
The sperm make this journey largely under the power of their own swimming ability. If the woman has an orgasm, contractions within her uterus provide some help. Sperm are able to survive for several days, much longer than an egg. They lie in wait at the outer end of the woman’s fallopian tube at the time she ovulates. Although many sperm stick to the outside of an egg, only one is usually successful in actually penetrating the coverings of the egg to fertilise it. Multiple embryos result from either multiple eggs (oocytes) being released and fertilised to give nonidentical twins or triplets or from a fertilised egg splitting into two – identical twins.
AFTER CONCEPTION
If the egg is fertilised, it begins to divide rapidly (forming a morula) and is moved towards the uterus by the cilia (hairs) which are present in the lining of the fallopian tube. The cells of the fertilised egg initially divide without increasing the overall size of the embryo. The cells get smaller as they increase in number and remain in their surrounding ‘egg shell’, called the zona pellucida.
By the fourth day, the morula has developed a fluid-filled area in its centre and is now called a blastocyst which contains 32 to 64 cells. A small number of cells in the centre are destined to be the embryo; the remainder will make up the placenta (afterbirth) and other parts of the tissues that support the pregnancy.
At about day 6 or 7 after fertilisation, the developing blastocyst reaches the cavity of the uterus, hatches out of the zona pellucida and implants (embeds itself) into the endometrium. The placenta invades the mother’s tissues and develops a firm attachment and blood supply for the ongoing pregnancy.
THE EFFECT OF AGE
Male and female fertility are both affected by age. Although recent evidence questioned this, we still consider that a woman’s ovaries contain all her eggs from birth and no more eggs will be produced during her lifetime. With increasing age, and particularly once a woman reaches her 40s, the number of follicles and eggs available in her ovaries have decreased to a degree where conception becomes less likely.
When a woman reaches the menopause, her ability to conceive is lost. At this time, her ovaries no longer contain any follicles capable of developing and maturing. However, her uterus is still capable of carrying a pregnancy, even after her own egg reserves have been exhausted. Well-publicised cases have shown this: women in their 50s and 60s have borne children as a result of treatment with donor eggs.
There are many tales of men fathering children into their 70s, and these are probably true. However, most men experience a gradual reduction in testosterone production and sperm production as they get older. They are also less able to sustain their erection to allow sexual intercourse. By 55 years, the quality of their sperm is considerably affected. There is also a steadily increasing risk of passing on chromosomal abnormalities such as Down’s syndrome after this age.
SEXUAL INTERCOURSE AND THE CHANCE OF CONCEPTION
The likelihood of conceiving depends on several factors. The younger the couple (particularly the woman), the higher the chance of success. The frequency of sexual intercourse is another important factor – if the frequency is consistently less than once weekly, a pregnancy is much less likely to occur. In addition, sexual intercourse needs to involve full vaginal penetration, so that sperm are deposited at the top of the vagina close to the cervix. However, for some couples, the time interval from trying to get pregnant to achieving a pregnancy may vary considerably without any obvious explanation.
