In vitrofertilisation is often held up as the prime example of healthcare rationing. Its availability has depended on the willingness of individual health authorities to fund treatment - often hedged with restrictions and eligibility criteria - or a couples ability to pay for treatment privately.

But this position is likely to change within the next couple of years.

Former health secretary Frank Dobson ordered a review , with the intention that everyone should be able to receive some IVF treatment.

Although this may still include some restrictions - for example, an age threshold for the mother - it will mean HAs have to reassess their spending priorities and, in some cases, purchase IVF for the first time.

IVF technology has moved on tremendously since Louise Brown, Britains first test tube baby, was born in 1977. She is now one of more than 30,000 children and young adults conceived using the technique in Britain.

Elsewhere, IVF is even more common - around one in 60 children born in Sweden are conceived in this way .

The technique is not a panacea for the one in six couples who have difficulty conceiving and was originally seen primarily as a method to help women with blocked fallopian tubes.

However , it is now also used in cases of male infertility, where sperm are of too poor a quality or are too low in number to fertilise an egg in the normal way . Women with cervical mucus problems or endometriosis, and couples with unexplained infertility, may also benefit from IVF .

Couples seeking IVF are likely to be carefully screened before treatment is offered. The woman will typically be given drugs to stop her ovulating, although some clinics prefer just to collect the eggs, which are released naturally.

Blood tests and ultrasound will confirm that the woman is not releasing an egg, and her ovaries will then be artificially stimulated to produce several eggs through daily injections or tablets of superovulatory drugs.

The growth of the eggs within fluid filled follicles is measured. When they are the right size a single injection of another drug - human chorionic gonadotropin (HCG) - is given to ripen the eggs and prepare them for removal.

The ripened eggs are then removed, under general or local anaesthetic, together with painkillers and sedatives. A thin needle is inserted through the vagina into the ovaries while they are scanned on ultrasound. The fluid within each follicle is sucked out and searched for eggs under a microscope.

In some cases the eggs will be removed using a laparoscope inserted through the abdomen under general anaesthetic.

A few hours later the woman's partner will be asked to produce a semen sample. The most vigorous sperm can be selected - using special washing techniques - and put into a dish with both the eggs and a nutrient-rich fluid to aid fertilisation.

If fertilisation occurs it should be detectable the following day , and on the second or third day after egg retrieval the eggs will be put into the woman s uterus. This involves inserting a catheter through the cervix, but is relatively simple, and no painkillers or anaesthetic are usually needed.

In some cases embryos can be frozen and transferred to the woman later, although this has a lower success rate.

For the next fortnight the women will be given hormones, either as injections or pessaries, to help establish a pregnancy . A pregnancy test will confirm whether the transfer has been successful and ultrasound scans may be given to establish how many embryos are growing.

Sperm or eggs donated by volunteers may also be used in IVF treatment, depending on the cause of the infertility.

The main side-effect of IVF treatment is ovarian hyperstimulation syndrome, which can cause the woman to suffer from pain, swelling, nausea and vomiting.

Severe cases will need hospitalisation, and it is relatively common for treatment to have to be abandoned because of the woman s adverse reactions.

The major advance in IVF treatment in the 1990s has been the introduction of intra-cytoplasmic sperm injection (ICSI), where a single sperm is injected directly into an egg. It was initially used to help couples where the man had a very low sperm count and where donor sperm would otherwise be the only way to achieve a pregnancy .

But its use has extended rapidly - as well as ejaculated sperm, surgically obtained immature sperm and spermotids from the epididymis and testes have been used.

When ICSI was first introduced in the mid-1990s it had a success rate of around 4 per cent per treatment cycle: this has increased to 21.6 per cent, higher than with standard IVF.

The safety of IVF has been extensively researched, and there is little evidence that babies born through this method fare worse than babies conceived naturally . But because several embryos are transferred to the mother s womb, IVF tends to lead to multiple births.

In Britain in 1996-97, for example, there were 262 triplet or quadruplet births attributed to IVF: half of all triplets born in Britain are conceived using IVF . Multiple births are associated with low birth weight, prematurity and an increased risk of death during infancy .

A Swedish study showed IVF babies were five times more likely to be born prematurely and have low birth weight than non-IVF babies - which explains their slightly higher death rate.

1 But the study also revealed a slightly increased rate of malformations among singleton births, which was hard to account for .

Multiple and premature births have cost implications for the NHS, as well as posing a risk for the babies, and there has been strong pressure on clinics to reduce the number of embryos transferred.

The Human Fertilisation and Embryology Authority , which regulates clinics carrying out IVF, limits the number of embryos transplanted to three. But there are arguments that this should be further reduced to two, to lessen the risks of multiple births. Many clinics still transfer three embryos, although the difference in success rates between transplanting two and three is slight.

Experts disagree about whether ICSI carries extra risks. A report suggesting that children conceived by it showed minor development delays by the time they were a year old has been criticised by other specialists.

2 However , it is possible that genetic disorders which led to low sperm counts or less mobile sperm could be passed on to a son.

Over the next few years there are likely to be considerable advances in IVF. One area which is already attracting controversy is the ability to identify genetic disorders and other characteristics once the embryo has been fertilised, but before implantation.

This could stop embryos which are likely to miscarry being put in the womb but could lead to baby selection. Tweaks in techniques may both help to raise the take home baby rate and offer treatment to couples whose infertility was thought to be incurable. Cases where sperm were thought to be incapable of breaking through the eggs outer coverings and fertilising it have already been overcome with methods such as zona drilling and partial zona dissection.

IVF could also allow couples to have their families later in life: one possibility is the removal of immature eggs from a young woman which could be stored for years, fertilised and then implanted, allowing women to have their own children beyond the menopause.