Almost 24 years after Louise Brown became the first so-called test tube baby, the in vitro fertilisation (IVF) techniques that brought her into the world have run into trouble.

A white mother has given birth to black twins, following a laboratory mix-up. Though the first mistake of its kind to come to light - and then, worryingly, only because of the skin-colour discrepancy - the rules which govern the fertilisation of eggs outside the womb are again under scrutiny.

Test tube births doubled in the 1990s

During the 1990s, the number of women undergoing IVF almost doubled. In 1992-3 some 14,996 women received treatment, according to Human Fertilisation and Embryology Authority (HFEA) figures. By 1998-9 the number had climbed to 27, 151.

Despite improvements in the techniques used to harvest eggs, introduce them to sperm and replace them in the womb, the proportion of babies being born has not increased significantly.

In 1992-3 some 3,343 test tube babies were born, compared to 8,498 in 1998-9.

This is due to social trends, rather than medical ones, says Dr Brian Lieberman, director of reproductive medicine at St Mary's Hospital, Manchester.

"The single most important factor in successful IVF treatment is the age of the mother, that's followed by the duration of infertility. That more women are delaying having children has mitigated improvements in IVF."

High expectations

Stories of "miracle births" have raised couples' expectations despite the fact that the success rate of IVF procedures has remained "stubbornly low" at around 17%, according to Ruth Deech, a former chairman of the HFEA.

Dr Lieberman says that while some prospective IVF candidates arrive at his NHS clinic well-informed about the procedures, others have little understanding of the difficulties or are so desperate to conceive that they "take in the positive information, but ignore the negative".

More than 70 clinics offer IVF in the UK, with around 80% of them charging private patients �3,000 for each "cycle" of treatment.

The success rate of IVF is much debated

With the six leading private clinics estimated to be making an average of �1.8m each a year, concerns about the already controversial IVF technique are growing.

Some fertility experts - Dr Lieberman included - have queried the way in which the HFEA's patient data is presented, fearing that couples may be attracted to a particular private clinic's success rate without taking into account other factors - such as the age of its patients or the techniques used.

One concern expressed by many in the field is that the number of multiple births has increased in recent years - the chances of parents having IVF triplets rose from one in 200 to one in 20 during the 1990s.

With multiple births increasing the risk of premature delivery, the possibility of a caesarean and the chance deformities and disabilities, UK clinics were last year directed not to return more than two embryos to a patient's womb.

Some have suggested even tighter rules which would see only a single fertilised egg placed in the womb, as is the case in Scandinavia.

Such a move would certainly do little to improve IVF's success rate, but Dr Lieberman says that for many in his field "success is measured only by the birth of a healthy, non-handicapped baby".