Infertility doctors have long been aware of "spontaneous conception" - a naturally occurring pregnancy after the birth of a child conceived through in vitro fertilisation.

Now a Japanese study points to who is most likely to benefit from the effect.

It found that overall there was an 18% chance of spontaneous conception after IVF treatment, but that the likelihood rose to 37% among women under the age of 27.

Unexpected pregnancy

The study was performed by Dr Yasushi Shimuzu and colleagues at Akita University School of Medicine, and was published in the journal Fertility and Sterility.

Modern techniques have made fertility treatments more successful

The authors wrote: "When a spontaneous conception unexpectedly occurs after the birth of an infant conceived through IVF, the couple may suspect that the previous IVF treatment was unnecessary."

However, rational explanations for such incidents are usually impossible, they said, "especially in view of the long-standing infertility before conception".

The study aimed to examine the incidence of such cases of spontaneous conception, and to identify clinical factors that might cause it.

It looked at 142 women aged 24 to 40 who had had a baby through IVF. Twenty-five of them later conceived spontaneously.

The scientists found that "patient age was the most important clinical variable related to the occurrence of spontaneous conception.

Added benefits

Professor Ian Craft, director of the London Fertility Clinic, said the research supported earlier studies.

He said that people who had undergone successful fertility treatment once were more likely to benefit from it again.

In some way, he said, this indicated that one pregnancy could help clear up problems that had previously caused infertility.

However, spontaneous conception was unlikely to occur in cases where the man had "poor sperm performance" - a low sperm count or slow-moving sperm - or where the woman had stopped producing eggs.

There may be problems with people seeking fertility treatments before it was clear that they really needed them, he said.

"Most people under the age of 27 wait at least one year and usually two years trying to get pregnant."

He said there was a 75% chance of a successful pregnancy after two years of trying.

Pressure to bear children

Different people had different pressures, and some cultures would find it unacceptable to wait for a baby, he said.

But "over-enthusiastic fertility treatment" such as IVF was unnecessary at an early stage in people who have "no other known abnormality".