A Scottish couple whose chances of conceiving were as low as one in 20 have had a baby boy after becoming the first in the world to use a new fertility drug.

The treatment mimics the natural processes that take place in fertile women more closely than other conventional medicines.

Doctors said their success marked a new chapter in the care available to people struggling to start a family.

The couple, who had been trying for a child for three years and had already experienced a failed attempt through IVF, eventually conceived using the new drug and their son was born last month. In statement the couple, who did not wish to be identified, said: "It is an amazing feeling having him in our lives and we could not be happier now."

In IVF eggs are removed from the ovaries and mixed with sperm in a laboratory in the hope some will fertilise and can be transplanted back inside the mother. The woman is given drugs to stimulate the growth of several eggs in the ovary at once, so a number can be extracted and increase the chances of success.

The main component of these drugs is the follicle stimulating hormone (FSH). Traditionally the medication was made using FSH purified from the urine of menopausal women. It was a significant step forward when biochemists managed to produce the hormone in a laboratory.

However, in fertile women FSH does not act alone to encourage the growth of an egg each month. The pituitary gland releases another chemical, luteinising hormone.

Now a drug called Pergoveris has been produced which combines both luteinising hormone and FSH. There is hope that using the two hormones together may improve the quality of the embryo created and increase its chances of survival.

Professor Richard Fleming, scientific director of the Glasgow Centre for Reproductive Medicine (GCRM), the private fertility clinic where the treatment was successfully used for the first time, said: "It is trying to replicate what happens in nature more than the pure FSH drug."

GCRM doctors believed the 36-year-old woman who gave birth after using the treatment, which typically costs around %500 for a 10 to 12 day course, stood a lower than average chance of conceiving even through IVF because of her hormone profile.

Professor Fleming said: "Usually, we expect a pregnancy rate of 35% in patients of this age. But in a case like this we would expect a success rate more in the region of 5% to 10%. Tests before treatment showed that her ovarian function was below normal, with only a limited supply of eggs available. Most patients like this do not respond well to fertility drugs."

Ideally five to 15 eggs are harvested for use in IVF. Even with Pergoveris only two were available in this case, but still the couple beat the odds.

Professor Fleming said it was always thrilling at GCRM when a patient gave birth, but added: "When it happens for a patient whose chances are limited there is a smile on everyone's face."

Doctors are increasingly able to find out more about a patient's fertility - such as the number of eggs the woman has in reserve - before embarking on IVF. Professor Fleming said success with drugs such as Pergoveris marked a new chapter in which patients could be offered more individual treatment.

Dr Marco Gaudoin, medical director of GCRM, said: "The important bit is being able to tailor treatment to individual women. Cancer specialists are talking about giving tailored cancer therapy once they know about cancer genetics. We can actually do something like that now with infertility treatment."

The couple said: "Staff at the GCRM were fantastic throughout the whole process and we cannot thank them enough."