Nicola and Nigel Dawson have experienced the frustration and heartbreak of infertility. The couple tried for several years to conceive naturally, but without success. "We learned that Nigel had problems with both the quality and quantity of his sperm, and that I had some irregularities in my hormone levels," recalls Nicola, a project manager.

That led to them starting fertility treatment. At first that involved three cycles of intracytoplasmic sperm injection, a technique widely used to tackle male infertility, which cost £15,000. Again, that did not work. "At the end of that we found out that the doctors thought I had premature ovarian failure – basically that I was heading for an early menopause, even though I was just 34," says Nicola, who is now 37. "My ovaries weren't working normally, my egg supply was diminishing and the quality was poor."

A doctor suggested that she try donor treatment. But when they went to one of London's most-respected private fertility clinics, they learned that it would take between 18 months and two years to receive donated eggs anywhere in the UK. "We knew the waiting times here were horrendous. But the consultant told us they were much, much shorter abroad, and mentioned Spain," adds Nicola.

Spain is a centre of so-called fertility tourism because it has good clinics and, crucially, women there are paid €900 for their eggs, so there is no shortage.

So the Dawsons opted to have donor treatment at the IVI clinic in Barcelona, which has a steady stream of British patients. "We only had to wait five weeks and their success rate was 65%, far higher than the 40-45% at the British clinics we checked out," says Nicola. The couple had their shape, height, hair colour and skin tone analysed, as well as their blood group and rhesus status, to try to ensure as good as possible a match with the anonymous Spanish donors, one female and one male. In January 2008 Nicola was "matched" with an egg and a sperm donor and had two embryos implanted.

That September, with the arrival of twins, the couple finally realised their dream of starting their own family. "It was just amazing when Mia and Hannah were born. They mean absolutely everything to us. We couldn't love them any more if they were genetically ours. They are the family we always wanted," she says. The whole process, including flights, accommodation, blood tests and scans, came to £9,000.

Their happiness is tempered with frustration. "I don't feel we were forced to go to Spain. But the shortages of eggs and sperm here, and long waiting-times, have certainly pushed people to go abroad," says Nicola. "If someone's had years and years of IVF but hasn't had a child, then they find out that they need donor treatment, and then learn that there's an 18-month wait until you can start and then, if that fails, another 18-month delay, it could be many years until you get a child, and that's if it works. It's awful to be needing donor eggs in the first place. Not to be able to have a family because your eggs are no good is devastating. Women needing eggs is a very common problem."

Indeed it is. One in seven British couples has trouble conceiving, though not all undertake donor conception. And not everyone can afford anything like the sums which the Dawsons spent to finally become parents. Their only regret is that Spain's system of donor anonymity means their daughters will never be able to find out who their natural parents are, which is a frustration. "Mia and Hannah's genetics are Spanish, but we hope their roots will be with us because they will grow up with us," adds Nicola.

The Dawsons were helped by Infertility Network UK, the country's leading support service for people who have problems conceiving.

Donor conception explained

Donor conception involves having a baby or babies after undergoing fertility treatment using someone else's eggs, or sperm, or both, or another woman's embryo.

In 2008 a total of 1,600 children were born in the UK as a result of donated sperm (977), eggs (541) or embryos (82). That represented 11% of the total of 15,237 births that year due to either in vitro fertilisation (IVF) or donor insemination (DI) – the highest number ever.

"The experience of people who have had donor-conceived children shows that this can be a positive way to create a family," says the Human Fertilisation and Embryology Authority (HFEA).

In 2008 the HFEA registered just 396 sperm donors – far fewer than experts say is needed to help women conceive. Although it was the highest since 1996, fewer men donate sperm than previously, mainly because donors lost their right to anonymity in 2005. Children born as a result of a donation are allowed to find out who their biological father is when turning 18.

Sperm donors must be aged between 18 and 45, though the upper limit is coming down to 40 from next April. Men must undergo counselling and medical screening before being able to become donors.

Egg donors have to be aged 18 to 35, as women's fertility starts to drop after 35. As with men, screening tests are used to identify anyone likely to pass on diseases or abnormalities as a result of the woman's donation. Both sexes have to give written consent.

Payment of both sperm and egg donors is banned. But both types of donor can claim back "reasonable" out-of-pocket expenses, such as for food and travel costs, and also up to £55.19 per day in lieu of loss of earnings – but only up to a maximum of £250 – each time they donate. Some donors do not claim the £250 as they see their action as entirely altruistic, and thus not needing any reward.

However, HFEA rules do allow a woman who takes part in an egg-sharing scheme to get her own fertility treatment free or at a big discount in return for donating half her eggs to the clinic– a benefit potentially worth several thousand pounds. The clinic then uses her surplus eggs to help treat another patient. In 2008 some 400 women entered into egg sharing arrangements, while 784 donated altruistically.

In 2008 some 1,306 women were treated with donated eggs – the lowest number since 1995 – and 204 with donated embryos. Another 1,916 received donated sperm – a huge drop on the 4,496 of a decade earlier.

Any donor-conceived child, when they turn 16, can ask the HFEA for details of their donor – such as their height, weight, hair colour, occupation and year of birth – and the same for any half-siblings they have as a result of that donation, though not their identity. But they are able to find out the identity of their donor once they reach 18, and make contact with them if they wish.