Couples are being warned about seeking fertility treatment abroad.
Experts fear UK couples are looking abroad for treatment
Since laws came in last year removing the right of anonymity for UK donors, European fertility clinics have seen a boom in UK customers.
But the Human Fertilisation and Embryology Authority is urging people to think twice about fertility tourism.
The watchdog said standards in many countries do not match those in the UK, adding women could put themselves at risk by going abroad for treatment.
Dame Suzi Leather, chairman of the HFEA, said: "We know that a relatively small number of people choose to travel abroad to undergo fertility treatment and that sometimes the treatment is packaged as a 'holiday' where the patient can convalesce in the sun.
"However, we are concerned that people who choose to have their treatment abroad should know about the potential risks.
"We have heard of some clinics which offer treatment to patients that is so dangerous that it has been banned in the UK - for example implanting five embryos which significantly raises the chance of multiple pregnancy - the biggest risk of IVF for both mothers and babies.
"It is very sad when we receive complaints from patients about their treatment abroad and we are not able to help or reassure them.
"We would urge patients to think twice and consider the risks and implications before going abroad for treatment."
The HFEA, which regulates clinics in the UK, said there were a number of issues patients should check before seeking treatment abroad, such as the level of patient confidentiality and what rights they would have if treatment goes wrong.
The HFEA said it did not have any figures about the number of people heading abroad, but said patient groups had been warning people were becoming attracted to foreign markets.
Clare Brown, chief executive of patient support group Infertility Network UK, said she shared the HFEA's concerns about fertility tourism.
"There is no doubt that the care and treatment in UK centres is of the highest standards and that these standards may not apply in some foreign clinics.
"However, some patients are being forced to travel overseas as they are unable to access the treatment they need in this country.
And she added patients would not travel abroad for treatment if they could access it in the UK, but there were "unacceptably long waiting lists" in some areas and some preferred to have treatment from anonymous donors.
But British Fertility Society secretary Dr Allan Pacey, of the University of Sheffield, said while couples should give "due consideration" to safety when going abroad for treatment, the vast majority of clinics, particularly in Europe, have as good a standard of care as the UK.
Although he did acknowledge the problem in finding treatment in the UK was a concern.