Couples that seek to have twins through in vitro fertilization (IVF) are putting the health of mothers and babies at risk, British health experts said.

Fertility clinics were warned late Thursday to restrict the number of twin pregnancies they provide after figures showed that carrying more than one child at a time was the single highest risk factor for the success of a pregnancy and the health of mother and infant.

Lisa Jardine, chairwoman of the Human Fertilization and Embryology Authority (HFEA), said many couples tried for twins through IVF because they thought it was their best chance of having a bigger family.

But she warned that the "hidden side" of multiple pregnancies was an increased chance of miscarriage and health complications.

Infant mortality for twins within the first month of life is 19 per 1,000 babies, compared with only three deaths per 1,000 for single babies.

The risk of cerebral palsy climbs from 1.7 cases per 1,000 live births for single babies, to 6.2 cases per 1,000 for twins.

Twins make up around 22 percent of the 12,200 IVF births in Britain each year, compared with around one percent in the overall birth rate.

During IVF, a woman's egg is fertilized by a partner's or donor's sperm, and then the embryo created is implanted into her womb. More than one embryo can be produced and transferred back into the mother during this process.

The HFEA recommended that younger women should be implanted with one embryo, with a second frozen and used as a back up only. But older women, or those with poor quality embryos could still receive the double embryo transfers, the group said.