Pregnant women undergoing tests for foetal abnormalities could be up to twice as likely to miscarry if they attend a small hospital unit, according to new research.

Pregnant women undergoing tests for foetal abnormalities could be up to twice as likely to miscarry if they attend a small hospital unit, according to new research.

Some 30,000 women a year in the UK undergo amniocentesis or chorionic villus sampling (CVS) to detect possible problems with a pregnancy.

Amniocentesis indicates the likelihood of the baby developing chromosomal conditions, such as Down syndrome or Edward's syndrome.

CVS tests are usually carried out earlier in pregnancy in an attempt to detect serious foetal problems. They are particularly recommended for those with a family history of genetic disorders or who are over 35.

The new study, involving more than 64,000 pregnant women, found that miscarriage rates were higher at smaller hospital units.

Overall, the miscarriage rate following amniocentesis was 1.4%, and 1.9% following CVS. However, women treated in departments carrying out fewer than 500 amniocentesis tests over an 11-year period had a higher chance of miscarriage than in those attending larger units.

The risk of miscarriage was 2.2 times higher in these small units than in those performing 1,500 or more tests during the same period.

The risk of miscarriage following CVS was also 40% higher in departments that performed 500 to 1500 procedures, compared with those carrying out more than 1500.

The research, by experts at Copenhagen University Hospital, was published in the Ultrasound in Obstetrics and Gynaecology journal.

The study's Lead author, Professor Ann Tabor, said: "The very clear variations in miscarriage rates between units with different levels of expertise demonstrate that the invasive procedures play a role in miscarriage rates."

Alan Cameron, professor of foetal medicine at The Queen Mother's Hospital, Glasgow, said staff needed to keep up their skill level and that some units in the UK performed only a few procedures a year.

"It certainly would be my perception that the more you do in a centre, the lower your miscarriage rate would be.

"There's quite a clear association between how experienced staff are and the risk of miscarriage."