Cancerous cervical cells are detected through smear tests

Researchers from the Royal Preston Hospital, Lancashire, analysed 27 studies looking at various treatments.

Laser ablation was the only treatment not linked to any pregnancy related risks, the Lancet study found.

But UK experts said women who needed treatment should go ahead, as the risk of pregnancy complications was small.

Where cervical smear tests detect worrying cells, women are sent for further investigation.

It may be decided that the abnormal cells, and possibly some around them, should be removed.

'Halt progression'

There are a number of techniques which are used to remove or destroy the abnormal cells.

Women in the UK should not be too worried by this report, as the risk of developing serious side effects during pregnancy is very small But Professor Peter Sasieni, Cancer Research UK

Ablation uses a laser to destroy the cells.

Cold knife conisation involves using a scalpel to remove a cone-shaped wedge of tissue around the abnormal cells. Laser conisation uses a laser to do the same thing.

But the most common treatment used is LLETZ (large loop excision of the transformation zone) which uses a small tool and electric current to remove the area containing the abnormal cells from the cervix.

The techniques are equally successful at preventing progression to cervical cancer.

Dr Maria Kyrgiou and her colleagues looked at the existing studies to assess what impact each treatment had on future pregnancies.

They found that cold knife conisation increased the risk of giving birth prematurely, of having a low birthweight baby and of having to have a Caesarean.

LLETZ also increased the relative risk of having a premature or small baby. But it also raised the chances of a woman's membranes rupturing prematurely.

Laser conisation was found to have similar adverse effects, although not to the same degree.

No increase in pregnancy-related problems was seen after laser ablation.

Counselling

Writing in the Lancet, Dr Kyrgiou said: "Evidence clearly indicates that excision of the transformation zone is associated with a small but real increase in risk of pregnancy-related morbidity.

"This information should be considered when counselling women before their consent to treatment and lends support to the philosophy of doctors not treating young women with mild abnormalities."

But Professor Peter Sasieni, a Cancer Research UK expert on cervical screening, said: "Women in the UK should not be too worried by this report, as the risk of developing serious side effects during pregnancy is very small.

"If a woman has been told she needs treatment for any abnormalities of the cells, it could be essential that she has the necessary treatment to prevent cervical cancer from developing, but she should discuss all the options with her doctor."

He added: "It is important to have published information so we understand more about the side-effects that treatment for cervical abnormalities can cause.