New Zealand women have been warned to take care with popular painkillers during pregnancy, after a European study found a link to potential fertility problems in baby boys.

The Danish study found that boys born to women who had used aspirin or ibuprofen (which is often sold as Nurofen in New Zealand) during pregnancy, especially during the fourth to sixth months, were more likely to have undescended testes, a condition known as "cryptorchidism". Paracetamol showed the same trend, but the evidence was weaker.

Although these results arose only in part of the study - among women from Denmark and not those from Finland - they are reinforced by experiments which found exposure to paracetamol reduced production of the male sex hormone testosterone in rats.

"Studies from the 1980s have suggested a link between prenatal exposure to mild analgesics and reduced masculinisation in animals," the Copenhagen University researchers say in the journal Human Reproduction.

"This is of concern, because several reports have indicated an increase in the incidence of male reproductive disorders over recent decades ..."

They found the greater the use of mild painkillers in pregnancy, the greater the risk of congenital cryptorchidism.

"In particular, use during the second trimester increased the risk. This risk was further increased after the simultaneous use of different analgesics."

Undescended testes fail to develop normally. They are incapable of normal sperm production and increase the risk of testicular cancer, although surgery can correct cryptorchidism.

The New Zealand Self-Medication Industry, which represents suppliers of non-prescription medicines, said that painkillers - and all medicines - should be used during pregnancy only on the advice of a doctor. The organisation's executive director, Tim Roper, said aspirin, ibuprofen and paracetamol were available without a prescription at low doses for short-term use.

Aspirin and ibuprofen packaging labels already contained warnings against use during the last three months of pregnancy and suggested they should be used earlier in pregnancy only on the advice of a doctor.

Mr Roper referred inquiries about Panadol, a form of paracetamol, to the supplier, GlaxoSmithKline, which referred the Herald to him.

GSK's Panadol website makes no mention of pregnancy. A Health Ministry-approved GSK data sheet for Panadol says paracetamol has been taken by a large number of pregnant women without any proven observed increase in the rate of harmful effects on the fetus. Adults should use Panadol for no more than a few days - and children for no more than 48 hours - except on medical advice,

The ministry's medicines safety division Medsafe noted the Danish researchers acknowledged that further investigation was required.

"Medsafe considers that the results of this study do not imply that any woman who has taken a mild analgesic medicine during pregnancy will have adversely affected the health of their baby.

"It is already advised that some types of analgesics - such as aspirin or ibuprofen - should not be taken during pregnancy. Medsafe advises pregnant women to seek advice from their doctor or pharmacist before taking any medicine."