Being exposed to the pesticide DDT in the womb could delay a woman becoming pregnant as an adult, researchers suggest.

The chemical has already been linked to premature births and low birthweights.

DDT was banned in the USA and UK around 30 years ago after concerns were raised over its effects on the environment, animal and human health.

But it is still used in some developing countries to kill malaria-carrying mosquitoes.

US researchers began following a group of pregnant women in California in the early 1960s.

They took blood samples from them while they were pregnant, and were then able to follow them and their children through the following decades.

For this research, the team from the Centre for Research on Women's and Children's Health at the Berkeley Public Health Institute looked at maternal blood concentrations of DDT and DDE, a by-product of the chemical in 289 women.

They then compared them with how long it took their daughters to become pregnant around 30 years later.

The daughters, who were aged between 27 and 31, were asked about the number of menstrual cycles during which they had used no contraception to establish the period in which they could have become pregnant.

The researchers then devised a "fecundability ratio" in which the probability of becoming pregnant in each menstrual cycle was compared with levels of exposure to the DDT.

They found the higher the concentrations of DDT in their mother's blood, the longer it took the daughters to become pregnant.

The daughters' probability of becoming pregnant fell by 32% for each 10 microgrammes of DDT per litre of blood.

Delays

However, the daughter's chances of becoming pregnant increased by one- sixth per 10 microgrammes per litre increase of DDE concentrations in maternal blood.

The researchers suggest this may be because DDE offsets the harmful effects of male hormones called androgens which have been linked to polycystic ovaries, a cause of infertility.

So although DDT may delay pregnancy, DDE may protect against this cause of infertility.

The researchers say this may be why large variations in human fertility have not been seen since the introduction of DDT.

Delays

Dr Barbara Cohn, who led the research, told BBC News Online: "This is the first scientific report to make a link between DDT and reproductive impact on women exposed in the womb.

"It opens up the potential for studying the effect of exposure to many other substances, such as other environmental chemicals."

She said more work would also be needed to see exactly how DDT might affect the reproductive system.

Dr Cohn added: "While it is reassuring that possible harmful effects of DDT may be reduced by its conversion to DDE, women still experienced delays in becoming pregnant.

"Our findings could eventually lead to new understanding about the causes and prevention of sub-fertility."

But Dr Richard Sharpe, a senior scientist at the Medical Research Institute, told BBC News Online:"This is very interesting and intriguing research.

"But it's just not immediately obvious how DDT is exacting that effect."

The research is published in The Lancet.