Women who live in areas with high levels of pollution are more likely to have twins, research suggests.
Living near incinerators may increase the chance of twins
German scientists found the rate of twin births in areas with high environmental pollution were double that of other areas.
The rate was highest among women living near a toxic waste incinerator, the authors say in the Occupational and Environmental Medicine journal.
They recommend further research to find out why this might be.
The University of Hamburg team compared twin birth rates among mothers living near an incinerator in Hesse and mothers living in two other regions of Germany.
Proximity to pollution
One region was 20km north of the incinerator and the other was a non-industrial area much further away.
In areas where people lived close to the incinerator or other heavy industries, the twin birth rate was about twice as high as other areas.
In Hesse, 5.3% of mothers had twins compared to 1.6% 20km away and 2.3% in the non-industrial area.
Dr Nadia Obi-Osius and colleagues then checked the trend was not due to fertility treatment, which is known to increase the likelihood of multiple births.
Fertility treatment was more common in the non-polluted areas than in the polluted regions, and the twinning rate was no higher among women undergoing fertility treatment.
In the UK, the proportion of multiple births has increased by 20% in the last 10 years, according to the Office for National Statistics.
In 2002, the multiple birth rate was 15 per 1,000 women giving birth, compared with 12.5 in 1992.
In Scotland in the early 1980s women living near to incinerators showed higher twinning rates.
Belgian researchers have found similar trends.
But Swedish scientists found no increase in twinning rate around 14 incinerator plants.
Given the increased health risks of multiple births, such as low birth weight, the researchers say further research is needed to find out why this might occur.
Professor Nick Fisk, professor of Obstetrics
and Gynaecology at Queen Charlottes' Hospital in London, said there were a couple of possible mechanisms behind the trend.
He said it might be that the women in the study were at lower risk of miscarriage, but he thought that was unlikely.
He said it was possible that toxic waste had an effect on female hormones.
"It is possible that something in the toxic waste suppresses oestrogen levels in women. Therefore, they have higher gonadotrophin levels - they are the hormones that kick start the egg forming," he explained.
If you have higher gonadotrophin levels you are more likely to make more eggs that could become fertilised at the same time, resulting in non-identical twins.
"There is a plausible mechanism but that would only apply to non-identical twins."
"It's very different from identical twins which occur when the fertilised egg splits very early on," he said.