Gary Steinman, an obstetrician specializing in multiple-birth pregnancies at Long Island Jewish Medical Center in New Hyde Park, New York, suggests that milk drinkers are five times more likely to have twins than women who ate no animal products.

Since 1975 the numbers of twins in the world has increased in some countries by over 50%.

Scientists have previously suggested fertility treatments and women delaying pregnancy may explain the rise but the latest research suggests that diet can also play an important part.

In a study twin rates of women who ate a diet including milk were compared with women who followed a vegan, or no animal products diet.

Vegan women were found to have about a 13 percent lower level of IGF in the blood than women who consume dairy.

The researchers say they believe that a protein found in the livers of animals, Insulin-like Growth Factor or IGF, may be the reason; it is found in cow's milk and other animal products.

It apparently makes women's ovaries more sensitive and increases the number of eggs produced; higher levels of IGF also improve the survival chances of an embryo in the early stages of development.

This effect says Steinman is likely to be greater in countries such as the United States that allows growth hormones to be fed to cattle.

It has been found in cattle that regions of the genetic code that control the rate of twinning are close to the IGF gene.

Stienman says that multiple births are more prone to complications such as premature delivery, congenital defects and pregnancy-induced hypertension in the mother than single pregnancies, and women contemplating pregnancy might consider substituting meat and dairy products with other protein sources, especially in countries that allow growth hormone administration to cattle.

The research is published in the Journal of Reproductive Medicine.

The drinks with excessive benzene were Safeway Select Diet Orange; Cadbury Schweppes Crush Pineapple; Giant Light Cranberry Juice Cocktail by Ahold, AquaCal Strawberry Flavored Water Beverage, and Kraft's Crystal Light Sunrise Classic Orange.

The American Beverage Association said the FDA's findings were consistent with past reviews of benzene in soft drinks.

The FDA has issued guidelines to manufacturers on ways to minimize benzene but does say that benzene has been found in several foods.

It seems that bananas, ground beef, cheese, butter, eggs, avocados and popsicles are among foods that have been found at times to have excessive benzene levels greater than the water standard of 5 ppb.

The FDA has not issued health concerns for those foods.

The FDA also points out that Benzene can form in soft drinks that contain vitamin C and chemicals called benzoate salts when they are exposed to light or high temperatures.