According to the figures released by the National Center for Health Statistics in the U.S., the twin birth rate, the number of twins per 1,000 births, has risen by around 76% over the past three decades, from one twin being born in every 53 babies in 1980 to one in 30 by 2009. Twin birth rates rose by at least 50 percent in 43 states and the District of Columbia, while the twin birth rate in the five states including Connecticut, Hawaii, Massachusetts, New Jersey and Rhode Island have risen by more than 100 percent. Epidemiologist said the largest increase occurred among non-Hispanic white moms, but the overall increase has crossed race, and age. So what are the causes of the rise in twin births? Researchers say that around one-third of the twin birth rate can be explained by an increase in older women having children, as women in their 30s and 40s naturally have more chances to give birth to twins than younger mothers. The other two-thirds of the increase can be attributed to the rise of infertility treatments. Even though twin births only make up about 3% of all births in the U.S., experts are expressing their concern over the trend. Researchers said that multiple births have greater health risks, and higher care costs because they are more likely to be born prematurely and 10 % of the twins born are smaller than single babies. In addition, mothers of twins and the babies are more likely to be hospitalized longer.