An Ohio school has 13 sets of twins -- unofficially tying the 2010 Guinness World record for the most sets of twins in a single grade, the principal said.

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Keven Fancher, the principal at Loveland Primary School in Loveland, said, "I knew we had a lot of twins, but I never paid attention to how many. I'm sure if I put it in someone's hands, the staff would want to pursue (sharing a world record)," the Cincinnati Enquirer reported Saturday.

The official Guinness record of 13 sets of twin in a grade was set in January last year at Pomperaug High School in Southbury, Conn., the Enquirer reported.

Doctors said the number of multiple births has risen sharply in the last few decades due to fertility drugs, medical advances leading to high survival rates for premature infants, and women delaying bearing children.

The Twins Foundation, a non-profit organization that disseminates research information about twins, said twins comprise 2 percent of the population nationwide.

Earlier and wider use of fertility drugs such as clomiphene, which stimulates ovulation, has played a big part in increasing the number of multiple births, said Dr. Michael Marcotte, an obstetrician specializing in high-risk pregnancies and the medical director for the Quality for Maternity Services at TriHealth hospital programs in Cincinnati.