Ten sets of twins have been born at St. Mark's Hospital in Salt Lake City, Utah over the course of a week, including four sets within a 21-hour period.

Of the 59 babies at the hospital on Thursday of last week, 20 were twins.

Dr. Gary A. Dilby, director of maternal-fetal medicine at the hospital, says he has no idea why there were so many twins this week.

Typically twins make up about 1 percent of deliveries in the United States. At St. Mark's, that means two or three sets a month. But 10?

One St. Mark's doctor joked it was like being on Noah's Ark: "They keep coming, two by two."