By GILLIAN MOHNEY

Source: http://abcnews.go.com

An Indiana couple is celebrating an extra-special arrival with the birth of their identical triplet daughters.

Ashley and Matt Alexander of Greenfield, Indiana, were surprised weeks ago when they learned they were expecting three new additions to their family during a routine sonogram, according to ABC affiliate WRTV-TV in Indianapolis, Indiana.

"She was checking [Ashley] and right away there were twins, and she goes, 'Let me check for a third,'" Matt Alexander told WRTV-TV in an earlier interview. "I'm like, she's just joking. I said, 'You're joking,' and she said, 'No, we don't joke about this stuff.' So [Ashley] about came off the table."

The couple, who already have a son, had conceived the triplets naturally, so they were not expecting to see three heartbeats on the sonogram.

Ashley Alexander told WRTV-TV she has a plan to tell the girls apart.

"I'm painting their nails," she said. "One's going to be pink, one purple, and the other probably pale blue."

Dr. William Gilbert, the director of women's services for Sutter Health in Sacramento, California, said in an earlier interview with ABC News there was no definite rate for the number of identical triplets born every year.

"It's hard to calculate a conservative estimate," Gilbert said about the rate of naturally conceived identical triplets. "One in 70,000 - that would be on the low end. The high end is one in a million."