Two sets of identical twins: Texas woman Tressa Montalvo delivers two sets of identical twins

A Texas woman got a quadruple Valentine's Day gift this year, giving birth to four babies -- two sets of identical twins.

Ace, Blaine, Cash and Dylan Montalvo were born at the Woman's Hospital of Texas in Houston to Tressa Montalvo, 36, and her husband, Manuel, 43, of Houston.

Ace and Blaine were first, born at 8:51 a.m. and weighing 3 pounds, 10 ounces, and 3 pounds, 15 ounces, respectively, the hospital said. Cash and Dylan were a minute later and weighed 2 pounds, 15 ounces and 3 pounds, 6 ounces. All four were born by cesarean section.

"We tried to stick to the A-B-C-D theme when naming them," Tressa Montalvo said in a statement the hospital released.

The twins were not the result of fertility treatments, the hospital said. Tressa Montalvo learned she was carrying twins at 10 weeks.

A third heartbeat was found at a later doctor's visit, and the Montalvos were referred to a maternal fetal medicine specialist, Dr. Brian Kirshon.

"We couldn't have been more surprised when Dr. Kirshon told us we were having four babies and that they were two sets of twins," Manuel Montalvo said.

Each pair of twins shared a placenta, the hospital said.

Identical twins result when a fertilized egg splits into two embryos. Twins occur in about 2% of all pregnancies, according to the University of Pennsylvania Health System. Of those, 30% are identical twins.

The odds of having two sets of twins at once is about 1 in 70 million, Dr. Alan Penzias, associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School, told ABC News. Attempts by CNN to reach Penzias on Tuesday were not immediately successful.

The boys were all born at 31 weeks' gestation, so they face some health risks, as their immune systems aren't fully developed, said Krista Cato, a nurse in the pediatric intensive care unit of Children's National Medical Center in Washington.

Manuel Montalvo said he and his wife were trying to have "one little brother or sister" for their 2-year-old son, Memphis.

"We didn't expect it," Tressa Montalvo said. "We were trying for just one and we were blessed with four. ... We planned the pregnancy -- I guess we just succeeded a little too much."

Still, the Montalvos say, they are planning on trying again. "We want a girl," Manuel Montalvo said.