Identical Twin Quadruplets: Texas Mother Gives Birth to Two Sets of Twin Boys

One Texas couple trying to give their two-and-a-half year old son a baby sister faced an unexpected twist -- they had two sets of twin boys instead.

Tressa Montalvo, 36, gave birth to quadruplets Ace, Blaine, Cash and Dylan on Valentine's Day, according to a statement from The Woman's Hospital of Texas in Houston issued to ABC News. While Ace and Blaine shared one placenta, Cash and Dylan shared the other. "We tried to stick to the A-B-C-D theme when naming them," Montalvo said in the statement. "We didn't expect it, we were trying for just one and we were blessed with four."

Montalvo and her husband Manuel Montalvo, Jr., thought they were having twins until their doctor detected a third fetal heartbeat. When they went to a specialist, the doctor there discovered a fourth heartbeat.

"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 in the statement. "We were trying for one little brother or sister for our two-and-a-half year old son, Memphis."

Roughly 2 percent of all pregnancies result in one set of identical twins, when a fertilized egg splits into two embryos. But the chances of having two sets? Even lower, Dr. James Grifo, director of the NYU Fertility Center, told ABC.

"The chance of this outcome is approximately one in 10,000," Grifo said of two embryos splitting after IVF. "This could also occur in a natural conception, but the chance of that is much [rarer]."

The Montalvo's case falls on the extremely rare side. "No fertility drugs were used," Tressa Montalvo told ABC. "We planned the pregnancy - I guess we just succeeded a little too much!"

Risks abound with having multiple children at once; twin-to-twin transfusion syndrome, a rare disease that results in one twin getting more blood than the other, is one of them. "Having multiples certainly adds risk to a pregnancy in and of itself," Dr. Kimberly Gesci, an obstetrician at UH Case Medical Center in Cleveland, Ohio, told ABC. "One of the biggest risks is preterm delivery and growth restriction."

The four new Montalvos were born by C-section at 31 weeks -- weighing between three pounds, 15 ounces and two pounds, 15 ounces.

Although the Montalvo's case is extremely rare, theirs isn't the first one. In April, 2011, Miranda and Josh Crawford of Charlotte, N.C. gave birth to identical twin boys and girls after undergoing in-vitro fertilization. And in 2002, like the Montalvos, Christina Tetrick of Wichita, Kan. was pregnant with two sets of twin boys.

Despite these four additions to the household, the Montalvos say they're not finished yet.

"We want a girl," Manuel Montalvo said.