Allison and Kevin Carlson have gotten an early Christmas present this year – the healthy birth of rare monoamniotic twins named Kate and Annie.

The baby girls were delivered on November 7 at just 30 weeks old and have so far beaten the odds.

Sharing the same amniotic sac and occurring in less than 1 percent of all U.S. twin pregnancies, monoamniotic twins carry serious risks – including cord entanglement, which can cut off the blood flow from the placenta to the fetus, according to doctors at Lucile Packard Children’s Hospital.

And that’s what caused some extremely anxious moments in early November.

With Kate and Annie, the girls’ cords created a perfect but alarming knot and their heart rate was dropping – leading to an emergency cesarean delivery. “Having a set of monoamniotic twins can be dangerous and unpredictable,” said Dr. Susan Crowe, who led the delivery team and noted that around 20 percent of these twins die from complications. “With no membrane dividing them, the obstetrician has to balance the risk of prematurity with the risk of a cord event.”

As first-time parents, Allison and Kevin thought they were only having one baby, but were thrilled to learn at week 17 that Allison was pregnant with twins.

While their excitement quickly turned to a bit of fear when they learned about the dangers involved with monoamniotic twins.

Though the team at Packard was hoping to get Kate and Annie to at least a November 30 due date to minimize the risk of lung disease and other complications associated with prematurity, the fateful November 7 day came when doctors determined that the twins’ rapidly decreasing heart rate required they enter the world immediately.

“One of the nurses held my hand and spoke to me in a soothing voice as I was getting my anesthesia,” Allison said. “She calmly walked me through the entire procedure.”

The babies began life weighing in at just 3 pounds for Kate and 3 pounds, 2 ounces for Annie.

“It was a testimony to the skill of the obstetricians to allow the twins to grow in the womb as much as they did, so that their lungs were able to mature,” Neonatologist Dr. William Rhine said. “They perfectly weighed the balance of being born early versus creating a heightened risk of cord compression and entanglement by letting them stay in utero.”

Now, as the girls approach their first Christmas and a date to take the girls home to Menlo Park, around the New Year, Kate and Annie are already displaying their personalities.

Kate is the “calm one,” and Annie the “wild woman,” according to mom and dad.