Walking into her doctor's office for a prenatal checkup, Natalia Herrera had a 1 in 80 chance of finding out she was pregnant with twins and a 1 in 3 chance of hearing those twins would be identical.

But the chance that her twins would be identical, share a placenta and have a rare condition called Twin-to-Twin Transfusion Syndrome? That's 1 in 3,600, according to Dr. Michael Ruma, one of Herrera's physicians at Perinatal Associates of New Mexico, a clinic of high-risk pregnancy specialists.

The odds were against the mother of one and her husband, AJ, the day doctors told the couple their twins suffered from TTTS, an in-utero condition in which one twin transfers more nutrients than is healthy to the other twin because blood vessels on the placenta don't bring blood back to the same baby. Without treatment, TTTS carries a 90 percent mortality rate for babies.

The 28-year-old mother wasn't expecting to have twins, let alone unhealthy ones, but she had known something was wrong about six weeks into the pregnancy.

"It was like a train hit me. I had recently run a marathon, and all of a sudden I couldn't even handle the elliptical," Natalia Herrera said. "If you think something is wrong, it probably is. A mother knows."

On her way into work at Los Alamos National Laboratory one day, Herrera started having excruciating pains. She immediately went to a hospital, but doctors didn't notice anything out of the ordinary. It was soon after, during a routine prenatal appointment at Perinatal Associates, that specialists pointed out the telltale sign of TTTS: Her ultrasound showed one of her twins moving freely and the other "stuck."

The phenomenon occurs when the "donor" baby has transferred away so many nutrients to the "recipient" that it produces less urine. If untreated, the donor baby could suffer enough loss of blood that it has to shut down its kidneys to stay alive. A sign of a more serious stage of TTTS is when a bladder is not visible in an ultrasound of the donor baby, meaning that twin is not creating any fluid.

"[The amniotic sac is] like a swimming pool. When you have a normal amount of water, kiddos can swim and move freely. If there's no fluid, they're stuck in a small pool with no fluid to move around in," said Ruma, who worked closely with the Herrera family throughout the pregnancy. In Natalia Herrera's case, Kayla, the recipient, had about 13 cm of fluid, and Jocelyn, the donor, only 2 cm.

Ruma said Perinatal Associates only sees about four cases of TTTS per year, making it a "very rare diagnosis" for the specialists there. He estimated about 5,000 cases of the syndrome per year nationwide, and said only about 15 centers in the U.S. service TTTS patients.

As recently as 10 years ago, doctors would have given Natalia Herrera and other women with TTTS diagnoses a bleak outlook on their twins' survival. But thanks to a relatively new laser-ablation procedure, Natalia Herrera is among a growing group of women whose twins survive TTTS.

Natalia Herrera and her husband, AJ, were already the parents of a 2-year-old son named Diego at the time of her twin pregnancy. The young couple said the TTTS process was a "five-month grind," with the scariest moment coming during the diagnosis at Perinatal Associates.

"At one point, we were sitting in the waiting room, and the doctor told us we might lose the babies. That was the lowest point we had gone through," AJ Herrera said.

After being diagnosed with an early stage of TTTS, the Herreras were flown immediately to Texas Childrens Hospital in Houston, which has performed the laser-ablation procedure in hundreds of TTTS cases. But because Jocelyn, the donor, still was showing a bladder in the ultrasound, doctors said the procedure wasn't yet necessary. After a few days of close monitoring, the couple flew back to Santa Fe unsure about what was next.

Natalia Herrera said during the waiting period, she became "an expert at reading ultrasounds." The mother still has a box full of them at her Santa Fe home, categorized like business files by date. Some show the "stuck" baby, while others from later in the pregnancy reveal tiny, curled hands or little feet.

A week after the emergency Houston trip that ended in no surgery, the couple was back in Santa Fe. Doctors immediately noticed that Natalia Herrera's cervix had shortened significantly — so much so that the surgery that could save her babies' lives also now ran the risk of throwing her into premature labor. But by week 20, doctors said it was clear she needed the laser ablation after all, and the couple flew back to Houston.

"The doctors had told us, 'You're going to lose the babies unless we get you over to Houston,' so it was a no-brainer," AJ Herrera said of the couple's second emergency trip to the childrens hospital, which resulted in a successful surgery.

According to Dr. Michael Belfort of the Texas Childrens Hospital, the laser ablation involves inserting a fetoscope into the uterus to help locate the abnormal blood vessel connections along the placenta. Once those connections are located, the doctor burns them shut with a laser.

The mother is given a local anesthetic during the surgery, which Natalia Herrera remembers lasting only about 45 minutes. With the laser ablation, the odds of survival are 90 percent for one baby and 70 percent for both, Belfort said. Hospital figures also state about 85 percent of children who receive the laser intervention to correct TTTS show normal neurological function by age 3.

Major risk factors include the chance that membranes will rupture or that the mother will go into premature labor.

Two days after the surgery, Natalia Herrera was well enough to travel home. Ruma at Perinatal Associates said that within a week of the procedure, "there were basically no outward signs of TTTS. It was a complete resolution." And for Natalia, the biggest sign of things being back on track was that soon after the ablation, more than 20 weeks into her pregnancy, she felt her girls kick for the first time.

Like many women pregnant with twins or who receive the TTTS procedure, Natalia Herrera was placed on bed rest to prevent premature delivery. But for Natalia Herrera, the hospitalization from April to June paid off: On June 4, at 31 weeks, four days into her pregnancy — just under eight months — Natalia Herrera delivered two healthy girls. The twins were kept at Presbyterian Hospital for three weeks as "feeder growers," but had no significant health issues.

She said since their birth, the twins have rebounded to a healthy weight and are "doing everything they're supposed to be doing." That includes keeping the couple up at night, said husband AJ Herrera, the soccer coach at Santa Fe High School.

With the girls about three months old, Natalia Herrera is making plans to get back to work at the lab.

She said come November, her mother will care for the girls while Diego attends preschool.

Both Natalia and AJ say the roller-coaster ride was well worth the turmoil.

"At the end of the day, even though it was so hard and so tiring, in the back of our minds we were happy because we knew we were in good hands. ... This is all worth it in the end because our girls are healthy," he said.

Contact Emily Hubbell at ehubbell@sfnewmexican.com.