A set of 13-month-old twins who are physically attached at the cranium have been given a new chance to live after doctors' successful attempt at separating them. After more than twenty hours of nerve-wracking surgery, Jadon and Anias McDonald are no longer conjoined.

Craniopagus twins make up only 2 percent of conjoined twin cases, according to Live Science. One in every 200,000 live births are conjoined twins. Unfortunately, not many of these cases survive their first day of life.

Nicole McDonald was 17 weeks pregnant when she discovered that she was pregnant with twins, according to Today. She was in utter shock when she found out that the twins are attached at the head. A little over a year after the twins were born, the whole McDonald family moved closer to New York City in hopes of finding a solution to their problem. That is where they met Dr. James Goodrich at Montefiore Medical Center.

Dr. Goodrich successfully separated another set of craniopagus twins in 2004. Though the craniopagus-separation case was deemed highly successful for the McDonald twins, there is no certainty about how successful their recovery would be. Despite the operation's success, the boys are still facing critical times.

Craniopagus is a rare type of conjoined twins. As written in Live Science, the embryos of identical twins begin to detach within eight to 12 days after they are formed. A theory once stated that if the separation process was initiated at a later time -- around 13 to 15 days after fertilization -- a malformation happens, resulting in conjoined twins.

Another theory proposes that conjoined twins can result from an egg that separately completely then later rejoins back together. According to the statement released by the Montefiore Medical Center, "the twins still face 'a long road of recovery and rehabilitation.'" The McDonald's surgery was the 59th successful case in the world since 1952.