Erin and Abby Delaney were born in 2016 joined at the head -- a rare condition called craniopagus. 

Conjoined twins occur when, in the early stages of development, an embryo only partially separates to form two babies. 

Conjoined twins are rare, occurring in about one out of every 200,000 live births, experts say. But surgical teams are increasingly capable of separating many such pairs, putting the rare condition in the spotlight.

Keep clicking to see pictures of these remarkable individuals ...

Erin and Abby Delaney were born joined at the head. The connection extended deep into their brain tissue, making separation especially difficult.

The Delaney twins were separated on June 6, 2017, by a 30-member surgical team at Children's Hospital of Philadelphia. The surgery took 11 hours. 

In October, 2017, Erin had been discharged but Abby remained hospitalized.

"They're such happy, content little girls for what they've gone through," said their mother, Heather Delaney. 

After 485 days at Children's Hospital of Philadelphia, Erin and Abby Delaney headed home with their family in November 2017, just in time for Thanksgiving.

"Riley and I are so grateful for the care our girls have received here and so excited to take them home -- just in time for the holidays," said the twins' mother, Heather Delaney.

Conjoined twins Vani and Veena are pictured at the Niloufer Hospital in Hyderabad, India, in April 2006. 

Vani and Veena, seen here in in Hyderabad, India, in 2007, are craniopagus twins, which means they were born joined at the head -- making separation risky.

Indian sisters Vani and Veena use a chalkboard at the Niloufer Hospital in Hyderabad on March 8, 2007. 

In 2017, Veena and Vani moved to the State Home for Children in Hyderabad after spending 11 years at Niloufer hospital. 

Maria Paz and Maria Jose, 10-month-old twins from Chile, were successfully separated in an 18-hour surgery on Dec. 14, 2011. Sadly, doctors announced that Maria Jose had died of organ failure on Dec. 18, 2011.

The Chilean girls were born sharing internal organs and a urinary system, providing an especially complicated situation for doctors.

Angelina and Angelica Sabuco were born in the Philippines in August 2009.

At the age of 2, they were scheduled to undergo separation surgery on Nov. 1, 2011.

Maria and Teresa Tapia were joined at the lower chest and abdomen, sharing a liver, pancreas and portion of the small intestine.

Maria and Teresa Tapia were separated in a 20-hour surgery.

These Chinese boys share a heart and a liver. They were abandoned three days after birth in December 2004, and were sent to a hospital in China's Anhui province to recover. This photo was taken on February 28, 2005.

At the hospital in Anhui, the boys' health was not in jeopardy as they awaited donations for a separation surgery.

The separation surgery for the boys was planned to take place in a hospital in Shanghai.

Sisters Ladan (left) and Laleh (right) Bijani died shortly after being separated in an operation at 29 years old. Here, they speak during a news conference at Raffles Hospital in Singapore on June 11, 2003.

Xin Yue and Xin Chen, sisters from the Shanxi Province in China, lie in bed prior to their separation surgery on June 1, 2005, in Shanghai.

The girls were successfully separated after being connected at the chest and stomach.

Ayara (left) and Jayara (right) Ratun, also known as Ganga and Jamuna, rest at the Dreamland Fun-Fair in Ahmedabad, India. 

These twins earn their living by exhibiting themselves to the public in traveling fairs such as this one.

Anggi (left) and Angeli (right) lie in an intensive care unit at a hospital in Jakarta, Indonesia, on Feb. 26, 2004. Born via caesarian section, these twins shared an anus, genitalia and three legs.

The two Indonesian twin baby girls were successfully separated in a ten-hour operation in Singapore in May 2005.

These girls were born in China's Sichuan province on May 9, 2011.

Newborn twins lie at a pediatric ward in Fatima al-Zahraa Teaching Hospital in Baghdad, Iraq - facing almost insurmountable odds as part of a poor family in a war-torn nation.

Iraqi twins Fatima (left) and Zahra (right) sleep on their hospital bed in Riyadh, Saudi Arabia.

One-month-old Angela Corina and Angela Leticia are pictured at the Roosevelt Hospital in Guatemala City on July 28, 2006. The twins, joined at the abdomen, underwent surgery despite warnings from doctors that Angela Corina had no chance of survival and Angela Leticia only had a 30 percent chance.

A few hours after surgery, Angelina Corina died from cardiac arrest.

Egyptian twins Ahmed and Mohamed Ibrahim lie together in this undated photo in Dallas, Texas.

The surgery to separate Ahmed and Mohamed at 2 years of age was a success, according to hospital officials.

In this photo provided by Children's Medical Center, father Ibrahim Mohamed Ibrahim, mother Sabah Abou Al Wafa and older brother Mahmoud Ibrahim spend time with Mohamed Ibrahim and Ahmed Ibrahim after they were separated in Dallas. 

The brothers were finally able to see each other for the first time since being born joined at the head.

An Afghan baby boy with his headless twin attached at the chest lies on a bed at the main hospital in Kunduz on September 14, 2009. The otherwise healthy infant was born on Sept. 10 with the torso, legs and hands of male a twin attached.

Moroccan twins Hafsah (left) and Ilham (right) are seen on Jan. 4, 2006 at Riyadh's National Guard Hospital.

One-year-old twins Maria Luiza (left) and Maria Luana (right) are seen at their home in the Beberibe neighborhood in northeastern Brazil, on Nov. 10, 2009.

Here, Maria Luiza (L) and Maria Luana rest on the lap of their mother Larissa at their home.

Conjoined twins are pictured in the neonatal intensive care unit at a hospital in Manila, Philippines, on July 29, 2009.

A pair of one-week-old girls lie on a bed at a hospital in Jinan, northeast China's Shandong province on Sept. 7, 2010.

Douaa (left) and Ihsane (right) are pictured at the Avicenne hospital in Rabat, Morocco, on Nov. 2, 2007. Born on Oct. 17 with one brain, the twins were not separated.

Conjoined twins drink milk at an Air Force General Hospital on Dec. 21, 2007 in Beijing, China. The two boys from Tianjing Municipality, were born with a  separate triplet sibling on Nov. 5, 2007.

Conjoined twin boys sleep at the hospital in Medan, North Sumatra on Jan. 19, 2009. Doctors in Indonesia said they planned to separate the twins but had to wait for them to grow strong enough to survive the complex surgery.

These twins were born on July 25, 2003, in Indonesia with two legs, one sex organ and no anus.

This photo, taken on July 24, 2009, shows Indonesian conjoined twins at the Arifin Achmad Hospital in Pekanbaru. The baby boys died on July 28, 2009, within two minutes of each other, due to heart and lung dysfunction five days after they were born, according to hospital officials.

In this picture, taken on Feb. 20, 2007, Indian boys Ram (left) and Laxman (right) stand with their mother Premvati at the family home in Raipur. At 10 months of age, the twins were successfully separated on May 29, 2007.

Moroccan conjoined twins al-Safa (right) and al-Marwa (left) lie in bed at the National Guard's King Abdul Aziz Medical City in Riyadh on July 10, 2008.

Three-month-old Banya Flood (left) and Barsha Rain (right) are pictured at the Bangabandhu Medical College Hospital in Dhaka on July 22, 2008. Banya and Barsha were born in northern Bangladesh on March 24, joined at the stomach and chest.

Maria de Jesus and Maria Teresa Alvarez were born conjoined at the head in Guatemala in 2001. They underwent separation surgery in 2002, at Mattel Children's Hospital UCLA.

The girls returned to UCLA in 2015 to visit with the medical staff who had cared for them in 2002.

The Egyptian twins Hassan (left) and Mahmud (right) are seen at the National Guard hospital in the King Abdulaziz Medical City in Riyadh on Feb. 27, 2009, the day before their separation surgery. King Abdulaziz Medical City is internationally acclaimed for the separation of conjoined twins - Hassan and Mahmud marked the 21st separation to take place.

Rital and Ritag Gaboura were born in Sudan with the tops of their heads conjoined. Although separation surgery is very risky, the condition can lead to serious medical problems so the girls' parents asked a charity for help funding the surgery.

Rital and Ritag were successfully separated at London's Great Ormond Street Hospital for Children in September 2011, after the family received donations from the British charity Facing the World.

Conjoined Bangladeshi twins Tofa and Tahura are treated before surgery to separate them at Dhaka Medical College Hospital in Dhaka in 2017. They were born joined at the spine and rectum.

The twins were successfully separated on August 2, 2017. Surgeons in Bangladesh called the procedure, their first successful separation of conjoined twins, a "groundbreaking" and complex operation.