Mum's joy as her 10-month-old conjoined twins are successfully separated.

Ten-month-old conjoined twins have been successfully separated by surgeons during an 11-hour operation at a US hospital.

The Children’s Hospital of Philadelphia said on Tuesday sisters Erin and Abby Delaney, of North Carolina, are recovering in the intensive care unit after last week’s procedure.

“This past week has been one of the most intense weeks of my entire life. It included being excited, nervous, terrified, relieved, worried, stressed, tired, overjoyed, overwhelmed, and curious,” the girl’s mum Heather wrote on the family’s blog.

The hospital says the surgery was carefully orchestrated, down to marking monitors and equipment with green or purple tape, one colour assigned to each girl.

The girls were joined at the top of their heads, described as the least common type of conjoined twins. They were born 10 weeks premature last July by caesarean section.

Heather explained on her blog that she and husband Riley received hourly updates on their daughters' progress and that the decision on whether to go through with the separation could only be made after the surgery began and doctors assessed the situation.

Listen: Anne Stephens talks about how she raised two sets of twins. (Post continues.)

She said in the moment they learned the separation was going ahead they were "flooded with emotions".

"This went from a fairly straight forward surgery to a very complicated one in a matter of minutes."

Heather shared her fears knowing Abby was at the greatest risk of death.

"My mind immediately went to Abby. My heart ached and all I wanted to do was be able to hold their hands and tell them that they could do it. That they would make it through the surgery. That they were so big, and strong, and brave, and that they could do anything," she recalled.

"But I couldn't. All I could do was sit, and pray, and wait. And so that is what we did."

Despite Abby losing a lot of blood, both girls survived the surgery.

Heather described what it was like to see her daughters finally separated and have the full gravity of the situation hit.

"At that moment I stood in the middle of the room in-between two beds and realised what had happened. We had two girls. Two separate girls independent of each other."

"They were on their own. They were fighting on their own. They each looked almost lonely in their beds without the other. It almost felt wrong in a way that they couldn't be with each other."

"I realised that I was now going to have to split myself between two children. Before I could stand in one spot and read them both a story. I could change Erin's diaper while chatting with Abby. I realised I couldn't do that anymore. I couldn't kiss them at the same time, and it blew my mind."

The Delaney twins were in good hands - it's the 23rd time the hospital has separated conjoined twins.

- With AAP