Abby Delaney can roll onto her stomach, hold up her head and turn pages in her favorite books.

Her sister, Erin, can now sit up on her own, and she is starting to think about crawling - learning to hold herself up on her small hands and knees.

More than four months after the formerly conjoined twins were separated in a rare surgery at Children's Hospital of Philadelphia, their mother said there have been rewarding but "terrifying" moments as the 15-month-olds recover.

"I know that when you see stories of conjoined twins being separated it's so exciting and everyone is so happy," Heather Delaney wrote Sunday in a blog post.

"I wasn't able to have that moment for a while."

But, she said, the twin girls are making tremendous progress and are preparing to head home.

Children's Hospital of Philadelphia

Delaney provided details Sunday about the Mooresville, N.C., family's long, emotional journey since her daughters' birth.

"I know we took a while to share pictures and share more of the girls' story," she wrote in the blog post.

"We just wanted them to be well, and the stress to calm down before we let the world back into our lives."

Heather and her husband, Riley, learned that their then-unborn daughters were conjoined last year when Heather Delaney was about 11 weeks pregnant.

Heather started traveling to Pennsylvania, later moving to a unit at the Children's Hospital of Philadelphia for mothers carrying babies with complex congenital conditions.

On 24 July 2016, Abby and Erin Delaney were delivered prematurely by C-section, each weighing about two pounds, and doctors started putting together a plan to try to separate them, according to the hospital.

Nearly a year later, on June 6, about 30 doctors, nurses and other medical personnel worked for 11 hours to separate them - untangling blood vessels and separating the brain's outermost membrane and the sagittal sinus, which carries blood to the heart, according to the hospital.

The hospital said at the time that it had separated 22 other pairs of conjoined twins over the past 60 years but never a pair of craniopagus twins, those who are connected at the head.

With identical twins, an embryo splits in two early in a pregnancy. But with conjoined twins, the embryo does not separate all the way and the twins remain connected, according to the University of Maryland Medical Centre.

Conjoined twins are uncommon, occurring once in about every 200,000 births - and craniopagus twins are the rarest form, accounting for about 2 percent of all conjoined twins, according to the medical centre.

"This is one of the earliest separations of craniopagus conjoined twins ever recorded," Jesse Taylor, a plastic and reconstructive surgeon at the Children's Hospital of Philadelphia, said in a statement.

"We know that children heal better and faster the younger they are, therefore our goal for Erin and Abby was separation as soon as possible with minimum number of surgeries."

The twins will need additional surgeries over the next several years to replace missing bone on top of their heads and help mend their hairlines, according to the hospital.

Heather Delaney recounted the twins' recovery in the heart-wrenching blog post.

The mother posted pictures showing the infants tangled in a mess of tubes and wires, recalling moments her knees went weak, such as when Erin would stop breathing and her heart rate would drop, or when Abby would scream and her tiny body would start shaking because her brain was bleeding.

"Feeling helpless was the new normal, and I hated it," Delaney wrote.

"The worst part is everyone keeps asking if you are ok. That's when you know things aren't good. When you have person after person asking if you're ok, if they can get you anything, trying to hug you or pat you on the back to offer some support. It's moments like that were you just want everyone to go away and leave you alone.

"All you want to do is will your child to get better because that is all you can do."

Delaney said the twins have made strides. Soon after their first birthday they started rehabilitation - learning how to sit up and play with their toys.

But it has not been easy. In addition to continuing their rehab and recovery from surgery, the girls have been battling illnesses as their delicate immune systems adjust to an outside world.

"Each time it is heart wrenching because I know it is a setback," Delaney wrote on her blog.

"Whenever they aren't in rehab getting their therapies it is a set back as to when we can go home. And poor Abby just can't seem to catch a break. She has had a respiratory virus three times (Erin twice, me once), Erin had the flu (Abby and I escaped that one thank God), and then just this past week Abby got a weird blood infection that caused her to go into septic shock, landing her yet another stay in the ICU. This last one scared me I am not going to lie."

Earlier this month, Erin was discharged from the hospital, her mother said.

"Abby still has a little ways to go till she is able to be discharged," she added. But she said after a few more weeks of rehab, "hopefully, God willing, we will be headed home for the holidays!!!"

Delaney called her young daughters "inspiring".

"As their parents, it is very neat for Riley and me to have a front row seat to this and watch them overcome these incredible obstacles," she said in a statement.

"We cannot wait to see what their future holds!"

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This article was originally published by The Washington Post.