Mom Mourns Death of 6-Month-Old Daughter Who Was Both a Quadruplet and Identical Twin: 'So Proud of Her'

After giving birth to two sets of identical twins six months ago — a rarity that an expert in obstetrics previously described as "like 1 in 10 million" — Massachusetts mom Ashley Ness had settled into a new family routine that included caring for her newborn quadruplets.

As her partner Val Bettencourt shuttled the couple's three other children to school, Ashley packed up three of her four babies for trips to the Neonatal Intensive Care Unit at Mass General for Children hospital in Boston, where they would visit the fourth, a little girl named Chesley.

"Miss Chesley" — as Ashley, 36, called her smallest daughter — had struggled more than her newborn siblings since they arrived about 12 weeks early on July 28.

"Just everything is happening to her," Ashley told PEOPLE on Jan. 25. "And she takes it like a champ. She opens her little eyes and just stares at you, like, 'Hi.' She doesn't look miserable. Just like, 'Okay, mom, I'm going to be fine.' "

"It breaks my heart," Ashley said a little more than a month ago.

Ashley Ness quadruplets
Ashley Ness quadruplets

Ashley Ness

On Wednesday, Chesley died.

"My beautiful daughter, Chesley May, was called home to Heaven after a courageous six months of life," Ashley says in a statement shared Monday with PEOPLE. "Her passing leaves a huge emptiness in the hearts of all of us; her parents, her siblings, her family and the NICU team at Mass General hospital."

Ashley Ness quadruplets
Ashley Ness quadruplets

Ashley Ness

The girl who underwent several surgeries and never left the hospital after she was born "weighing only 1 pound 7 ounces," according to Ashley's statement, "was tiny, but so mighty!"

"Day in and day out she fought and fought and continued to beat the odds," Ashley continues. "Her strength was unwavering. Each time she overcame an obstacle we were so excited thinking that finally all the quads would be home together soon. We were positive that she would live a long and healthy life."

The statement adds: "We will forever miss her laughing face, her funny little air leak sounds, and the sparkle in her big beautiful eyes. We are so proud of her and know that she is at peace."

RELATED: Mom Shocked to Learn She's Pregnancy with Two Sets of Identical Twins: 'This is Going to Be Crazy'

The twin-quad phenomenon occurs when two eggs are fertilized at the same time and then each fertilized egg divides again, according to Dr. Ahmet Baschat, director of the Center for Fetal Therapy and a professor in the Johns Hopkins Medicine Department of Gynecology and Obstetrics.

Because each split egg remained encased in its own placenta — unlike traditional quadruplets, where each of the four would represent an individually fertilized egg and be encased separately — two sets of identical twins were the result, explained Dr. Baschat, who was not involved in Ashley's care.

Ashley Ness quadruplets
Ashley Ness quadruplets

Ashley Ness

Much had unfolded in the lives of Ashley, who put her work as a hair stylist on hold during her pregnancy, and Val, 48, a small-engine mechanic, since PEOPLE broke the news of the quadruplets' July 28 delivery.

The couple were already sharing their three-bedroom home in Taunton, Mass., with Ashley's daughter Chanel, 9, and Val's two youngest sons, 11-year-old Isaiah and Zayden, 8.

For now, Chesley's twin sister Chatham, who was released from the hospital on Oct. 4, and brothers Chance and Cheston, who went home on Oct. 17 and 18, respectively, each have their own bassinet in their parents' bedroom.

A GoFundMe campaign was launched to help the parents raise money for a possible home addition for their expanded family.

The couple had since traded a seven-passenger minivan for a newer — but still used — eight-person vehicle. But even that wouldn't have provided enough room to safely hold everyone for the day they hoped Chesley would come home.

RELATED: Mom Delivers 2 Sets of Identical Twins After Being Surprised by Pregnancy with Quadruplets: 'I Feel Amazing!'

Doctors performed surgery on Dec. 28 to insert a tracheostomy tube that helped Chesley breathe, and it would have been a fixture of her life for the near future, Ashley was told.

Diagnosed with severe chronic lung disease, Chesley was scheduled for surgery last November that was postponed when the anesthesiologist deemed it unsafe safe to put the infant under, Ashley said. Chesley also had pulmonary hypertension.

While praising the family's "strength and resiliency" that "continues to inspire us," a hospital spokesman said in a statement to PEOPLE last month that Chesley was being treated for "ongoing lung complications related to prematurity, which can be made significantly worse by quadruplet gestation and severely low birth weight."

Ashley Ness quadruplets
Ashley Ness quadruplets

Ashley Ness

Because of the little girl's struggles, Ashley had held Chesley in her arms only five times since she was born.

But Chesley still got pampered with certain privileges, including a hospital room all to herself, where Ashley laid out a blanket atop a floor mat so her three siblings could play while mom fussed over the fourth.

Meanwhile, the newborns had begun to display their own personalities.

"The boys are the biggest flirts ever," Ashley said last month of identical twins Chance and Cheston. "Every time they see anybody, and especially females, their eyes open so wide, and they start smiling and talking to them. I'm like, 'Oh boys, stop, please.' "

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from celebrity news to compelling human interest stories.

And Chatham is "a tough nut to crack, that one," Ashley said. "She has the most serious face on all the time. The boys will give smiles out left and right, and you're lucky if you get one smile from her."

At home, older daughter Chanel had risen to the occasion as an assistant to her parents. "Chanel is truly such a little mother hen, she is right there by my side helping me, you would think she's the mom," Ashley said last month.

Now, the entire family is grieving together.

"Chesley loved spending time with her siblings," Ashley says in her statement. "She loved taking turns cuddling with her big sisters and brothers. She loved when her identical twin, Chatham, laid with her babbling. I would look at them laying together thinking of all the tricks they are going to play on everyone when they get a little bit older because they look identical."

Ashley Ness quadruplets
Ashley Ness quadruplets

Ashley Ness

"She also enjoyed her time with Chance and Cheston," Ashley adds. "She always seemed so content when they were with her. It was like she knew her big brothers were there to comfort her."

Though she was only six months old and had health challenges, Chelsey was a charmer who seemed to love music and giggling, according to her mother.

"Chesley stole the hearts of every single person on the NICU team," Ashley's statement says. "Oftentimes I would make my 2 a.m. phone call to the nurses to check on how her night was going and they would remind me how she constantly keeps them on their toes. If she seemed agitated, the nurses would play her favorite soundtrack, Beauty and the Beast, to help comfort her. She loved listening to nurse Abby sing Copacabana and Lizzo during bath time ... It always made her laugh."

RELATED: One of the Quintuplets Born to Mom of 7 in Poland Dies at 3 Days Old: 'We Are Devastated,' Parents Say

In her statement, Ashley expresses her whole family's gratitude for the care they and Chelsey received during the past six-plus months. "I could never thank the team enough for their dedication, love and support they provided to my daughter, myself and my family," she writes. "Chesley was one extremely lucky girl to have the best nurse aunties and uncles."

With a smile that "lit up the room," Chelsey "left a mark on everyone that was lucky enough to meet her," Ashley says of her daughter.

That includes her mom Ashley.

"Chesley has shown me more about life, love and serenity than I ever knew," she writes. "We love you sweet Chesley May and will forever share your courageous legacy!"