Britney Alba remembers the day she and her husband, Frankie Alba, received the news they were expecting twins. It had only been six months since they had welcomed their twin sons.

"Frankie and I were not trying but it just happened. We just found out we were pregnant … so we go and get our ultrasound," the mom of four recalled to "Good Morning America." "And Frankie was like, 'Are you sure it's not two?' He was joking around and [the ultrasound technician] was like, 'Let me check one more time.' And she checked again and sure enough, there was a second heartbeat."

"I was definitely in shock," Frankie Alba, 25, recalled. "But I was excited at the same time. They're definitely a blessing."

PHOTO: Britney Alba said Luka and Levi are "great" and "very sweet" with their younger sisters.
The University of Alabama at Birmingham
Britney Alba said Luka and Levi are "great" and "very sweet" with their younger sisters.

The Albas said twins don't run in either of their families and for them to expect twins twice was astonishing.

"I would have never guessed in a million years that I would have one set of twins, much less a second set," Britney Alba, 27, added. "So we were super excited. Super excited. But it was like a laughing-and-crying-at-the-same-time moment."

The Albas' first set of twins – Luka and Levi – were monochorionic-diamniotic (MoDi) identical twins, meaning they shared the same placenta but separate amniotic sacs. But their second set of twins – Lydia and Lynlee – turned out to be monoamniotic-monochorionic (MoMo) identical twins. The girls ended up sharing both the same placenta and amniotic sac.

PHOTO: Luka and Levi, now 18 months, are monochorionic-diamniotic identical twins so they shared the same placenta but separate amniotic sacs.
The University of Alabama at Birmingham
Luka and Levi, now 18 months, are monochorionic-diamniotic identical twins so they shared the same placenta but separate amniotic sacs.

"It was already rare for there to be identical twins back to back, but particularly just my twins because MoMo twins make up 1% of all twin pregnancies. It's very rare," Britney Alba said of what her doctors told them. "So just their pregnancy alone was rare. But then to have identical twin brothers right before them made it even more rare, so definitely a unique situation."

There is not much research data available on the rarity of such twin pregnancies, but one Journal of Perinatology study estimates MoMo twin pregnancies happen once out of every 10,000 pregnancies, while another comparative study estimates MoDi twin pregnancies occur in 0.3% of all pregnancies.

In addition to being rare, a MoMo twin pregnancy is considered high risk. Having to share the same placenta and amniotic sac increases the risk for problems with blood flow to each twin and increases the risk that the umbilical cords could become entangled. Britney Alba was admitted to the University of Alabama at Birmingham Women and Infants Center at 25 weeks and spent over 50 days there before the girls were delivered.

PHOTO: Lydia and Lynlee are monoamniotic-monochorionic identical twins, meaning they shared the same placenta and amniotic sac.
The University of Alabama at Birmingham
Lydia and Lynlee are monoamniotic-monochorionic identical twins, meaning they shared the same placenta and amniotic sac.

The Albas welcomed Lydia and Lynlee in Oct. 25, 2022 and the newborn girls spent their first six weeks in the neonatal intensive care unit under the care of nurses and doctors.

They were able to go home to Tuscaloosa, Alabama, for the first time on Dec. 7, according to the Albas, and today, the family of six is settling into what they call their "new normal."

PHOTO: The Albas say although life now with their four kids is "nonstop" and "fast paced," they feel "extremely grateful" at the same time.
The University of Alabama at Birmingham
The Albas say although life now with their four kids is "nonstop" and "fast paced," they feel "extremely grateful" at the same time.
PHOTO: Britney and Frankie Alba are parents to two sets of twins - twin boys and twin girls.
The University of Alabama at Birmingham
Britney and Frankie Alba are parents to two sets of twins - twin boys and twin girls.

"It is nonstop, fast paced, but we love it," Britney Alba said. "The boys are great with the girls. Their main focus is just to play and have fun but when they do see the girls and they're around them, they're very sweet. They have gentle hands, sweet hands. And they're just really good around their sisters."

Both Albas say they feel "extremely blessed" and "extremely grateful" for their family.

"Even on those nights where we feel like we want to pull our hair out, and none of our children want to sleep, we just have to remind ourselves how lucky and how blessed we are to be able to have these children," Britney Alba said.

"Life has definitely changed dramatically and very quickly but it's a huge blessing for us and we are just looking forward to what the future holds with these babies because they're special," Frankie Alba added. "We can't see life without our babies."