Couple Welcomes 'Extremely Rare' Identical Quadruplets: 'I Was Relieved Once I Could Hear Their Little Cries'

Mercedes Sandhu delivered four healthy baby girls on May 1, just in time for Mother's Day

Mercedes Sandhu received her Mother’s Day gift a bit early — on May 1, she welcomed four healthy identical baby girls!

“I was just relieved once I could hear their little cries,” Sandhu tells PEOPLE. “It was just a sense of relief really knowing that they were here."

Born at 29 weeks and three days, Hannah Grace, Lucy Marie, Rebecca Claire and Petra Anne are continuing to grow stronger in the NICU at Texas Children’s Hospital, according to the Houston mother, 34, and her husband Jonathan, 37.

They’re also statistical marvels, says Sandhu’s doctor, maternal-fetal medicine specialist Dr. Manisha Gandhi.

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In addition to the fact that the couple did not use any reproductive technology to conceive, Gandhi tells PEOPLE that all four girls shared one placenta, which is known as a monochorionic pregnancy.

“They both wanted numbers,” Dr. Gandhi says of the couple, who both leaned on their training as engineers to process the news. “It's really, really rare. And that's all I keep saying because I don't have good numbers ... it's just extremely rare.”

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Gandhi goes on to explain that with monochorionic pregnancies, "there are much higher risks for complications than if each baby had their own placenta."

“So we watch these babies very closely, regular ultrasounds quite frequently to monitor for either growth issues in the baby or they're not sharing the blood supply equally," she adds.

The obstetrician/gynecologist adds that in the last week of the pregnancy, she was concerned about how one of the babies was receiving her blood supply, so the decision was made to deliver the girls early.

“Because it's a monochorionic pregnancy, if something affects one, it can really quickly affect the others,” she says. “So we have to be very vigilant, and that's what led to our recommendation to move towards delivery."

Fortunately, everything went “really well,” Gandhi says, with four neonatal teams dedicated to taking care of each baby following the cesarean delivery.

For the husband and wife, the early part of Mercedes’ pregnancy was actually routine.

With the couple already parents to Luke, 3, and Aaron, 18 months, Mercedes told her husband that he didn’t need to come to her seven-week ultrasound. Looking back, it's something the Kansas City-area native laughs at. “Mercedes had said, it's our third kid, it's seven weeks, don't worry about going,” Jonathan recalls.

As for what was going through her mind when she found out she wasn't just expecting her third child, but also her third, fourth, fifth and sixth child?

"Oh man, I was crying and laughing at the same time because I thought there's no way that this is happening," she tells PEOPLE. "I felt all these different emotions, but it was shock, that's for sure."

Related: Couple Welcomes Quadruplets 4 Years After the Birth of Their Twins: ‘It’s Been a Joy’ (Exclusive)

And when it came time to choose names for their four daughters, the couple pragmatically relied on a spreadsheet.

“We had a spreadsheet with a bunch of names, and for our first two boys, we used an app that's essentially like Tinder for baby names for your match with somebody, and you swipe left if you don't like it, and right if you do like it,” Jonathan tells PEOPLE. “And then when you get both, get a match, it puts it together."

He continues, “But for this round, it wasn't as helpful, so we just kind of tossed it out."

With their first Mother's Day as parents of six on the books, the proud dad is celebrating the woman who made all of it possible.

“Despite the fact I was carrying no children in this, it did not have a huge physical effect on me, I was the one that constantly had breakdowns throughout the entire pregnancy,” Jonathan says. “And Mercedes was the one that had to really help me get it together and just talk through, ‘Hey, we're all going to get through this. This will be okay.’ “

He adds, “But this is a crazy situation, as a lot of our friends and family have noted. I could think of few people better suited to handle it than Mercedes. As a mom of six little kids, it’s going to be chaos for the next whoever knows how many years.”

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