Nicole Ziesemer, 31, and her husband Matthew, 34, had just been introduced to their daughter Blakeley when the doctors realized that she was still in work.
They would have spotted "another head" while they were going to sew it after birth and the baby Cade was born 59 minutes later.
The couple had a "natural pregnancy" and had only undergone one sweep on the day of delivery – but it was thought that Cabe was "hiding" behind his sister, he did not have a baby. so was not seen.
Ziesemer, from Grand Haven, Michigan, is now home with her family and said she could no longer imagine having a single baby.
"It was crazy, we were on the moon but in shock," said Ziesemer, who worked in the insurance industry before becoming a homemaker. "There was really no sign that I was going to carry more than one baby, in fact, people were saying how small I was. We thought the biggest surprise would be the baby's sex, little did we know! "
"It was really scary when they said that there was something else inside of me," she said. "And when they told us it was another baby, I literally asked her if she was lying, I just could not understand everything, and I was still in such pain that it was almost impossible to treat."
"But I really can not imagine having a single baby now, it's like they're supposed to be," she said. "I like the idea that they have always exchanged themselves."
Ziesemer had planned a natural birth at home with only one midwife, but she had to travel to the nearest hospital – Gerber Memorial, Michigan – when her water broke in just 36 weeks.
The first mother had severe morning sickness throughout the pregnancy and decided to dispense painkillers for the benefit of breathing exercises, yoga ball and fresh showers.
She did not have any scans throughout her pregnancy except one on the day of delivery.
After giving birth for more than 45 hours, the expectant mother opted for an epidural and, after two more hours, Blakeley arrived at 22:06. December 30, weighing 4 pounds, 4 ounces.
But the doctors worried because the pain was diminishing, Ziesemer was still in agony after childbirth. While the doctors were trying to recover his placenta, it became clear that something was wrong and decided to do an emergency ultrasound.
Ziesemer gave birth again 59 minutes later to a little boy weighing 5 pounds.
Baby Cade, who was having trouble breathing on her own, was immediately taken to Spectrum Hospital in nearby Grand Rapids.
"Over the next few days, my husband was mainly with Cade, while my daughter and I stayed to make observations," Ziesemer said. "We were still in shock and were just trying to understand, but all we wanted was, first and foremost, that they were strong and healthy."
"The craziest thing is that on the day of delivery, I had a scan, but the only baby was detected," she said. "They think Cade had to hide right behind his sister."
When they were allowed to go home, the couple, who met online in 2012, went shopping to find duplicates of all the baby equipment that they had already bought for one.
Since then, the couple have learned that twins and triplets are running by her side.
"It was amazing to hear that it was a bit of a family tradition, my grandmother's cousins apparently had twins and triplets," she said. "People are asking us if we would have another baby" would we have exams this time? "But honestly, I do not think so … Unless there were clear medical alarm signals, I would do it the same way everywhere.
"It was such a surreal moment," said Matt Ziesemer, a chiropractor. "I could not believe my wife was carrying two kids all the time, my wife is doing amazing work with both of them, I really do not know how she does it every day."