Cara Winhold said she was told by her OBGYN that her infants had been conceived a week apart.
The rare medical phenomenon is known as superfetation.
Winhold said the newborns were her "rainbow babies" because she'd previously experienced three miscarriages.
After the pain of three recent miscarriages, Cara Winhold could barely bring herself to look at the sonogram which showed she was five weeks pregnant.
But, to her relief, everything seemed fine. She and her husband, Blake, 33, cautiously started planning for the baby's birth in the fall of 2021.
Then, to their surprise, a second ultrasound picked up two heartbeats a couple of weeks later.
"I was shocked," Winhold told Insider.
While it's not unusual for one twin to be discovered later than the other, Winhold said that her doctor studied the scans and found that, because of its noticeably smaller size, the second fetus was a week behind the first one gestation-wise.
"I didn't really understand what was happening," Winhold, of Fort Worth, Texas, said. "But, since I had a tracker app to tell me when I was ovulating, I knew when the first one was conceived."
"Then Blake and I had sex the following week when we assumed my ovulation was over," she added.
The medical term for conceiving a baby days or weeks after the other is superfetation
It turned out that the couple, who already have a three-year-old son, Wyatt, had gotten pregnant when, as Winhold said, they were "already pregnant."
The 31-year-old said their physician explained that the medical term for the rare occurrence was superfetation. It happens when a second egg is fertilized by sperm several days, or even weeks, after the first implants in the uterus.
"It felt like a miracle," Winhold said, adding that the three miscarriages she'd suffered had been so traumatic, she needed therapy.
Dr. Sheryl A. Ross, an OB-GYN in Santa Monica, California who has not treated Winhold, told Insider that someone would have to ovulate twice during one menstrual cycle to create a second embryo at a later stage. She said the phenomenon was "not medically described."
There have been "less than 10 cases in the literature" of superfetation in humans, according to a recent case report,
Ross said that if Winhold's experience was a "true superfetation, it goes against what we know medically about getting pregnant spontaneously."
"The truth is, not all medical phenomenons have a logical explanation, period," she said.
Winhold, who had 3 miscarriages, was considered high risk during her pregnancy
Winhold, who said she was delighted to discover that she was having two more boys, was closely monitored during her pregnancy, which was considered high risk.
"It was mostly because of the miscarriages and having had pre-eclampsia with Wyatt," the specialist math teacher said.
One complication occurred when the smallest, and youngest, twin was found to be growing at a slower rate than his brother, who appeared to be getting more nutrients. Winhold said the doctors believed the size difference was not only caused by the longer gestation period of the first baby but also its placenta being healthier than the other.
The babies were safely delivered on October 25, 2021, when Winhold went into premature labor and gave birth vaginally. The older boy, named Colson, weighed 6lbs 5oz and his twin, called Cayden, weighed 4lb 9oz.
They spent 14 days in the NICU before being discharged around two weeks before Thanksgiving. Cayden has remained a little smaller than Colson, but is doing well.
"I consider them my rainbow babies," Winhold said, adding that their arrival had eased some of her pain from the miscarriages.
She said she'd like to become pregnant again, this time with a girl. But she said she is convinced the superfetation won't happen a second time.
Read the original article on Insider