Every baby is a miracle. But once every 38 times, there's even more to it.
About 3 percent of the pregnancies in the United States are multiple, resulting in twins or more.
Once in 8,000 times, a mother has triplets.
Once in 64 million times, identical quadruplets are born.
Jennifer Clapp-Bennett and her husband, Daniel, got to see their sons on a video monitor, moving in 3-D images, months before they were born.
"To see your children smile before they're here, it's just amazing, you know, it's just an amazing experience," said Clapp-Bennett as her 10-month-old triplets scooted around the family living room near Washington, D.C.
The National Geographic Channel used her 4-D time-lapse ultrasounds and extrapolated from them to create a computer-generated chronicle of life in a very crowded womb.
(The program, "In the Womb: Multiples," has its first showing on the National Geographic Channel Sunday at 8 p.m. ET, 9 p.m. PT.)
Dr. Thomas Pinckert, chairman of obstetrics and gynecology at Shady Grove Adventist Hospital in Rockville, Md., is Clapp-Bennett's doctor, and saw the images in production.
"It's remarkable, and so accurate," he said. "I was totally enthralled to watch it. It was just amazing how accurate it was."
Today the Bennetts have three healthy 10-month-old boys -- Ashton, Roman and Colten.
But a multiple pregnancy can be complicated.
When a new embryo implants itself in the wall of the uterus, seven to 10 days after conception, it is smaller than a grain of rice. From there, it grows quickly.
By the end of the second month, it is a little more than half an inch long, roughly the size of a lima bean. By the end of the third month, it might be 3 inches long -- and with eyes, ears, arms, legs and most other body parts taking shape, it is considered a fetus.
But in the case of a multiple pregnancy, the uterus becomes very crowded by the fifth month. Each fetus, by then, is 7 to 9 inches long. But for lack of room, the pace of their growth slows.
Clapp-Bennett said she could tell things were busy inside her. "If one started kicking, they all just started going. It was like gymnastics in my belly."
The Bennett boys were delivered by Caesarean section about a month early, each weighing about 4½ pounds -- large for triplets. Today they are busy and curious, crawling all over the place.
By watching ultrasounds, doctors say they've seen remarkable things happen in a busy womb.
They've noticed siblings holding hands, grabbing each other's umbilical cords, sometimes kicking each other, sometimes covering their faces as in self-protection.
And some researchers think that in the uterus, one can see early signs of how twins or triplets will relate to each other after birth.
"Twins have an affinity for one another in infancy that seems to be an extension of the womb," says Penny Glass, director of the child development program at Children's National Medical Center in Washington. "They will reach out and touch one another, just as they did in the womb."
Having seen her boys so often by ultrasound, Clapp-Bennett says she learned to tell them apart -- and saw characteristics that are still true today.
"I think this guy right here, Ashton, was pretty dominant," she said, as she and her husband played with the boys. "They're all pretty much like their personalities in the womb."