(CNN) Amanda Burger and Amanda Prentice live more than 700 miles apart. They have never met, but they share something even more unusual than a first name.

Burger, 33, went to the hospital in September 2010 when she felt stomach cramps so painful that she could barely walk. Prentice, 34, had a seizure in April of this year, so her husband took her to the emergency room.

Each woman went home with a healthy newborn daughter. Before going into labor, neither woman had any idea she was pregnant.

Wait, how could a woman not know she's pregnant?

There's a lot of fascination surrounding these sorts of mysterious pregnancies, so much so that there's a show on TLC called "I Didn't Know I Was Pregnant" that showcases real-life situations in which mothers give birth to surprise babies.

Burger and Prentice have not been on the show, but their surprise deliveries made local news in their respective communities.

The phenomenon is indeed rare and hard to study scientifically. Some experienced obstetrician-gynecologists have never seen it happen; others have seen only a handful. One Serbian study estimates that one out of every 7,225 pregnancies is unknown to the mother until the moment of delivery.

It's more likely among people who are very overweight to begin with, and teenagers in denial about being pregnant, said Dr. Patricia Devine, director of labor and delivery at Columbia University Medical Center's department of obstetrics and gynecology.

"This is an extreme situation, and a very rare situation, because it is pretty hard to miss all of the signs of pregnancy," Devine said. Personally, she has seen it happen about five times since 1994. "Sometimes you doubt that they were completely unaware. Other times, it's completely plausible."

Pregnancy may unknowingly happen in women who are particularly ignorant about the signs and symptoms, Devine said. These include a missed period, breast tenderness, bloating, weight gain, constipation and nausea.

Obesity can affect a woman's menstrual cycle, which is why missing a period wouldn't send off warning signs. The added weight might not be a tip-off either.

If the placenta is on the front part of the uterus, people don't feel a baby's movement that much, said Dr. Kathleen Brennan, health science assistant clinical professor at UCLA Health System. A woman who doesn't realize she's pregnant would think it's gas.

And sometimes mothers aren't emotionally or mentally fit to have a child. There are also people who don't want to be pregnant and unconsciously deny a pregnancy, said Dr. Sabrina Sukhan, lead physician of laborist practice at Pennsylvania Hospital in Philadelphia.

Some women in their 40s who already have families aren't psychologically prepared to have an additional child. It's not that they are lying about not being pregnant; they genuinely aren't aware of it. Of course, some teenagers might deliberately conceal pregnancies from their parents until the last possible moment.

In eight years, Sukhan has only had one patient who unexpectedly gave birth: a woman who happened to be an emergency medical technician. She didn't particularly look pregnant, and was neither ignorant nor a teenager.

"She was in such utter shock," Sukhan remembered. "She kept saying, 'I can't believe I didn't know. I didn't have a crib ready.'"

The bottom line is that you'd need to have a "perfect storm" of factors in order to carry a baby to the point of labor without realizing it, Sukhan said. Apparently, a storm came for two women named Amanda.

Burger's story

Burger, of Cedar Falls, Iowa, was already a mother of an 11-year-old son when, unbeknownst to her, she became pregnant. She and her husband weren't using birth control and she had the attitude that "if it happened, it happened."

But she had no indication that pregnancy had happened except for a generally "weird" feeling that prompted her to take pregnancy tests over the course of several months. All three came up negative, perhaps they were outdated or Burger had drunk too much water beforehand.

She had no nausea, slept on her stomach and never felt the baby kicking. As for her menstrual cycle, Burger said she still experienced bleeding every month (experts say bleeding on a period-like schedule is highly unusual in pregnancy). While women who are pregnant don't have their periods exactly, it is possible to have bleeding related to hormonal changes of pregnancy, said Sukhan, who did not treat Burger.

Burger only gained about 15 to 20 pounds during the pregnancy. She's 5 feet 9 inches tall and weighed 170 pounds when she gave birth. "I'd go to the pool in a bikini," she said.

She was 36 weeks pregnant when she woke up around 4 a.m. one morning with severe cramping. She could barely walk. She and her husband went to the hospital, where medical staff prepared to give her a CT scan of her appendix. As part of standard procedure, they first tested to see if she was pregnant.

Since that particular hospital doesn't do births, a different hospital sent staff to her to deliver the baby.

"They had to break my water and then she pretty much just fell out," Burger said.

Her daughter McKinlee was born September 22, 2010, sharing the same birthday as Burger's uncle and grandfather McKinley, for whom the baby was named.

And her son started calling his friends' parents to tell them the news.

"He was calling everyone and telling them 'My mom's having a baby!'" she said. "And they're like 'No, quit playing jokes!'"

Prentice's story

Before this spring, Prentice and her husband had no children, and had been trying to have a baby for four years, without success.

She had always had irregular periods and thought she wasn't getting one because of stress. Her mother passed away in August of 2011, and she helped her father take care of paperwork.

The weight she gained, about 10 pounds, she attributed to drinking too much soda. She got sick once, but thought it was because of the barbecue she ate. As for the baby moving inside her, she probably thought it was gas.

One morning in April her husband came back from hunting to find Prentice in bed, unable to answer questions. There was blood on the floor from where she had bit her tongue during a seizure.

Her husband took her to a hospital in their hometown of Lawrenceburg, Tennessee. Doctors determined that Prentice appeared to be seven or eight months pregnant, and decided to send her to the larger medical center at Vanderbilt in Nashville. A helicopter took Prentice there. She doesn't remember any of this.

The staff at Vanderbilt performed a Caesarean section on the still-unconscious woman. She didn't wake up until two days later.

That's when her husband told her their daughter had been born.

"It made me so happy, but I was scared too, because I didn't know how far along I was, I didn't know if she was ok," Prentice said.

Prentice spent five days in the intensive care unit, while her daughter, Aleanna Makenleigh Rose, spent five days in the neonatal intensive care unit.

Before this, she was skeptical of women who aren't aware of their pregnancies; now, she watches "I Didn't Know I Was Pregnant" with a new perspective.

"I was one of these that was saying 'They've gotta know; they've gotta know.' Now: 'No, you don't have to know,' Prentice said. "If the good Lord wants it there, then he's going to put it there."

Risks of not knowing

There are several risks involved in having a baby if the mother didn't know she was pregnant for so many months. The baby hasn't been monitored properly, so doctors don't know which type of delivery would be safest; a large baby might be better served by a Caesarean section.

Doctors also wouldn't know about any abnormalities and complications associated with the fetus or the mother. The mom may need to take antibiotics for a bacterial infection, for example.

A mother who doesn't know she's expecting likely hasn't been monitored for hypertension, which was especially relevant for Prentice. On the day she went into labor she had eclampsia , seizures that may result as a complication of pregnancy; high blood pressure is a risk factor.

The Amandas say they would have done things differently if they had known they were pregnant. Burger wouldn't have drunk alcohol or smoked and would have taken prenatal pills and seen a doctor. Prentice doesn't smoke or drink, but she would have watched her blood pressure more.

Burger noted that her first pregnancy was also somewhat abnormal. She was 19 years old and didn't experience any symptoms then, either. She found out she was pregnant when she was five-and-a-half months along. Her son Benjamin was born at 32 weeks.

"I've never really had a 'real' pregnancy," she said. "I have two kids and no real pregnancy."