When 31-year-old Andrea Curry put on an extra 10 pounds last year, she attributed it to stress.
"I thought I was just gaining weight because I had been stressed out about a few things," Curry, a busy mother of a toddler-age son, Tyler, told ABC News.
And as Curry continued to menstruate, she figured everything in her body was normal.
"I had my period every month, so nothing shot up as a red flag in my mind," she said. "When I was pregnant with my first son, I found out I was pregnant because I stopped getting my period. I had morning sickness for the seven or eight of the months I was pregnant with him. I definitely felt pregnant."
Even when her mother told her she looked pregnant in the clothes she was wearing, Curry, who lives in Rochester, N.Y., didn't believe it.
"I said, 'Well, if I am, there are some big issues going on because I haven't felt anything that would make me feel like I was pregnant,'" she said. "I said to her, 'Oh, no, I am just getting fat.'"
It was not until one night in October, nine months after her initial weight gain, that Curry learned, the hard way, that her mother had been right.
"My back pain started in here [near her abdomen] when Tyler and I were playing," she recalled of the night she went into labor. "I couldn't sit. I was in so much pain. It was doubling me over at the kitchen counter."
Curry's fiance, Brad, called 911. The operator who answered believed Curry was having a miscarriage.
"I said, 'I don't think I am,'" Curry recalled. "I said that I never had a miscarriage before, and I don't know what it would be like."
Once the paramedics arrived and transported Curry into the waiting ambulance, it was clear that Curry was not having a miscarriage at all. She was giving birth.
"When we were in pursuit to the hospital, the lady in the back was telling me she saw an umbilical cord there," Curry said. "That is when she informed me that I was, in fact, in labor, and that is when it hit me that I was having a baby."
In that instant, Curry became part of a small, exclusive club of women who conceive and carry their babies all the way to labor and delivery with no idea that they were ever pregnant.
The medical phenomenon of surprise pregnancies garnered worldwide headlines in December 2009 when Chilean Olympic weightlifter Elizabeth Poblete gave birth while training, unaware she was pregnant.
And now the phenomenon has commandeered its own reality show, TLC's "I Didn't Know I Was Pregnant." The show is now its second season, with more than 10 episodes per season, each featuring the story of a woman who gave birth out of the blue.
"My main thought was 'Wait, what?'" Curry said. "I was in complete and utter shock. I thought maybe it was a dream I was having."
When Curry delivered a healthy baby boy, Alex, both she and Brad were immediately racked with guilt and self-doubt.
"Both of us wondered, 'How could we have not known?'" said Brad.
The nurses at the hospital, Curry believed, asked questions meant to second-guess the new mom, even though her baby bump the second time around was barely noticeable.
"I was thinking maybe I shouldn't have taken things so lightly, all my stress and weight gain," recounted Curry. "Instead of trying to do sit-ups, I should have looked more into it."
"I was definitely upset," she said. "I went through all the last how many months, thinking, 'Oh, I felt this,' and 'Maybe that is what this was' and 'Oh, why did I do that?'"
Dr. Ashley Roman, a clinical assistant professor at NYU School of Medicine and a specialist in high-risk pregnancies, said surprise pregnancies aren't so implausible after all.
"There really doesn't seem to be much scientific data on this topic, but I think if you were to poll every obstetrician, we've all seen it at some point in our career," Roman told ABC News in an interview last year. "We've seen the woman who comes in at term or near term and who didn't realize she was pregnant, and gives birth."
Roman said there are reasons a woman can carry a baby full-term and miss the signs.
"What's difficult, what many people say is, but how could you miss your growing belly? But some women tend to hide it well, their body just simply disguises it," she said. "They might chalk it up to the fact that they, maybe, they've put on a couple pounds and, 'I've gained weight for one reason or another.' There are a lot of ways to talk yourself out of this."
In addition to weight gain and, perhaps, denial, medical experts say factors like stress, dieting, a small or inactive fetus, obesity, a history of irregular cycles aor infertility or breakthrough bleeding during the pregnancy can also contribute to the "surprise pregnancy" phenomenon.
For Curry and her family, the trauma of the unknown pregnancy and surprise birth is just a memory, one now replaced by the joy of having another member of the family.
"Having Alex was a shock, but it was one of the greatest things in the world," she said. "We were overjoyed, and Tyler was overjoyed to have a new baby brother."
"He has taught me to not take anything for granted," she said. "Unfortunately, it had to be done in this way, but ever since he was in my life, I have had nothing but happiness and couldn't have asked for a better family."
The episode of "I Didn't Know I Was Pregnant," featuring Andrea Curry airs on TLC, tonight, Wednesday, Aug. 17, at 9:30 p.m., ET.