By the time 22-year-old Allyson Opfer and her doctors realized she was pregnant, she’d already been in labor for nearly 40 hours.

“I went to the hospital that day because I had terrible cramping,” Opfer, now 23, tells Reader’s Digest. “I knew cramps that bad had to be something serious.” They were—she was bringing a new life into the world.

“The ER doctor originally thought I had kidney stones,” she recalls, but “when they took me back for an ultrasound, instead of finding kidney stones, they found a baby!”

Even more shocking was the fact that the baby was full-term and in distress.

WOMAN WHO DIDN'T KNOW SHE WAS PREGNANT GIVES BIRTH WHILE IN COMA

The baby wasn’t the only one in trouble—Opfer was close to death herself.

“My blood pressure was through the roof,” she explains.

She had preeclampsia, a pregnancy-induced form of high blood pressure that can lead to seizures and strokes.

“They told me if I had waited any longer to come to the ER, I’d be dead,” says Opfer. “The only way to save my life was to deliver the baby via emergency c-section immediately.” Watch for these 11 subtle signs of preeclampsia.

Remarkably, Opfer had no idea she was pregnant. Women in their first trimester often experience fatigue and morning sickness, but Opfer didn’t have those symptoms. Throughout her pregnancy, she felt fine, and when she gained weight, she blamed it on the fact that she wasn’t eating right or exercising as much.

MOM WHO SUFFERED HEART ATTACK WHILE PREGNANT WARNS ABOUT RARE CONDITION

“I didn’t have any symptoms the entire pregnancy,” Opfer recalls.

She had even taken a pregnancy test when she realized she hadn’t had her period in several months. But the test was negative; she wasn’t surprised because her periods had always been irregular.

Experts explain that Opfer’s experience is more common than people realize.

“Depending on the body’s bone structure, the pelvis and rib cage may be shaped to more easily accommodate an enlarging uterus in an inconspicuous manner,” says Kyrin Dunston MD, FACOG, and host of “Her Brilliant Health Radio,” a podcast on iTunes. Read up on these 10 very early pregnancy symptoms you might miss.

For women with irregular periods, says Dr. Dunston, the absence of bleeding may not seem like a big deal. She explains that the vast majority of pregnancies can go nicely for both mother and baby—with no medical intervention or assistance required. But Dr. Dunston also says that Opfer’s case underscores the need for regular prenatal care to protect the health of the mother and the developing baby—and to manage potential complications such as the preeclampsia Opfer suffered.

CLICK HERE TO GET THE FOX NEWS APP

Everything turned out well for Opfer and her baby: 30 minutes after the mad rush to the operating room, she was holding her new son, Oliver, in her arms. She recovered from the preeclampsia, and a year later reports that she is thrilled with motherhood. But she isn’t the only one whose “diagnosis” turned out to be something totally different—these 11 people discovered their cancer symptoms completely on accident.

This article first appeared on Reader's Digest.