On Monday, Jenna Bush Hager shared the happy news that she is expecting her third child. But on Tuesday, she revealed that she had suffered a frightening complication with her first pregnancy: it was an ectopic pregnancy, which occurs when a fertilized egg attaches outside the uterus,

“Before I had Mila [her first child], I had a pregnancy in my fallopian tube,” the Today show co-host told Meredith Vieira during the show.

Bush Hager, 37, who is the daughter of former President George W. Bush and first lady Laura Bush, said she took a pregnancy test after feeling morning sickness, then went to a physician to confirm the positive results.

"I go to the doctor’s office and she said, ‘Yeah, you’re pregnant, we gave the blood test, but we can’t find the baby.’ "

“I had no idea what an ectopic pregnancy was,” Bush Hager added. “They looked ... and the baby was in my fallopian tube.”

Bush Hager underwent emergency surgery to remove her fallopian tube. The crisis, she said, was “very isolating,” especially because her husband, Henry Chase Hager, was out of town at the time.

But the couple went on to have Mila, now 6, and Poppy, 3. “There is joy and there is pain,” said the expectant mother.

Her candor offers an opportunity to share a brief overview of ectopic pregnancy, according to experts.

How does this problem occur?

According to the American College of Obstetricians and Gynecologists, more than 90 percent of the time an ectopic pregnancy outside the uterus happens when a fertilized egg grows in one of the two fallopian tubes, which normally transport an egg, fertilized or not, from the ovary to the uterus.

The condition is dangerous because the tube can rupture as the pregnancy grows, causing internal bleeding that can be life-threatening.

Are some women at higher risk?

About one in 50 pregnancies in the U.S., or 2 percent, are ectopic, according to the March of Dimes. Women who have already had an ectopic pregnancy are at slightly higher risk. Other risk factors include pelvic surgery, endometriosis, and certain sexually transmitted diseases.

How is it diagnosed?

At first, ACOG says, an ectopic pregnancy may feel like a normal one, with a missed period, nausea, and breast tenderness. Abnormal bleeding and low back pain may be early signs of trouble. A pregnancy test, pelvic exam, and an ultrasound exam are used to diagnose it.

How is ectopic pregnancy treated?

Because a fertilized egg can’t survive outside the uterus and can’t be relocated from the tube, it has to be eliminated. If the pregnancy is very early, an injection of methotrexate, an immune system supressant, can stop the cells from growing and the body absorbs the tissue over a period of about six weeks.

If the fallopian tube is damaged or ruptured, surgery under general anesthesia is required. It is usually done with laparoscopy, using a camera and instruments inserted through small abdominal incisions. A laparotomy, which involves a larger incision, may be necessary in an emergency.

What about having a baby in the future?

The chances of having a successful pregnancy are high, and studies suggest 85 percent of women have a healthy pregnancy within two years of an ectopic one, according to the Ectopic Pregnancy Trust, a London-based nonprofit that helps people with pregnancy loss.

While the removal of a fallopian tube does reduce fertility, it is not cut in half. The fallopian tubes have delicate finger-like structures that create a slight vacuum to suck an egg into the tube. When an egg is released by the ovary on the tubeless side, it manages to wind up in the tube on the other side about 20 percent of the time.