Each year in the United States, about 40,000 babies are born with congenital heart defects (CHD), more than any other type of birth defect, according to the U.S. Centers for Disease Control and Prevention. Most of the heart defects have no known cause, but now, because of the success of modern prenatal care and heart surgery for newborns, more of these children survive into adulthood than ever before. Childhood mortality linked to CHD dropped by nearly 60 percent from 1987 to 2005, found a study published in the Journal of the American College of Cardiology.

But many people born with congenital heart defects don't realize how extensively their hearts continue to be different from others' hearts. CHD is not just a pediatric health concern.

Here are 10 things you might not have heard from your doctor about congenital heart defects in adults.

1. Congenital heart defects are not all the same.

For each of the 1 in 100 babies born with a congeni tal heart defect — a problem with the structure of the heart — the condition may be unique. This is because there are more than 40 known types of congenital heart defects, notes the Children’s Heart Foundation, and even more that are one-of-a-kind. Throughout life, differences persist.

“No two adult patients have exactly the same symptoms; each one is their own rare disease,” explains 63-year-old Paula Miller of Memphis, Tennessee, a nurse born with a congenital heart defect. She now works as the member services manager for the Adult Congenital Heart Association (ACHA), providing support for adults with heart defects.

The most common heart defect is pulmonary valve atresia and stenosis, which occurs when a heart valve does not open completely, found a 2012 national survey of U.S. birth records. The second most common, according to the National Heart, Lung, and Blood Institute, is the complex defect known as tetralogy of Fallot. In people with this condition, the heart has a hole between the ventricles and a blockage where the heart normally would lead to the lungs, among other defects.

2. More adults than children are living with congenital heart defects.

You may not even know you were born with a congenital heart defect until you're diagnosed in adulthood. And as an adult with congenital heart disease, you may feel alone at times — but you aren’t. “For 49 years I never met another person who was an adult living with ACHD,” says Miller, “and I was a nurse.” Now, more than 1 million U.S. adults live with congenital heart defects, cardiologists estimate, while the number of children with CHD is under 1 million. The U.S. numbers are actually based on rates in Canada, where public health systems track patients throughout their lives.

“In the United States we don’t have accurate numbers because there is no system to follow up with adults. Children grow up and are lost to follow-up,” Miller says. This should be changing soon, because Congress passed the Congenital Heart Futures Act in 2010, a law that provides funding for CDC tracking across a patient’s lifespan. But for now, Miller says, “We don’t know who the oldest person living with ACH is. We have no way of knowing.”

3. Surgery for a congenital heart defect is not a cure.

If you had heart surgery as a child, you need follow-up as an adult. “Physicians use the word ‘fixed,' but you are not fixed. You are not cured,” says Miller, who adds this is the fact about CHD that surprised her the most. Adults with CHD can experience ongoing health issues like abnormal heart rhythms, stroke, heart failure, and even premature cardiovascular death, as well as tripled rates of hospitalization compared with other adults, according to the ACHA.

Symptoms are easily overlooked because they are not specific. You might feel heart palpitations, have difficulty exercising, or experience increasing fatigue and shortness of breath, according to Elizabeth E. Adams, DO, a congenital cardiologist at the Penn State Hershey Medical Center, in Hershey, Pennsylvania. People "may want to bury their head in the sand," says Dr. Adams, rather than face the possibility of a worsening heart problem.

4. If you were born with a congenital heart defect, your adult heart needs specialized care.

You should see a cardiologist with specialized training in congenital heart defects at least once, say American College of Cardiology and American Heart Association guidelines, and yearly for more complex heart defects. But according to the ACHA, under 10 percent of people born with congenital heart defects actually get the medical care they need as adults. The right provider is not the same as a general cardiologist, who may have had only six hours of lecture training in CHD, with no hands-on experience, Miller says. In contrast, as an adult congenital heart specialist, Adams had more than a year of training specifically in adult congenital heart defects.

This area will soon become a new specialty, with the first board exams scheduled for October 2015 by the American Board of Internal Medicine. A quick look at the U.S. map of adult congenital heart disease (ACHD) clinics maintained by the Adult Congenital Heart Association shows that these centers are few and far between in some regions. You might need to travel to get the specialized care you need. Miller is from Tennessee, which has only two ACHD clinics in the state. She suggests seeing a local doctor, but also using a team approach that includes an adult CHD specialist. “Find someone who will let you send records to them, and then will talk to your local doctor,” she says.

5. Living with a congenital heart defect doesn’t exempt you from other serious illnesses.

As an adult with CHD, you are still susceptible to other conditions of aging, says Miller, which surprises and disappoints people. “You think that this is what you have, and you’re not going to get anything else. But you can develop osteoporosis and the diseases of aging. These things still happen as you get older," she explains.

A diagnosis of uterine cancer shocked lifelong cardiac patient Alissa Butterfass, mother of two young children. Butterfass was born with a heart defect called a transposition of the great vessel. “I had a heart defect. I was done. I was safe,” Butterfass wrote in her personal story shared at ACHA, “but I am still not immune to cancer.”

6. Heart imaging tests look different for people with congenital heart defects.

Because of this, cardiologists who haven’t had specialized training in congenital heart defects could miss important signs of disease. The opportunity might pass right by at the very moment you urgently needed a heart valve replaced, for example. As Miller notes, “They are used to looking at a normally structured heart. We have funky hearts and need special treatment.” The defects in heart structure can be holes in the heart, missing vessels, malformed heart valves, or varied combination of changes in the normal heart layout.

7. As an adult with a congenital heart defect, you need to exercise.

“Growing up, I was never allowed to exercise,” Miller recalls. “Now they’ve found exercise is extremely important! They encourage people to walk and move.” Miller suggests working with your doctor to find a safe, appropriate exercise plan. The American Heart Association (AHA) notes that most people born with heart defects do not need to limit their physical activity.

RELATED: Congenital Heart Disease: A Surprising Adult Diagnosis

8. You might need a pacemaker as you get older.

“It had never been mentioned to me that I might need a pacemaker,” Miller says. “But at 42, I had a pacemaker put in, when I had two young children.” A pacemaker is implanted under the skin and uses an electrical impulse to keep the chambers of the heart beating regularly. An abnormal heart rate, called arrhythmia, can develop in many people living with a congenital heart defect. Tachycardia, a very rapid heart rate, is particularly common, notes a review in Circulation. And as Miller experienced, some CHD patients will have atrial fibrillation as adults.

Inserting pacemaker leads into the heart is more complicated for many adult patients with CHD, says Adams. "If you don't have all the right connections in the heart, it may be more complicated to thread the leads," she explains.

9. Drinking alcohol can be riskier for you if you were born with a congenital heart defect.

“Drinking alcohol can cause dehydration and can aggravate arrhythmia,” says Miller, who has uncontrolled atrial fibrillation — the most common heart rhythm abnormality. In addition to alcohol, smoking and stimulants like cocaine are particularly dangerous when you are living with a congenital heart defect. Using IV drugs puts your heart at an even higher risk, according to the AHA.

10. Finding a support network can be a life-changing event.

“Having a support network of people that understood you was life-changing for me. It lit up a passion in me,” says Miller, who now works with the ACHA. The discussion forum at ACHA has 8,400 members and counting, covering thousands of topics. And heart-to-heart ACHA ambassadors are available to guide patients and their families for added personal support along their health journey. The Support Network community at the AHA also has an online forum for parents of children with congenital heart disease.