-
Now Playing: Belly fat is bad for your heart, researchers say
-
Now Playing: Moms whose babies have heart defects may have long-term heart health risks
-
Now Playing: Study shows rear-facing car seats perform well in rear-end crashes
-
Now Playing: Health officials warn of 'nightmare' bacteria
-
Now Playing: Later school start times help teens' moods, study says
-
Now Playing: Night owls at high risk of poor health, study says
-
Now Playing: 35 reported cases of E. coli linked to romaine lettuce: CDC
-
Now Playing: Could caffeine be good for your heart?
-
Now Playing: Moms who have depression early on may have kids with lower IQ scores: Study
-
Now Playing: What is the definition of twins?
-
Now Playing: Even a mild head injury increases risk for Parkinson's disease, veterans study shows
-
Now Playing: Family history of heart disease doesn't mean you shouldn't exercise, study shows
-
Now Playing: Hot off the press: Can peppers cause headaches?
-
Now Playing: Twins suffering from liver disease, transplant saves one but other dies
-
Now Playing: Chewbacca tells boy he's getting new heart
-
Now Playing: 1 in 3 young adults ride with impaired drivers, according to new study
-
Now Playing: 10 measles cases reported in one state
-
Now Playing: Children more likely than before to draw scientists as women, study shows
-
Now Playing: Binge drinkers down 17.5 billion drinks a year: Study
-
Now Playing: Health benefits of exercise in short burst or longer bouts similar, new study finds