Now Playing: Moms who have depression early on may have kids with lower IQ scores: Study

Now Playing: 35 reported cases of E. coli linked to romaine lettuce: CDC

Now Playing: Could caffeine be good for your heart?

Now Playing: Study shows rear-facing car seats perform well in rear-end crashes

Now Playing: Health officials warn of 'nightmare' bacteria

Now Playing: What is the definition of twins?

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: Later school start times help teens' moods, study says

Now Playing: Moms whose babies have heart defects may have long-term heart health risks

Now Playing: Night owls at high risk of poor health, study says

Now Playing: Day care teacher loses foot to flesh-eating bacteria

Now Playing: Breast cancer testing may need to be tailored by race: Study

Now Playing: Binge drinkers down 17.5 billion drinks a year: Study

Now Playing: 1 in 3 young adults ride with impaired drivers, according to new study

Now Playing: Chewbacca tells boy he's getting new heart

Now Playing: 10 measles cases reported in one state

Now Playing: Could a pill help with peanut allergies?

Now Playing: Obesity epidemic grows in adults; perhaps levels off in youth

Now Playing: Health benefits of exercise in short burst or longer bouts similar, new study finds