The ECHO Cohort is a longitudinal prospective multi-site study aiming to understand how the environment and things that happen early in children's lives - even before they are born - affect their development, health, and well-being. This research program includes many study sites across the United States. ECHO will combine information about 20,000 pregnant participants, more than 50,000 children, and their families. With so many participants from across the country, researchers can answer important childhood health questions.