Twins Born in Guangzhou

This cohort study follows twin babies born in Guangzhou, China, from birth through childhood to understand how genetics and the environment shape growth, development, and health. Data from the mother's pregnancy is also used. You may be eligible if: - You are a set of live twins delivered at Guangzhou Women and Children's Medical Center - Your family plans to remain in Guangzhou for at least 3 years - Baseline pregnancy information is available from the mother You may NOT be eligible if: - Your family does not plan to stay in Guangzhou long-term - Pregnancy baseline data is not available Talk to your doctor to see if this trial is right for you.