Hear from filmmakers Tim Wardle and Becky read about the process of investigating the compelling story behind Three Identical Strangers in a special bonus episode.

Sanjay dives into the fraught history of domestic adoption in the United States. First, he'll speak with adoption historian Dr. E. Wayne Carp to get some context and hear about the role that secrecy has played in American adoption practices. Then, writer Rene Denfeld shares her heartwarming story of starting a family.

Can twins feel each other's pain? Do they share a special bond? Sanjay will attempt to answer these questions by speaking with twin astronauts Mark and Scott Kelly, the research subjects of a fascinating NASA experiment about their own twin relationship. He will also get a more scientific explanation of all that twins share from Dr. Nancy Segal.

Why are there so many multiples in American elementary schools right now? This episode explores the connection between multiples and fertility. First, hear from two CNN anchors you probably already wake up with, Alisyn Camerota and John Berman-who both happen to be the parents of twins-on what it's like to parent multiples. Sanjay also talks with Dr. Lucky Sekhon, a fertility specialist, who explains the rapidly changing medicine behind fertility treatments.

Can ethically questionable medical experiments ever lead us to groundbreaking scientific discoveries? Dr. Sanjay Gupta dives into the complex world of medical ethics with NYU's Dr. Barron Lerner. Sanjay also speaks with Lawrence Wright, the journalist who wrote the groundbreaking investigation behind Three Identical Strangers.

Does our genetic code matter more than our environments? Join CNN's Chief Medical Correspondent Dr. Sanjay Gupta as he wrestles with the age-old nature-nurture debate. First, Sanjay brushes up on genetics 101 with Dr. Ruth Tennen. Then, he discusses what studying twins can teach us about nature and nurture with twin expert and psychologist Dr. Nancy Segal.

Join CNN's Chief Medical Correspondent as he explores the themes that emerge in the award-winning CNN Film Three Identical Strangers.