'Switched at Birth' is one of This American Life’s most popular episodes, and rightfully so. It’s one of those crazy stories that host Ira Glass somehow manages to keep grounded and relatable.

In this episode we hear the stories of Mrs. Kay McDonald, who raised a daughter named Sue, and Mrs. Mari Miller, who raised a daughter named Marti.

In the summer of 1951 Marti Miller and Sue McDonald were born in a hospital in small-town Wisconsin. The babies were accidentally switched and went home with the wrong families.

Neither of the girls knew anything about it until 43 years later when Mrs. Miller sent a letter to both Marti and Sue that read:

'Dear Martha and Sue, have you ever suspected or been told that we took home the baby that belonged to Kay and Bob McDonald and they later took home the baby that belonged to us?'

So already, that’s crazy, but here’s the kicker. Mrs. Miller knew she had the wrong baby as soon as she got home from the hospital in 1951, but didn’t say anything. Primarily at the insistence of her husband Reverend Miller (he didn’t want to embarrass the doctor!) Mrs. Miller did not do anything about her suspicion. The other mother, Mrs. McDonald, didn’t suspect the daughter she was raising was not biologically hers.

The show is about the emotional fallout that followed Mrs. Miller’s letter. This is one of those episodes that I don’t want to spoil for you because hearing these women talk about why this happened and what happened after everything came out in the open is truly mind-blowing.

Bonus info: On July 9, 2015, The New York Times Magazine did a profile on two pairs of Colombian identical twins in Colombia who were switched at birth and raised as two pairs of fraternal twins. It’s a great read with an added twin level.