Which 1950s sitcom starring Lucille Ball featured a major character's pregnancy as a central plot point?

I Love Lucy

I Love Lucy made television history when Lucille Ball's real-life pregnancy was written into the show, though the word "pregnant" was forbidden by censors and replaced with "expecting." The episode where Little Ricky was born was so popular that it attracted more viewers than the inauguration of President Dwight D. Eisenhower.