The first baby born to a main character in Friends is Ben Geller, the son of Ross Geller and Susan Bunch.
Ben was born in the season 4 episode "The One with the Baby on the Bus." While Ross and Susan were married, they had a tumultuous relationship, and Susan unfortunately passed away before Ben's first birthday. Ross, however, took on the role of single father to Ben, with help from his friends, especially Rachel, who ultimately became Ben's stepmother.
