In an earlier episode, where Laura is seen going to the hospital to have Ritchie, Millie is also pregnant. However, here she is not and makes no mention of having a newborn of her own.
Even back then, before the HIPAA confidentiality laws, hospitals would never give out patients' addresses to other patients, such as when Rob repeated Peters telling him on the phone that the hospital had given him Rob's address.
Rob and Laura lived in a different house when Laura went to the hospital to have Ritchie.
Through the first half of the show, including at the hospital, Mr and Mrs Peters had a baby girl. When Rob was discussing the two babies with Jerry, she had changed to a baby boy, and remained a boy until the end of the show.