The book I am searching for may be from late nineteen forties through early sixties. The mother of two (a high-school boy and a married daughter) is the main character. The daughter, named Tilghman, has her first baby near the end of the story. Then the main character, now a grandmother, finds out she is expecting!
It may be something by Janet Lambert. She had quite a few different series about families growing from young teens to adults. Each series was about a different family, but the series sometimes crossed over when the characters married.
Thanks for your comment. I looked through all of her book descriptions that I could find and none sound familiar. But thanks anyway!