S4·E13 Matthew's Old Lady

When Matthew shows up at Grace's and announces he's about to marry Joan, a woman old enough to be his mother who also happens to be pregnant, Grace tries to keep her cool, especially when Matthew tells her his adoptive mother has no objections. But when Grace speaks to Emily, she learns that the only reason she didn't object was that Matthew seemed so adamant about marrying this woman. As the wedding day gets nearer, Joan confesses to Grace that although she definitely wants the baby, she doesn't want to marry Matthew in spite of how determined he seems to go through with the marriage. When Grace confronts Matthew about Joan's feelings, Matthew says he wants to marry Joan because he won't abandon this child. Grace understands that Matthew feels on some level that he himself was abandoned by her, and she tells him that he can still be very much a part of the baby's life without marrying the baby's mother.