mutual child
n. A child born to a couple who already have children from previous marriages.
Q. My wife is jealous of my 4-year-old from a previous marriage, who recently joined us. We are fine when our mutual child is in the room, but miserable when my son is. - Utah man.

A. When stepfamilies are formed, it is key to discuss the structure - who will live with whom. If this change was unexpected, your wife's resentment is understandable.

If, however, she knew this was a possibility, her resentment may be with you, not your child. Something else is going on here. Either talk with her directly for some understanding, or consult a therapist who specializes in stepfamilies.
—Karen S. Peterson, “Experts help smooth family rough spots,” USA Today, November 29, 1994
1986 (earliest)
Elizabeth Cronk, 34 years old, had served as stepmother through two previous marriages. Now she was eager to have a child of her own. But her current husband already had a 6-year-old son and was not quite ready for another child.

This worried Ms. Cronk. ''I kept saying to myself, 'Will he throw it up at me if I insist? Will he be as happy as I will?' '' Ms. Cronk, a stock trader, and her husband discussed their problem at a recent workshop organized by the New York Metropolitan Chapter of the StepFamily Association on making the decision to have a mutual child. Their dilemma was typical of those faced by the 26 other people present, all of whom were also in second or third marriages and thinking about having children…

Dr. Katherine Baker is a family therapist in Washington who has studied the effect of a mutual child on a stepfamily in her practice. ''A mutual child can provide a sense of cohesion that draws everyone closer together,'' she said. Others have also reported that it made their roles as stepparents easier to handle.
—Andree Brooks, “His, hers and their children,” The New York Times, December 01, 1986