Sunday Children (1977)

A middle-aged pair of newly weds is still without a child after two years of marriage. It's an embarassment so they decide to adopt a child. This takes time, however, and they cannot wait. The solution is to fake a pregnancy and make a deal with a young girl expecting an illegitimate child, who in turn does not want give the baby up to the adoption home. The deal is made, pregnancy faked and then the middle aged wife suddenly finds herself pregnant. Now the deal is off and the young expecting mother is facing a new decision. She does keep the baby in the end.