後腹 (atobara: afterpains, repercussions; consequences; child by one’s second wife) after + abdomen
The last definition is strange, no? Traditionally, the wife’s most important role was provide her husband with an “abdomen” (a womb) to bear a child. So in 後腹, the second kanji means “wife” or “woman.” As you can see, “child” has been omitted from the expression.
This compound has an antonym, 先腹 (sakibara: before + abdomen) meaning “child by one’s first wife.”