The first question was, what do we mean by an “unintended pregnancy”? This is a technical definition. It is what women themselves say about a recent pregnancy, that it either was unwanted or that it occurred two or more years earlier than they wanted.
Health risks in pregnancy are greatest if pregnancies occur too early in a woman's life, say, before she's 20, when her body is finally fully developed and ready to bear a child, if pregnancies are too closely spaced together, or if they occur too late in a woman's life. Many unintended pregnancies fall into one or more of those categories.
They involve very high health risks to the mother as well as poor prospects for the newborn and infant.