Childlessness is defined as the proportion of couples who have not had a live birth by the time of interview, despite at least five years of cohabitation and exposure to pregnancy and in the absence of contraception, breastfeeding or postpartum amenorrhoea. Unlike a couple with primary infertility, a childless couple also includes those who have successfully conceived but have failed to deliver a live birth. Similarly, secondary sterility in these studies refers to couples having difficulty bearing a second or higher order birth, despite usually five years of exposure, as in the definition above. The five year reference period is typically used, but not necessarily, in demographic surveys.