Background: Australia has remarkably detailed data on non-marital pregnancy dating from 1908. They both offer insight into long-term trends in childbearing resulting from non-marital sexual activity and reveal in historical context key features of the second demographic transition and its genesis.

Objective: Trends are traced in rates of non-marital conception of children ultimately born both outside and within marriage. A range of related indices is also presented in examining how demographic behaviour surrounding non-marital pregnancy (i) helped generate the second demographic transition and (ii) unfolded as a component of it.

Methods: Core indices are rates of non-marital conception partitioned into additive components associated with marital and non-marital confinement. Data on non-marital and early marital births (at marriage durations 0-7 months) are lagged back 38 weeks to a date of and age at conception basis to facilitate a common, unmarried, population at risk.

Results: Post-war weakening of parental oversight of courtship was a fundamental trigger to the broader rejection of normative and institutional values that underpinned the second demographic transition. In tandem with denying the unmarried access to oral contraception it generated rampant youthful non-marital pregnancy, which undermined Judeo-Christian values, especially once abortion law reform occurred.

Conclusions: Childbearing following non-marital conception transitioned rapidly after the 1960s from primarily the unintended product of youthful intercourse in non-coresidential relationships to mainly intended behaviour at normative reproductive ages in consensual unions. Family formation increasingly mixed non-marital births and premaritally and/or maritally conceived marital births.