Introduction Globally, over half of pregnancies in developed countries are unplanned. Identifying and understanding the prevalence and complexity surrounding pregnancy preparation among Australian women is vital to enable sensitive, responsive approaches to addressing preconception and long-term health improvements for these women with varying motivation levels. Aim This study evaluated the reliability and validity of a comprehensive pregnancy planning/ intention measure (London Measure of Unplanned Pregnancy) in a population of pregnant women (over 18 years of age) in Australia. Methods A psychometric evaluation, within a cross-sectional study comprising cognitive interviews (to assess comprehension and acceptability) and a field test. Pregnant women aged over 18 years were recruited in early pregnancy (approximately 12 weeks’ gestation). Reliability (internal consistency) was assessed using Cronbach’s alpha, corrected item-total correlations and inter-item correlations, and stability via a test-retest. Construct validity was assessed using principal components analysis and hypothesis testing. Results Six women participated in cognitive interviews and 317 in the field test. The London Measure of Unplanned Pregnancy was acceptable and well comprehended. Reliability testing demonstrated good internal consistency (alpha = 0.81, all corrected item-total correlations >0.20, all inter-item correlations positive) and excellent stability (weighted kappa = 0.92). Validity testing confirmed the unidimensional structure of the measure and all hypotheses were confirmed. Conclusions The London Measure of Unplanned Pregnancy is a valid and reliable measure of pregnancy planning/intention for the Australian population. Implementation of this measure into all maternity healthcare, research and policy settings will provide accurate population-level pregnancy planning estimates to inform, monitor and evaluate interventions to improve preconception health in Australia.