STUDY QUESTION In a country with supportive funding for medically assisted reproduction (MAR) technologies, what is the proportion of MAR births over-time? SUMMARY ANSWER In 2017, 6.7% of births were conceived by MAR (4.8% ART and 1.9% ovulation induction (OI)/IUI) with a 55% increase in ART births and a stable contribution from OI/IUI births over the past decade. WHAT IS KNOWN ALREADY There is considerable global variation in utilization rates of ART despite a similar infertility prevalence worldwide. While the overall contribution of ART to national births is known in many countries because of ART registries, very little is known about the contribution of OI/IUI treatment or the socio-demographic characteristics of the parents. Australia provides supportive public funding for all forms of MAR with no restrictions based on male or female age, and thus provides a unique setting to investigate the contribution of MAR to national births as well as the socio-demographic characteristics of parents across the different types of MAR births. STUDY DESIGN, SIZE, DURATION This is a novel population-based birth cohort study of 898 084 births using linked ART registry data and administrative data including birth registrations, medical services, pharmaceuticals, hospital admissions and deaths. Birth (a live or still birth of at least one baby of >= 400 g birthweight or >= 20 weeks' gestation) was the unit of analysis in this study. Multiple births were considered as one birth in our analysis. PARTICIPANTS/MATERIALS, SETTING, METHODS This study included a total of 898 084 births (606 488 mothers) in New South Wales and the Australian Capital Territory, Australia 2009-2017. We calculated the prevalence of all categories of MAR-conceived births over the study period. Generalized estimating equations were used to examine the association between parental characteristics (parent's age, parity, socio-economic status, maternal country of birth, remoteness of mother's dwelling, pre-existing medical conditions, smoking, etc.) and ART and OI/IUI births relative to naturally conceived births. MAIN RESULTS AND THE ROLE OF CHANCE The proportion of MAR births increased from 5.1% of all births in 2009 to 6.7% in 2017, representing a 30% increase over the decade. The proportion of OI/IUI births remained stable at around 2% of all births, representing 32% of all MAR births. Over the study period, ART births conceived by frozen embryo-transfer increased nearly 3-fold. OI/IUI births conceived using clomiphene citrate decreased by 39%, while OI/IUI births conceived using letrozole increased 56-fold. Overall, there was a 55% increase over the study period in the number of ART-conceived births, rising to 56% of births to mothers aged 40 years and older. In 2017, almost one in six births (17.6%) to mothers aged 40 years and over were conceived using ART treatment. Conversely, the proportion of OI/IUI births was similar across different mother's age groups and remained stable over the study period. ART children, but not OI/IUI children, were more likely to have parents who were socio-economically advantaged compared to naturally conceived children. For example, compared to naturally conceived births, ART births were 16% less likely to be born to mothers who live in the disadvantaged neighbourhoods after accounting for other covariates (adjusted relative risk (aRR): 0.84 [95% CI: 0.81-0.88]). ART- or OI/IUI-conceived children were 25% less likely to be born to immigrant mothers than births after natural conception (aRR: 0.75 [0.74-0.77]). LIMITATIONS, REASONS FOR CAUTION The social inequalities that we observed between the parents of children born using ART and naturally conceived children may not directly reflect disparities in accessing fertility care for individuals seeking treatment. WIDER IMPLICATIONS OF THE FINDINGS With the ubiquitous decline in fertility rates around the world and the increasing trend to delay childbearing, this population-based study enhances our understanding of the contribution of different types of MARs to population profiles among births in high-income countries. The parental socio-demographic characteristics of MAR-conceived children differ significantly from naturally conceived children and this highlights the importance of accounting for such differences in studies investigating the health and development of MAR-conceived children. STUDY FUNDING/COMPETING INTEREST(S) This study was funded through Australian National Health and Medical Research Council (NHMRC) grant: APP1127437. G.M.C. is an employee of The University of New South Wales (UNSW) and Director of the National Perinatal Epidemiology and Statistics Unit (NPESU), UNSW. The NPESU manages the Australian and New Zealand Assisted Reproduction Database with funding support from the Fertility Society of Australia and New Zealand. C.V. is an employee of The University of New South Wales (UNSW), Director of Clinical Research of IVFAustralia, Member of the Board of the Fertility Society of Australia and New Zealand, and Member of Research Committee of School of Women's and Children's Health, UNSW. C.V. reports grants from Australian National Health and Medical Research Council (NHMRC), and Merck KGaA. C.V. reports consulting fees, and payment or honoraria for lectures, presentations, speakers, bureaus, manuscript, writing or educational events or attending meeting or travel from Merck, Merck Sparpe & Dohme, Ferring, Gedon-Richter and Besins outside this submitted work. C.V. reported stock or stock options from Virtus Health Limited outside this submitted work. R.J.N. is an employee of The University of Adelaide, and Chair DSMC for natural therapies trial of The University of Hong Kong. R.J.N. reports grants from NHMRC. R.J.N. reports lecture fees and support for attending or travelling for lecture from Merck Serono which is outside this submitted work. L.R.J. is an employee of The UNSW and Foundation Director of the Centre for Big Data Research in Health at UNSW Sydney. L.R.J. reports grants from NHMRC. The other co-authors have no conflict of interest.
