Prevalence of microcephaly in an Australian population-based birth defects register, 1980-2015.

OBJECTIVES: To describe the prevalence and characteristics of microcephaly in a geographically defined Australian population.

DESIGN, SETTING AND PARTICIPANTS: Descriptive epidemiological study of microcephaly cases ascertained by the Western Australian Register of Developmental Anomalies, 1980-2015, defining microcephaly as an occipito-frontal head circumference below the third percentile or more than two standard deviations below the mean sex- and age-appropriate distribution curve.

MAIN OUTCOME MEASURES: Microcephaly prevalence (per 10 000 births) was calculated for cases with known and unknown causes, and by demographic characteristics. Temporal trends were analysed, and prevalence ratios (PRs) and 95% CIs calculated for Aboriginal and non-Aboriginal births.

RESULTS: For births during 1980-2009 (ie, with at least 6 years' follow-up and therefore complete case ascertainment), 416 cases were identified, a prevalence of 5.5 per 10 000 births (95% CI, 4.95-6.02), or 1 in 1830 births. There was no significant temporal trend in prevalence (P = 0.23). Most cases were in live-born children (389, 93.5%), and other major birth defects were diagnosed in 335 of the affected children (80%). Prevalence was higher in Aboriginal births (PR, 4.5; 95% CI, 3.55-5.73). A cause of microcephaly was identified in 186 cases (45% of cases), and more often for Aboriginal (64 cases, 70%) than non-Aboriginal births (122 cases, 38%). The most frequent known cause of microcephaly in Aboriginal births was fetal alcohol spectrum disorder (FASD; 11 per 10 000 births); monogenic (0.68 per 10 000) and chromosomal conditions (0.59 per 10 000 births) were the most common causes in non-Aboriginal births.

CONCLUSIONS: These data provide a baseline for prospective surveillance of microcephaly. We identified a high proportion of cases without known cause, highlighting the need for clinicians to carefully investigate all possibilities, including emerging infections. FASD is an important cause of microcephaly in the Aboriginal population.