A mass shooting at Port Arthur, Tasmania, Australia: a study of its impact on early pregnancy losses using a conception time-based methodology.

Study Question: Does an acute calamity in a community cause early miscarriage and is this association the same for male and female fetuses?

Summary Answer: Estimated losses of 29.5% of first trimester pregnancies in the affected region could be associated with an acute calamity, with no statistically significant difference in estimated losses by fetal sex.

What Is Known Already: There are very few studies on the impact of a calamity on early pregnancy loss and its differential effects on male and female fetuses. A decline in the human sex ratio at birth associated with the events of 9/11 in New York has been documented.

Study Design, Size, Duration: This is a retrospective descriptive study of birth register data in Tasmania, Australia, from 1991 to 1997, covering the period in which the calamity occurred. The register contains data on all pregnancies that proceeded to >20 weeks gestation. The conception date was calculated by subtracting gestational age from birth date. We estimated that 40 318 pregnancies were conceived in the period 1991-1996 inclusive. These were aggregated to 4-weekly blocks classified by region and sex.

Participants/materials, Setting, Methods: The acute calamity was at Port Arthur, Tasmania, Australia. On 28 April 1996, a gunman opened fire on visitors and staff in a tourist cafe. A very stressful 20 h period, ended with 35 people dead and 22 injured. A negative binomial regression model was used to assess the association between this calamity and pregnancy loss. This loss is evidenced by a shortfall in the registration of pregnancies that were in their first trimester at the time of the calamity.

Main Results And The Role Of Chance: We estimated a shortfall of 29.5% or 229 registered pregnancies among those in the first trimester at the time of the calamity (P < 0.001), in the region surrounding the calamity site. There was no sex effect in this shortfall (P = 0.911). There was no corresponding shortfall in other parts of Tasmania (P = 0.349).

Limitations, Reasons For Caution: The study is descriptive and cannot produce causal inferences. These first trimester miscarriages are estimated statistically and it is understood that gestational age is an estimate. The use of maternal residential postcodes at birth as a surrogate for geographic area or space assumes that the mother has not moved into the postcode area after the calamity and before the reporting of a birth.

Wider Implications Of The Findings: The results of this study suggest that calamities bring about significant pregnancy loss affecting both sexes. The methodology presented of inferring conception date from birth date and using this for analysis, provides a more accurate assessment of first trimester pregnancy losses than raw birth data or sex ratio at birth.