Estimating the Gestational Age of Spontaneous Abortions Identified via Database Algorithms: An Empirical Analysis Using Interrupted Time Series

PURPOSE: To estimate the average gestational age for algorithm-identified spontaneous abortions in Norway using interrupted time series analysis. METHODS: We used an algorithm to identify pregnancies registered in Norway from 2010 to 2020 and restricted to spontaneous abortions identified from primary and specialist care registers, and births from the Medical Birth Registry of Norway. For births, we calculated the first day of the last menstrual period (LMP; start of pregnancy) by subtracting the recorded gestational age from the birth date. We assigned spontaneous abortions an LMP corresponding to gestational ages ranging from 7 to 11 weeks. We identified prescriptions from 70 days before to 97 days after LMP and calculated the number of antidepressant prescriptions per 10 000 pregnancies per day. We applied two-sample interrupted time series analysis to compare spontaneous abortions versus births. We compared antidepressant prescription trends between intervention points at 28 and 55 gestational days, the time when most pregnancies are recognized. RESULTS: In our interrupted time series analysis including 122 495 spontaneous abortions and 631 929 births, the 7-week assumption showed no post-intervention trend, suggesting underestimation. The 9-week assumption closely matched the trend for births (-0.051 prescriptions/day, 95% CI -0.090 to -0.013 vs. -0.056, 95% CI: -0.067 to -0.046). The 8, 10, and 11-week assumptions showed less precise alignment. The best alignment occurred with the 64-day assumption (9.1 weeks). CONCLUSION: Our study provides an empirically derived 64-day average gestational age for algorithm-identified spontaneous abortions in Norway, supporting future research, and this approach could help inform assumptions in other contexts.