Detecting and Dating Early Non‐live Pregnancy Outcomes: Generation of a Novel Pregnancy Algorithm From Norwegian Linked Health Registries

ABSTRACT Purpose Pregnancies ending before gestational week 12 are common but not notified to the Medical Birth Registry of Norway. Our goal was to develop an algorithm that more completely detects and dates all possible pregnancy outcomes (i.e., miscarriages, elective terminations, ectopic pregnancies, molar pregnancies, stillbirths, and live births) by using diagnostic codes from primary and secondary care registries to complement information from the birth registry. Methods We used nationwide linked registry data between 2008 and 2018 in a hierarchical manner: We developed the UiO pregnancy algorithm to arrive at unique pregnancy outcomes, considering codes within 56 days as the same event. To estimate the gestational age of pregnancy outcomes identified in the primary and secondary care registries, we inferred the median gestational age of pregnancy markers (45 ICD‐10 codes and 9 ICPC‐2 codes) from pregnancies registered in the medical birth registry. When no pregnancy markers were available, we assigned outcome‐specific gestational age estimates. The performance of the algorithm was assessed by blinded clinicians. Results Using only the medical birth registry, we identified 649 703 pregnancies, including 1369 (0.2%) miscarriages and 3058 (0.5%) elective terminations. With the new algorithm, we detected 859 449 pregnancies, including 642 712 live‐births (74.8%), 112 257 miscarriages (13.1%), 94 664 elective terminations (11.0%), 6429 ectopic pregnancies (0.7%), 2564 stillbirths (0.3%), and 823 molar pregnancies (0.1%). The median gestational age was 10 +1 weeks (IQR 10 +0 –12 +2 ) for miscarriages and 8 +0 weeks (IQR 8 +0 –9 +6 ) for elective terminations. Gestational age could be inferred using pregnancy markers for 66.3% of miscarriages and 47.2% of elective terminations. Conclusion The UiO pregnancy algorithm improved the detection and dating of early non‐live pregnancy outcomes that would have gone unnoticed if relying solely on the medical birth registry information.