Ultrasound Procedure Codes Enable Timely Surveillance of Ongoing Pregnancies: A Nationwide Population-Based Register...

Pregnant women in Norway are offered routine ultrasound examinations in the first and second trimester during gestational weeks 11-13 and 17-19, but attendance rates are unknown. With the introduction of maternal pertussis vaccination, we sought to investigate whether ultrasound attendance during pregnancy could identify the target population for maternal vaccination. Our study aims to investigate if pregnancy-related ultrasound codes alone can provide reliable estimates of ongoing pregnancies before data is available in birth records.

In this nationwide population-based register study, we linked individual-level data from women with births in the Medical Birth Registry of Norway (MBRN) and first or second trimester ultrasound codes in the Norwegian Patient Registry (NPR) during 2018-2023. We calculated the proportion of pregnancies with pregnancy-related ultrasound examination codes, with trimester-specific codes from 2022, and median gestational age at ultrasound stratified by code.

For the entire study period (2018-2023), we retrieved data on 323 549 pregnancies for 249 915 women. Of the 59 739 pregnancies identified through MBRN in 2022-2023, 57 416 (96.1%) had at least one recorded routine trimester-specific ultrasound. The second trimester ultrasound was recorded in 92.9% (55 503/59 739) of pregnancies, with 87.7% (48 672/55 503) at the recommended time. The first trimester ultrasound code was recorded in 61.6% (36 799/59 739) of pregnancies, and 58.4% (34 886/59 739) had both codes.

The high attendance rate of at least one routine ultrasound examination during pregnancy (96.1%) in 2022-2023 confirms the reliability of trimester-specific ultrasound codes as an indicator for ongoing pregnancies. This provides valuable data for timely assessment of interventions targeting pregnant women, before information is available in birth registries.