Between 2003 and 2017, maternal mortality in the USA appears to have doubled. A shocking trend, given the fact that the indicator has been in global decline throughout the 20th century. What happened?
Originally the US counted cases of maternal death based on the cause of death listed on death certificates. However, the procedure overlooked relevant cases. The WHO recommended the introduction of a "pregnant" checkbox on the death certificate to correct this "underreporting". Since 2003, all deaths with the box ticked have been included in the nationwide maternal mortality statistics. States rolled out the new procedure one by one over a period of 15 years.
The staggered introduction of the new recording methodology means that data on maternal mortality in the US cannot be meaningfully compared to previous data points in the time series or to data from other countries.

Only content-identical indicators may be summarized across different geographical units. If a coding scheme (especially that of a long-running international data collection) is changed, it is essential to clearly communicate the application of new elements. In particular, structural breaks should be implemented at the same time within an observation area. Alternatively, the old and new measurement methods can be applied in parallel for a transitional period to simplify aggregation between different geographical units.