Technical Notes

Nature and Sources of Data

Maternal mortality rates are based on death and birth records received and processed by the National Center for Health Statistics (NCHS) as of a specified cutoff date. The cutoff date is generally the first Sunday of the month in which the update occurs. National provisional estimates include deaths and births among U.S. residents which occurred within the 50 states and the District of Columbia. NCHS receives the death and birth records from state vital registration offices through the Vital Statistics Cooperative Program. Rate estimates are based on final mortality data for 2019-2022, and provisional mortality data for 2023-2024, and final natality data for 2019-2023, and provisional natality data for 2024. The timeliness of provisional mortality surveillance data in the National Vital Statistics System database varies by cause of death. The lag time (i.e., the time between when the death occurred and when the data are available for analysis) is longer for maternal deaths compared with other causes of death. Thus, provisional estimates of deaths from maternal causes are reported 3 months after the date of death.

Provisional maternal mortality rates presented in this data visualization are for “12-month ending periods,” defined as the number of maternal deaths per 100,000 live births occurring in the 12-month period ending in the month indicated. For example, the 12-month ending period in June 2020 would include deaths and births occurring from July 1, 2019, through June 30, 2020. The 12-month ending period includes all seasons of the year and is insensitive to reporting variations by seasonality. Rates for the 12-month period ending in the same month of the previous year are shown for comparison. These provisional maternal mortality rates are provided for public health surveillance and monitoring of emerging trends. Provisional death data are incomplete, as death data may be revised when updates to death certificates are received from jurisdictions and when data are reviewed for coding and accuracy. Consequently, rates may be underestimated or overestimated based on provisional data relative to final data and are subject to random variation.

Maternal mortality rates and corresponding trends can fluctuate from month to month because of the relatively small number of these events, and there may be errors associated with the reporting of maternal deaths on death certificates (1-3). For example, erroneous entries in the pregnancy checkbox item on the death certificate may lead to overestimates of maternal deaths, which research has noted is especially problematic for women aged 40 and over (3). Manual review of and correction to death certificates with marked pregnancy checkboxes has been implemented to reduce potential misclassification. However, internal evaluation of maternal mortality data suggests that overestimation may persist. Efforts to improve data quality are continuing, and the data will continue to be evaluated for possible errors.

Cause-of-death classification and definition of maternal deaths

Mortality statistics are compiled in accordance with the World Health Organizations (WHO) regulations specifying that WHO member nations classify and code causes of death with the current revision of the International Statistical Classification of Diseases and Related Health Problems (ICD). ICD provides the basic guidance used in virtually all countries to code and classify causes of death. It provides not only disease, injury, and poisoning categories but also the rules used to select the single underlying cause of death for tabulation from the several diagnoses that may be reported on a single death certificate, and regulations on use of the classification. Causes of death for data presented on this report were coded according to ICD guidelines described in annual issues of Part 2a of the NCHS Instruction Manual (4).

NCHS reports maternal mortality using the WHO definition of a maternal death, which is “the death of a woman while pregnant or within 42 days of termination of pregnancy, irrespective of the duration and the site of the pregnancy, from any cause related to or aggravated by the pregnancy or its management, but not from accidental or incidental causes” (5). Consistent with other NCHS reports, the number of maternal deaths does not include all deaths occurring to pregnant or recently pregnant women, but only those deaths with the underlying cause of death assigned to International Statistical Classification of Diseases, 10th Revision (ICD-10) code numbers A34, O00–O95, and O98–O99 (6–8).

Differences between Final and Provisional Data

There may be slight differences between provisional and final data for a given data year (e.g., 2021). Provisional maternal deaths are reviewed periodically throughout the year and may be recoded upon review.  As a result, the provisional number of deaths may fluctuate with each data update.

Additionally, in July/August following the end of a given data year, the final file for a data year is ‘closed’ in July/August following the end of that given data year in order to produce the historical and public use files and reports. After it is considered closed, the final file is no longer updated, even if additional updates or records are received from the jurisdictions. The result is that some records that are in the final annual file may still list an unknown cause of death or manner of death indicating ‘pending investigation.’ The provisional data continues to be updated as additional cause of death information is received, and the counts may increase in response to these changes. As a result, provisional maternal death counts published by Vital Statistics Rapid Release may differ from the counts published using the final data.

Source

NCHS, National Vital Statistics System. Mortality estimates for 2023 and 2024 are based on provisional data. Mortality estimates for 2019-2022 are based on final data. Natality estimates for 2024 are based on provisional data. Natality estimates for 2019-2023 are based on final data. Data available at: http://wonder.cdc.gov.

References

1. Hoyert DL, Uddin SFG, Minino, AM. Evaluation of the pregnancy status checkbox on the identification of maternal deaths. National Vital Statistics Reports; vol 69 no 1. Hyattsville, MD: National Center for Health Statistics. 2020. Available from: https://www.cdc.gov/nchs/data/nvsr/nvsr69/nvsr69_01-508.pdf
2. Hoyert DL, Minino AM. Maternal mortality in the United States: Changes in coding, publication, and data release, 2018. National Vital Statistics Reports; vol 69 no 2. Hyattsville, MD: National Center for Health Statistics, 2020. Available from: https://www.cdc.gov/nchs/data/nvsr/nvsr69/nvsr69-02-508.pdf
3. Rossen LM, Womack LS, Hoyert DL, Anderson RN, Uddin SFG. The impact of the pregnancy checkbox and misclassification on maternal mortality trends in the United States, 1999-2017. National Center for Health Statistics. Vital Stat 3(44). 2020. Available from: https://www.cdc.gov/nchs/data/series/sr_03/sr03_044-508.pdf
4. National Vital Statistics System. Instructions for classifying the underlying cause of death. In: NCHS instruction manual; Part 2a. Published annually.
5. World Health Organization. International statistical classification of diseases and related health problems, 10^th revision. 2008 ed. 2009.
6. Hoyert DL. Maternal mortality rates in the United States, 2019. NCHS Health e-Stats. 2021. DOI: https://doi.org/10.15620/cdc:103855
7. Hoyert DL. Maternal mortality rates in the United States, 2020. NCHS Health e-Stats. 2022. DOI: https://dx.doi.org/10.15620/cdc:113967
8. Hoyert DL. Maternal mortality rates in the United States, 2021. NHCS Health e-Stats. 2023. DOI: https://dx.doi.org/10.15620/cdc:124678