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Perinatal Data Center Calculations

Calculations performed by the Perinatal Data Center generally follow the guidelines provided by the National Center for Health Statistics (NCHS).


Rates calculated by the Perinatal Data Center are processed using a series of programs written in SPSS software. All US total rates are based on the 50 states and the District of Columbia. Some rates are presented as aggregates for a combination of years (e.g. 2005-2008). Due to insufficient numbers some rates can not be shown on PeriStats. When rates for single years can not be shown, three-year aggregates are shown if available. If the three-year aggregates are not sufficient, that indicator will not be provided in order to ensure confidentialty.

When an item on a birth or death certificate is illegible or missing, it is coded by NCHS as "not stated" or "unknown." Denominators used in rates and ratios on PeriStats exclude values not stated.

Maternal Race/Ethnicity and Age

Many indicators are shown by race, race/ethnicity, and maternal age. Data provided by race and race/ethnicity reflect the race and ethnicity of the mother as indicated on the birth certificate. Race categories shown on PeriStats include: White, Black, Native American, and Asian, consistent with those reported by NCHS. Race/Ethnicity categories include: non-Hispanic white, non-Hispanic black, non-Hispanic Native American, non-Hispanic Asian/Pacific Islander and Hispanic. Race and ethnicity are reported separately on the birth certificate. When race of the mother is missing from the birth certificate, NCHS imputes race using race of the father, if available, or by assigning the specific race of the mother on the preceding record with a known race of mother (5.3% of live births in 2008).

While some states report the mother's age directly, most states do not, and maternal age is calculated by NCHS using the difference between the mother's and infant's dates of birth as reported on the birth certificate. From 1964 to 1996, births reported to occur to mothers younger than age 10 or older than age 49 years had age imputed according to the age of mother from the previous record with the same race and total birth order. After 1997, this range changed to mothers reported to be younger than age 9 or older than age 55. For records where age of mother was not reported, maternal age was imputed as described (0.01% of live births in 2008).

State Departments of Health

While one strength of PeriStats is the ability to make comparisons between States/local areas or between any state/local area and the U.S., the Web site is only a starting point for obtaining state and local data. We encourage users to work with their state health departments to analyze data in order to gain a deeper understanding of maternal and infant health issues specific to their area. Web site links for all state health departments are available on PeriStats.

Data provided on PeriStats may differ from rates obtained by state health departments and vital statistics agencies. This could be due to multiple causes. As part of the Vital Statistics Cooperative Program, states are required to send the National Center for Health Statistics (NCHS) natality and mortality data for a given year by a specific date. Sometimes states receive data after this date, which may result in slight differences in the rates calculated using NCHS processed data and state-processed data. Another reason rates may differ could be differences in the way NCHS and the States calculate variables and impute missing data.