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Last Edited: 8/11/2010

Infant and Fetal Mortality: Measurement and Limitations

Click below to learn how infant and fetal mortality is measured and calculated:


How is infant mortality measured?

Infancy is defined as the period beginning at birth and ending at age 12 months.  Infant mortality, therefore, is any death occurring during the period of infancy.  In California, staff at the Office of Health Information and Research (OHIR) screen mortality records for any death prior to the age of 12 months.  These death records are then matched to birth records on the basis of all identifying information in the record to ascertain that the infant was born in California and that no deaths are counted more than once (duplicate records).  This matching allows OHIR to construct a cohort file with which researchers can consider mortality for infants grouped according to when they are born.

How are rates of infant mortality calculated?

Most commonly, infant mortality rates are expressed as numbers of deaths per 1,000 births. This is the number of infant deaths in a population, divided by the number of live births, multiplied by 1000.

In the data on this website, we refer to infant mortality rates calculated in this way as “conventional rates.” Due to the statistical properties of these numbers, rates based on small populations are subject to uncertainty. The degree of uncertainty for any given rate is represented by its confidence intervals. Researchers utilizing these data may wish to know that we used a common modification of Wilson’s approach for the calculation of these intervals.

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How is neonatal mortality measured?

The neonatal period is defined as the period beginning at birth and ending at 28 days of age.  Neonatal mortality, therefore, is any death occurring during the neonatal period.  In California, staff at the Office of Health Information and Research (OHIR) screen mortality records for any death that occurs during infancy.  These death records are then matched to birth records on the basis of all identifying information in the record to ascertain that the infant was born in California and that no deaths are counted more than once (duplicate records).  This matching allows OHIR to construct a cohort file with which researchers can consider mortality for neonates grouped according to when they are born.

How are rates of neonatal mortality calculated?

Most commonly, neonatal mortality rates are expressed as numbers of deaths per 1,000 live births. This is the number of neonatal deaths in a population, divided by the number of live births, multiplied by 1000.

In the data on this website, we refer to neonatal mortality rates calculated in this way as “conventional rates.” Due to the statistical properties of these numbers, rates based on small populations are subject to uncertainty. The degree of uncertainty for any given rate is represented by its confidence intervals. Researchers utilizing these data may wish to know that we used a common modification of Wilson’s approach for the calculation of these intervals.

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How is postneonatal mortality measured?

The postneonatal period is defined as the period beginning at the age of 28 days and ending at the age of 12 months. Postneonatal mortality, therefore, is any death occurring during the postneonatal period.  In California, staff at the Office of Health Information and Research (OHIR) screen mortality records for any death that occurs during infancy.  These death records are then matched to birth records on the basis of all identifying information in the record to ascertain that the infant was born in California and that no deaths are counted more than once (duplicate records).  This matching allows OHIR to construct a cohort file with which researchers can consider mortality for postneonates grouped according to when they are born.

How are rates of postneonatal mortality calculated?

Most commonly, postneonatal mortality rates are expressed as numbers of deaths per 1,000 live births. This is the number of postneonatal deaths in a population, divided by the number of live births, multiplied by 1000.

In the data on this website, we refer to postneonatal mortality rates calculated in this way as “conventional rates.” Due to the statistical properties of these numbers, rates based on small populations are subject to uncertainty. The degree of uncertainty for any given rate is represented by its confidence intervals. Researchers utilizing these data may wish to know that we used a common modification of Wilson’s approach for the calculation of these intervals.

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How is perinatal mortality measured?

The perinatal period is defined as the period beginning prior to birth when 28 weeks of pregnancy have been completed (a full term pregnancy is considered to be anything more than 37 completed weeks); the perinatal period ends seven days following births. Perinatal mortality, therefore, is any death occurring during the perinatal period. 

In California, staff at the Office of Health Information and Research (OHIR) maintains two datasets that contribute to the monitoring of perinatal mortality.  One of these is the birth cohort database described above, although frequently births and deaths are reported at the same time since live births that die in the perinatal period are by definition less than a week old, so the maintenance of cohort records is simpler.  The second dataset is the fetal death records, which includes stillbirths. 

How are rates of perinatal mortality calculated?

Most commonly, perinatal mortality rates are expressed as numbers of deaths per 1,000 live births. This is the number of deaths among newborns prior to seven days of age plus the number of stillbirths occurring after 28 completed weeks of pregnancy, divided by the number of live births, multiplied by 1000.

In the data on this website, we refer to perinatal mortality rates calculated in this way as “conventional rates.” Due to the statistical properties of these numbers, rates based on small populations are subject to uncertainty. The degree of uncertainty for any given rate is represented by its confidence intervals. Researchers utilizing these data may wish to know that we used a common modification of Wilson’s approach for the calculation of these intervals.

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Where do these data come from?

In the US, states are responsible for issuing birth and death certificates and recording and maintaining the data included in them.  Birth certificates are considered one type of vital record (others include deaths, fetal deaths, and marriage).  In California, the Office of Health Information and Research (OHIR, http://www.cdph.ca.gov/programs/OHIR/Pages/default.aspx) is responsible for stewardship and distribution of vital statistics data and provides written reports and data tables analyzing these data. Since several of the important functions of the CEHTP include the analysis and processing of these records, we maintain our own databases consisting of records produced by OHIR and subject them to further processing, most notably regarding address and other geographic information fields.

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Limitations

It should be noted that for many pregnancies the last menstrual period date is not known or not accurately recorded,1 and that such inaccuracies can have substantial impacts on calculated rates and disparities between rates. Information such as maternal race and ethnicity or place of residence are generally provided by hospitals and other providers of obstetric services throughout the state.  As such, the methods of collecting this information and the categories chosen may vary.  Even when geographic residential information is accurate, it may not serve as a reflection of where the mother spends the majority of her time during or after her pregnancy or be useful when inferring exposures to environmental hazards.

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1.    Vahratian A, Buekens P, Alexander G. State-specific trends in preterm delivery:  Are rates really declining among non-Hispanic African Americans across the United States? Maternal and Child Health Journal. 2006;10(1):27-32.
2.    Wingate M, Alexander G, Buekens P, Vahratian A. Comparison of gestational age classifications:  Date of last menstrual period vs. clinical estimate. Annals of Epidemiology. 2007;17:425-430.