All too often people on all sides of the health care debate, to quote 19th century Scottish writer Andrew Lang, “use statistics as a drunken man uses lampposts – for support rather than for illumination.” So, I will attempt to shed some illumination on one of the reoccurring statistics of the American health care debate: infant mortality rates –and on why, in general, we must interpret statistics with care. Today I want to talk about how differing definitions have made infant mortality statistics incomparable across countries. Later this week I will address the role played by other factors.
As most have heard by now, the US has a significantly higher infant mortality rate than its peers, which is presented as a sign that its health care system is lacking. The first problem with international comparison of infant mortality rates is that definitions of live birth and stillbirth are not fixed and may be ambiguous or subjective. The US simply uses the World Health Organization definition, which classifies as a live birth any case in which there is “complete expulsion or extraction from its mother of a product of conception, irrespective of the duration of the pregnancy, which, after such separation, breathes or shows any other evidence of life - e.g. beating of the heart, pulsation of the umbilical cord or definite movement of voluntary muscles - whether or not the umbilical cord has been cut or the placenta is attached.”
But in the UK, while live birth goes essentially undefined, the legal definition of a still birth is “a child which has issued forth from its mother after the twenty-fourth week of pregnancy and which did not at any time after being completely expelled from its mother breathe or show any other signs of life.” The question of what constitutes a sign of life is left open, subject to the interpretation of medical personnel. Second, the fate of babies born earlier is left in a gray area. Again the judgment of doctors is crucial in determining which preemies are counted as still births and which are considered live births.
Similarly, in Germany, the definition of a live birth follows closely, but not exactly, the WHO criteria. Under German law, “[a] live birth…exists when in a child after the separation from the mother’s body either the heart beats or the umbilical cord pulses or the natural breathing of the lungs begins.” This omits, however, voluntary movement. Like in the US, weight is not a criterion for live birth, although a threshold of 500g birth weight is used to differentiate a stillbirth or death during birth from a miscarriage in cases where the child shows none of the three above signs of life.
Switzerland only uses two of the four WHO criteria, the two most obvious signs of life, respiration and heart beat. The oft cited figure of 30 cm as the required length for registering a live birth is not universal in Switzerland but is present in some cantons. Studies have found significant underreporting of premature births in Switzerland, including babies included in either local or national listings but not both, which can alter the overall mortality rate by more than a percentage point.
Need more proof?
Check out the following chart of restrictions on the gestation at which births must be reported as still or live:
Selected statistics from EURO-PERISTAT Project, with SCPE, EUROCAT, EURONEOSTAT, “European Perinatal Health Report,” 2008.
These variations in definition are important because they mean that some babies are not counted in the statistics or are counted differently depending on the country. The broader the criteria for live birth, the higher the infant mortality rate will be because you will be including more babies who ultimately do not survive. Restrictions based on gestation or birth weight, which are present in some countries, exclude precisely the babies that are most likely to die and artificially bringing down the mortality rate. The fact that criteria for registering stillbirths also vary means that some babies are not counted at all and also makes it impossible to correct the statistics or to use alternative statistics, like fetal or perinatal mortality, to produce a numbers that are more representative and comparable across countries.
As most have heard by now, the US has a significantly higher infant mortality rate than its peers, which is presented as a sign that its health care system is lacking. The first problem with international comparison of infant mortality rates is that definitions of live birth and stillbirth are not fixed and may be ambiguous or subjective. The US simply uses the World Health Organization definition, which classifies as a live birth any case in which there is “complete expulsion or extraction from its mother of a product of conception, irrespective of the duration of the pregnancy, which, after such separation, breathes or shows any other evidence of life - e.g. beating of the heart, pulsation of the umbilical cord or definite movement of voluntary muscles - whether or not the umbilical cord has been cut or the placenta is attached.”
But in the UK, while live birth goes essentially undefined, the legal definition of a still birth is “a child which has issued forth from its mother after the twenty-fourth week of pregnancy and which did not at any time after being completely expelled from its mother breathe or show any other signs of life.” The question of what constitutes a sign of life is left open, subject to the interpretation of medical personnel. Second, the fate of babies born earlier is left in a gray area. Again the judgment of doctors is crucial in determining which preemies are counted as still births and which are considered live births.
Similarly, in Germany, the definition of a live birth follows closely, but not exactly, the WHO criteria. Under German law, “[a] live birth…exists when in a child after the separation from the mother’s body either the heart beats or the umbilical cord pulses or the natural breathing of the lungs begins.” This omits, however, voluntary movement. Like in the US, weight is not a criterion for live birth, although a threshold of 500g birth weight is used to differentiate a stillbirth or death during birth from a miscarriage in cases where the child shows none of the three above signs of life.
Switzerland only uses two of the four WHO criteria, the two most obvious signs of life, respiration and heart beat. The oft cited figure of 30 cm as the required length for registering a live birth is not universal in Switzerland but is present in some cantons. Studies have found significant underreporting of premature births in Switzerland, including babies included in either local or national listings but not both, which can alter the overall mortality rate by more than a percentage point.
Need more proof?
Check out the following chart of restrictions on the gestation at which births must be reported as still or live:
Selected statistics from EURO-PERISTAT Project, with SCPE, EUROCAT, EURONEOSTAT, “European Perinatal Health Report,” 2008.
These variations in definition are important because they mean that some babies are not counted in the statistics or are counted differently depending on the country. The broader the criteria for live birth, the higher the infant mortality rate will be because you will be including more babies who ultimately do not survive. Restrictions based on gestation or birth weight, which are present in some countries, exclude precisely the babies that are most likely to die and artificially bringing down the mortality rate. The fact that criteria for registering stillbirths also vary means that some babies are not counted at all and also makes it impossible to correct the statistics or to use alternative statistics, like fetal or perinatal mortality, to produce a numbers that are more representative and comparable across countries.
