Monday, September 11, 2006

Why you cannot directly compare national neonatal mortality rates

Every few days someone brings up the fact that many Western European countries have lower neonatal mortality rates than the US and triumphantly declare that it must be because they have homebirths or midwives. And every few days I point out that there are many components to neonatal mortality that differ from country to country and that is why direct comparisons are invalid.

I recently came across two paper that emphasize the fact that differing neonatal mortality rates cannot be ascribed to midwives.

Perinatal death in ethnic minorities in The Netherlands; A van Enk, SE Buitendijk, KM van der Pal, WJ van Enk and TW Schulpen; Journal of Epidemiology and Community Health, Vol 52, 735-739, 1998.
OBJECTIVES: To investigate differences in perinatal death rate and associated obstetric risk factors between ethnic groups in the Netherlands. DESIGN: Retrospective cohort study based on the 1990-1993 birth cohorts in the National Obstetric Registry. SUBJECTS: 569,743 births of which 85,527 were for women belonging to ethnic minorities. MAIN OUTCOME MEASURES: Perinatal death occurring between 16th week of pregnancy and 24 hours after birth. METHOD: Bivariate and multivariate analysis of perinatal death rate per ethnic group. A total of 42,282 women living in the three main cities of the Netherlands were classified on the basis of postal code districts into four socioeconomic (SES) classes for analysis of the relation between SES, perinatal death, and preterm birth. RESULTS: Black mothers had the highest perinatal death rate compared with indigenous Dutch (odds ratio 2.2, 95% CI 1.9, 2.4) followed by a group "others", consisting of women of mixed or unknown ethnicity (odds ratio 1.8, 95% CI 1.5, 2.0), Hindustani (odds ratio 1.4, 95% CI 1.2, 1.6), and Mediterraneans (odds ratio 1.3, 95% CI 1.2, 1.4). Asians (excluding West Indian Asians) and non-Dutch Europeans did not have higher rates than Dutch women. The increased rates of black and Hindustani women could be explained fully and that of the group "others" partially by higher rates of preterm birth. Controlling for age and parity lowered the odds ratio of the Mediterraneans slightly. The risk of ethnicity was independent of SES. CONCLUSION: Ethnic minorities in the Netherlands except immigrants from Asia and other European countries have higher rates of perinatal death than indigenous Dutch women. With a twofold increase, black women had the highest rate, which was related to an equally large increased rate of preterm birth.

Perinatal mortality in the Netherlands; Backgrounds of a worsening international ranking, a 40 page report published in 2004 investigating why the Dutch have been steadily worsening in lists of neonatal mortality rates, which begins:
"Perinatal mortality rates have dropped sharply in the past few decades, in the Netherlands as well as in all other European countries. However, as the decrease has generally slowed down since the 1980s, the Netherlands has lost its prominent position in the international ranking of countries with favourable perinatal mortality rates. This lower ranking is not only the result of the dialectics of progress, but also the consequence of a relatively restrained use of antenatal diagnostics. In addition, the Netherlands is among the European countries scoring highest on a number of important risk factors. This article examines the effect on perinatal mortality rates of known risk factors, in particular the presence of non-western foreigners, multiple births and older mothers. With respect to the latter factor, it is concluded that children of older mothers run a significantly higher risk of foetal mortality, whereas babies of young mothers(including women in their early twenties) run a higher risk of infant mortality. For babies of non-western mothers, infant mortality rates are higher, although there are substantial differences between ethnic backgrounds. First week mortality is most unfavourable for Surinamese and Antillean/Aruban children, and post-neonatal mortality is highest among Turkish and Moroccan babies. The fact that relatively many non-western foreigners from countries with relatively high risks of perinatal mortality have settled in the Netherlands, is one of the reasons for the fall in the international ranking. Lastly, the increase in the number of multiple births has been stronger in the Netherlands than in most other countries. The higher incidence of assisted reproduction explains most of this increase."
In the entire 40 page report, there is no mention of midwifery as being in any way responsible for low neonatal mortality rates.

It is not a coincidence that most of the high ranking countries in Europe are the "whitest" in the world. Unfortunately, race is a risk factor for perinatal mortality. In the Netherlands, black mothers had perinatal mortality rates more than double that of whites. The same thing is true in the United States, but here the proportion of black women is much higher. Indeed, perinatal mortality rates for white women in the US are comparable to almost anywhere else in Europe.

There is no evidence that midwifery or homebirth leads to lower perinatal mortality rates. Even the Dutch don't think so.


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