Department of Genetics and Microbiology `A.Buzzati-Traverso', University of Pavia, via Ferrata 1, 27100 Pavia, Italy
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Abstract |
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Key words: sex ratio/stillbirth/urban areas
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Introduction |
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The upward trend is commonly accepted to be principally accounted for by improvement in living conditions and in ante- and neonatal care, which increase the survival chance of male fetuses who are more susceptible than females to detrimental conditions.
On the other hand, possible determinants of the downward trend have been widely discussed and the debate is still open. It is generally accepted that a decline in the sex ratio at birth occurs in selected samples of parents exposed to specific chemicals and to environmental hazards affecting the reproductive success and health of the fetus (James, 1996, 1997
; Davis et al., 1998
). However, a widespread debate arose when, on the basis of trends observed in the USA and some European countries, the declining sex ratio was postulated to signal a decline in the health of the general population. It has been suggested (Davis et al., 1998
) that the reduced male to female ratio at birth might be taken as a `sentinel health indicator . . . that signals changes in avoidable factors', i.e. an expression at the demographic level of factors which have begun to affect the population health. Their suggestion has been discussed by several authors, who have maintained that the sex ratio decline is casual and in any case not general (James, 1998
; Parazzini et al., 1998
). Moreover, in an analysis of data for several European countries, no decrease in male newborns in polluted urban areas in comparison with rural areas was observed (James, 1998
).
No decrease in the proportion of males was found for Italy (Parazzini et al., 1998), in contrast with what had been observed for other European countries.
Our hypothesis in initiating a study on this discrepancy was that the relative weight of demographic, behavioural and biological factors might have been changing and affecting metropolitan and non-metropolitan living conditions in contrasting ways. In particular we postulated that in the last few decades, the metropolitan area, where the highest level of medical care has been available since the 1970s, might have undergone the environmental deterioration typical of highly developed countries. On the other hand, the non-metropolitan area, which might have enjoyed less polluted conditions, still benefits from progressively better medical care, both factors being advantageous mainly to male fetuses. Therefore, if there were indeed an increase in reproductive hazards in the big cities, a decreasing sex ratio could be expected for them, whereas an increase might be found for the non-metropolitan areas of Italy.
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Materials and methods |
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The Italian provinces of Turin, Milan, Rome, and Naples, those most densely inhabited and/or industrialized, were assumed to be representative of the living conditions typical of the metropolitan environment. Data concerning the four provinces, referred to as the metropolitan area, were compared to the rest of the country, referred to as the non-metropolitan area.
Trends in male proportion and stillbirth rate were analysed by means of logistic regression models. Statistical and graphical analyses were performed by means of SAS V6.10 procedures (SAS Institute Inc., Cary, N.C., USA)
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Results |
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During this period, the overall stillbirth rate underwent a rapid and general decrease, but the male proportion among stillborn babies was nearly always higher in the metropolitan than in the non-metropolitan area. Stillbirth rates and the relevant quotas of males at 35-year intervals, compatible with the available data, are reported in Table II. Comparisons of the annual rates for the areas are shown in Figure 2a and b
, for males and females respectively. The trends are well fitted by a quadratic logistic regression model (stillborn proportion = constant + b1xarea + b2xyear + b3xyear2) and a significant difference between the areas was found for males, P = 0.002), but not for females (P = 0.07).
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Discussion |
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Fetal loss early in pregnancy and not detectable or naturally occurring later in intrauterine life, voluntary pregnancy interruption following unfavourable prenatal diagnosis, and stillbirth are expected to occur more frequently in males than in females, especially where environmental hazards exist. Consequently, the decrease in male proportion among liveborn neonates observed in several countries during the last decades has been postulated to be associated with increasing impact of environmental pollution (Williams et al., 1992, 1995
; Mocarelli et al., 1996
) and to signal reproductive hazards (Davis et al., 1998
). However the hypothesis was not sustained by a study of sex ratio temporal trends in some European countries when urban and rural environments were contrasted (James, 1998
).
The results of our analysis confirmed an overall decrease in male proportion, over the last 15 years, in nine out of the 12 European countries for which data were available (Table I). Even if we cannot exclude that the pattern, statistically not significant, is simply due to oscillation in the stabilizing process of the population sex ratio (James, 1997
), there is a suggestion that indeed a common phenomenon has been occurring. With respect to the positive trends shown in the Table, it should be noted that a significant decline was reported for the Netherlands over a longer period (Van der Pal-de Bruin et al., 1997
), and random fluctuations may thus account for the increase we observed. As for Greece and Italy, which suffer two of the highest stillbirth rates in Europe (7.9 and 6.6 per 1000 live births respectively), the improvement in prenatal care, reflected in the 55% decrease in overall infant mortality from 1980 to 1994, has probably favoured the survival chance of male fetuses.
Our study showed also that in Italy, in contrast to the increasing male quota observed in the non-metropolitan areas, the most densely and industrialized provinces, comparable to the highly developed European countries, have been witnessing a decreasing quota of liveborn males. Furthermore, in spite of the overall rapid decrease down to a very low value, stillbirth rate among males was predominantly higher in the metropolitan compared with the non-metropolitan area. Only deeper investigations will clarify the weight of the two phenomena, their relationships and their demographic, behavioural, or biological determinants.
Among the demographic and behavioural factors, the tendency to reduce family size and to delay parenting is to be considered, sibship size and parental age being related to fetal sex ratio and health risk. Sex ratio has been shown to decrease both with birth order and parental age (Rostron and James, 1977; James, 1980
, 1998
; Ruder, 1985
; Juntunen et al., 1997
), and risk at birth to be higher in first- than in second-borns (Astolfi et al., 2000
) and in babies of aged mothers (Fretts et al., 1995
). The differences observed between the Italian metropolitan and non-metropolitan areas can be at least in part accounted for by the tendency to delay parenting by more highly educated parents and employed mothers, probably more frequent in the big cities. In fact the quota of mothers aged
35 years, which in 1979 was 10.1 versus 9.8% in metropolitan and non-metropolitan areas, in 1995 reached 16.3 and 14.2% respectively.
The metropolitan and non-metropolitan trends seem to support the hypothesis that different living conditions have an influence in the two areas, and that in the major Italian cities more than in the rest of the country, neonates are exposed to changes in environmental and behavioural conditions which disproportionately affect males. It is unlikely that the non-metropolitan mothers seeking care in the metropolitan area because of pregnancies at risk or in need of special care for sub-fertility pathologies might inflate the quota of adverse pregnancy outcomes to the extent that it reduces the proportion of live-born males.
In conclusion, the slight decline in the male to female ratio at birth and the higher stillbirth risk among male neonates might indeed indicate that, in the most densely inhabited and industrialized Italian provinces, living conditions are somehow impaired though not necessarily by chemical hazards. Even if this finding requires further support, including investigations in other countries, we do think that the problem should not be underrated.
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Acknowledgments |
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Notes |
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References |
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Submitted on July 27, 1999; accepted on September 20, 1999.