1 International Centre for Health and Society, Department of Epidemiology and Public Health, University College London, 119 Torrington Place, London WC1E 6BT and 2 Department of Social Policy, London School of Economics and Political Science, Houghton Street, London WC2A 2AE, UK
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Abstract |
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Key words: birth seasonality/fertility/socioeconomic factors
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Introduction |
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Although a number of explanations have been put forward, the causes of these seasonal variations are not fully understood. Perhaps most attention is given to physical environmental characteristics, such as temperature and light (Roenneberg and Aschoff, 1990a, b
; Lam and Miron, 1991
, 1996
). It has been suggested that temperature and light, possibly via hormonal changes, may affect the quality of semen, frequency of coitus, or length of menstrual cycle, and thus the ability to conception (Jongbloet, 1983
; Rojansky et al., 1992
; Centola and Eberly, 1999
; Gyllenborg et al., 1999
). It has also been proposed that fetal loss, if seasonally dependent, may underlie the seasonality of births (Jongbloet, 1983
; Weinberg et al., 1994
). Among cultural factors, seasonal changes of the rate at which women enter (or leave) the population at risk may play a role. For example, seasonality in marriages would affect the number of women at risk of conception, and thus result in seasonal changes in birth rates. This has been shown for a number of traditional populations (Fialova, 1995
; Stolwijk et al., 1996
). In modern societies, however, it is likely that the probability of conception depends more on the choice of the time of pregnancy (probably related to the use of contraception) than on climate or duration of marriage.
The vast majority of studies of birth seasonality so far examined aggregate data for whole populations. Several early studies used data aggregated by a social indicator or race within one population; the results suggested that seasonality of birth differs by social class (Pasamanick et al., 1960; Zelnik, 1969
; Erhardt et al., 1971
; James, 1971
; Chaudhury, 1972
; Warren and Tyler, 1979
), although the pattern of these differences was not uniform. However, individual data may prove more useful to study the independent effects of different socio-demographic factors. If the intentional timing of pregnancies is responsible for a part of the seasonal variation, it is plausible to assume that some social groups within populations would be more successful than others; this would lead to differently pronounced seasonality in different social and demographic groups within a given population. We tested this hypothesis on data collected by the Czech national birth register in 19891991.
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Materials and methods |
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Seasonality in births by socio-demographic group was first examined by visual inspection of the seasonal trends in the monthly ratios of observed/expected numbers of births (the numbers were corrected for different number of days by month). As the results were identical in all calendar years, the data were pooled. We quantified the extent of the seasonal variation as follows. First, we calculated the coefficient of variation by the month of birth. For each month, the number of births was recorded and the variation coefficient between the 12 months was calculated, for all the births and separately according to socio-demographic characteristics. Second, in each socio-demographic category, we calculated the ratio of the number of births in the month with the most and least births, and the ratio of the numbers of births in 3 months with most births (March to May) with the 3 months with the smallest number of births (October to December). Finally, we examined the independent contribution of the individual factors to the seasonal variation by building a logistic regression model on data restricted to high and low birth rates periods (March to May and October to December). The outcome variable was period of birth (being born in March to May was coded as 1 and being born in October to December as 0), and maternal age, education, marital status and birth order were the independent variables.
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Results |
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Discussion |
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The results are unlikely to be due to random error. We used complete data for the whole population; the numbers of births in the analyses were large, the seasonal differences were clearly pronounced and formed a consistent pattern across different socio-demographic characteristics. The results are also unlikely to be due to low quality data. The Czech national birth register is considered complete and reliable (Koupilova et al., 1998); more importantly, it is difficult to envisage a systematic error in birth registration that would produce the observed pattern of seasonality. Confounding by physical environmental factors is unlikely, as the whole population was exposed to the same degree. The pronounced seasonality in mothers pregnant with their second or third child suggests that timing of the marriage did not play a major role.
Our results are consistent with data from Britain in the 1960s in which birth seasonality was more pronounced in higher social classes (James, 1971). Seasonality pattern in France was also similar to our results but data from Italy and The Netherlands were not (Prioux, 1988
). The US data are largely inconsistent with the pattern found in the Czech Republic. A study in Georgia found that social variation in births increased with lower social status (Warren and Tyler, 1979
). In New York seasonality was more pronounced in non-whites and in illegitimate births (Erhardt et al., 1971
). Several analyses of data from Baltimore produced mixed results (Pasamanick et al., 1960
; Zelnik, 1969
; Chaudhury, 1972
), perhaps because of the relatively small number of subjects in these analyses. The differences in seasonal patterns and their relation to socio-demographic factors both between areas with similar climatic conditions and over time within populations (Hoffmann and Kawiani, 1976
; Prioux, 1988
; Lam and Miron, 1994b) seem to suggest that birth seasonality is influenced by place- and time-specific factors. Erhardt et al. suggested that such factors include psychological, behavioural and social influences (Erhardt et al., 1971
).
This proposition may appear at odds with the body of evidence suggesting that there is a biological basis for birth seasonality (Jongbloet, 1983; Kallan and Udry, 1989
; Smits et al., 1998
). However, the two explanations, biological and social, may be compatible. First, it is important to distinguish between different concepts related to childbearing: fecundability (the physiological ability to conceive), fetal loss, and fertility (the realization of the potential to reproduce) (Golden and Millman, 1993
). By analysing the distribution of births we study fertility. Factors that influence fecundability and fetal loss can influence fertility, but some factors that influence fertility, such as factors related to personal choice and behaviour, do not affect fecundity or fetal loss. Fecundability in humans appears to fluctuate by season (Kallan and Udry, 1989
; Smits et al., 1998
), although this finding may not be universal (Stolwijk et al., 1996
).
Factors that influence fecundity (e.g. temperature or light) can thus play a role in birth seasonality. However, it is likely that the effect of biological factors on seasonality in birth in developed countries is weaker, because in these countries fertility is largely determined by factors related to individual choice (Bongaarts, 1978). In populations with fertility levels much lower than natural fertility (because of the wide use of contraception), and at the time when most of the world population experience fertility below the replacement level (2.1 children per women), it is evident that individual choices are particularly important. At the population level, factors related to pregnancy choice and planning would override factors that influence the physiological ability to reproduce (Erhardt et al., 1971
). Although there is some seasonal variation in fetal loss, it is probably too small to be a major cause of seasonality of birth (Warren et al., 1986
; Weinberg et al., 1994
).
Second, there may be an interaction between physical environmental factors and socio-demographic factors. Although climatic conditions in the Czech Republic are mild, and there are no known socio-economic differences in the quality of housing or exposure to outdoor factors, it is possible that the effect of meteorological factors are modified by the social environment.
The observation that seasonality is differently pronounced in different population groups may be attributed to a combination of several factors (that may interact with biological influences on fecundability). The differences in birth seasonality between socio-demographic groups can be due to differences in family planning. The weak seasonal variation in women who were 19 or
35 years old, or unmarried, suggests less well or less successfully planned pregnancies. This proposition is plausible. Young women are more vulnerable to poor family planning. The vast majority of Czech unmarried women who gave birth were, at the time of the study, without a stable male partner; this situation is, again, suggestive of poor family planning. The absence of seasonality among mothers
35 years old may be related to smaller importance given to the time of conception (or birth) and the higher risk of fetal loss at higher maternal ages (Weinstein et al., 1993
; Nybo Andersen et al., 2000
). By contrast, better educated women in stable marriages, with one or two children already, are more likely to plan their next pregnancy. We do not have data on contraception use by all maternal characteristics used in this study, but there is a pronounced educational gradient in Czech women (unpublished data).
Summer holidays may be the best time to attempt a conception (James, 1971); Basso et al. reported that summer was the preferred time for starting pregnancy in several European countries (Basso et al., 1995
). Consistent with this explanation, the major peak in births in our data corresponds to conceptions in the summer and the minor peak (in September) to conceptions around the Christmas holiday. Preference of the date of birth/conception may have also played a role. It is possible that different social groups may have different preferences of the date of conception or birth. Our data do not allow firm conclusions on the contribution of different aspects. In either case, the social and demographic factors seem to play an important role in shaping the seasonal variation in births in this population.
It is possible that in different countries different socio-demographic factors predict birth seasonality differently, perhaps because the interactions between the biological and social variables are not constant across time and place. This question can be addressed by birth registration data that are available in most developed countries. The association between socio-demographic factors and birth seasonality, if confirmed, may be useful for further studies of seasonality in reproductive outcomes, but can also provide the link between the month of birth and subsequent fertility (Smits et al., 1999) or mortality (Doblhammer, 1999
).
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Acknowledgements |
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Notes |
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References |
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Submitted on December 8, 2000; accepted on March 12, 2001.