1 Department of Genetics and Microbiology A.Buzzati-Traverso, University of Pavia and 2 Department of Genetics and Molecular Biology C.Darwin, University La Sapienza, Rome, Italy 3 To whom correspondence should be addressed at: zonta{at}ipvgen.unipv.it
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Abstract |
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Key words: birth order/maternal age/population fertility/secular trend/twinning rate
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Introduction |
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Although a relatively rare event, twin births have always been present in all human populations, with frequencies varying from 0.6 to 4.5% in different ethnic groups. Inter-population differences in twinning rate can be mainly attributed to dizygotic (DZ) twins, since the frequency of monozygotic (MZ) twins shows little variability between different populations (Bulmer, 1970; Vogel and Motulsky, 1986
). Moreover, an investigation carried out on relatively isolated Finnish groups supported the hypothesis that differences in twinning rates are maintained by natural selection (Lummaa et al., 1998
).
That genetic components can be involved in DZ twinning has been suggested in several familial studies (Bulmer, 1970; Lewis et al., 1996
; Meulemans et al., 1996
), and more recently some evidence of the association of DZ twinning to a chromosome 3 region has been found (Busjahn et al., 2000
). Some authors have suggested that MZ twinning might also be under genetic influence (Parisi et al., 1983
; Bortolus et al., 1999
).
Besides showing inter-population variability, in most Western countries twinning rates have varied extensively over the past 50 years: the temporal trend, which began to decrease in the early 1950s, displayed a reversal of tendency in the late 1970s, thereby reviving the interest of epidemiologists and geneticists in twinning and its possible causes (James, 1972; Parisi and Caperna, 1981
; Allen and Parisi, 1990
; Parazzini et al., 1991
; Imaizumi, 1997
, 1998; Wood, 1997
).
As several studies have shown a positive association between twinning incidence, in particular DZ, and maternal age and parity, the decrease in mean maternal age at delivery and the reduction in the number of children have been considered to be at least partly responsible for the downward trend (Bulmer, 1970; Eriksson and Fellman, 1973
; James, 1975
, 1982; Elwood, 1978
; Bonnelykke, 1990
; Parazzini et al., 1991
; Campana and Roubiceck, 1996
; Bortolus et al., 1999
). The recent drastic increase in twinning and in multiple births has been mainly attributed to ovulation-inducing treatments and assisted reproduction techniques, which have been adopted to counter subfertility problems (Parazzini et al., 1994
; Jewell and Yip, 1995
; Imaizumi, 1997
, 1998; Wood, 1997
).
Some authors have suggested that DZ twinning incidence, in so far as it is an indicator of multiple conceptions, is related to the population fertility. However, to consider natural twinning and the ratio of dizygotic/monozygotic twins as reliable monitors of the population fertility, or even of the population reproductive health, is controversial (James, 1995, 1997, 1998; Tong and Short, 1998
; Tong, 2000
).
In the present study, the temporal trend of twinning rates in the Italian population was updated to the latest available year, and the pattern within the European context was examined. The changes in some biodemographic factors affecting population fertility were then analysed in order to investigate their possible role in the changes in twinning rate. In Italy, as well as in most developed countries, the cultural modifications in womens reproductive strategies over the past 50 years have drastically influenced the demographic scenario, first through shrinkage in offspring number, and second by childbearing postponement (De Sandre et al., 1997; Ulizzi et al., 1997
; Astolfi et al., 2000
). Therefore, the evolution of the relationship of twinning with maternal age and birth order in three critical periods was studied, corresponding to: (i) the beginning of the decreasing pattern; (ii) the minimum reached; and (iii) the recent situation.
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Materials and methods |
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The 195152 and 197981 twin data by mothers age and birth order were drawn from ISTAT publications, and the latest available data were acquired directly from ISTAT at the authors request. Data for two or three consecutive years were pooled so as to obtain comparable sample sizes and to minimize yearly fluctuations. Mothers age was recoded into five classes (<25, 2529, 3034, 3539 and 40 years), and births of third and higher orders were pooled.
Weinbergs rule was applied to estimate the proportion of DZ twins from the proportion of opposite-sex twins and the newborn sex ratio, as is usually carried out whenever sex-pairing is the only indication of zygosity (Cavalli-Sforza and Bodmer, 1971).
Second-order polynomials were fitted to the European yearly twinning rates to depict better the 197595 patterns; the rates of increase over the last 10 years were estimated by linear regression analysis (SPSS 10.7 statistical package).
The t-test was applied on arcsin transforms to verify the differences between two twin rates (Sokal and Rohlf, 1981).
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Results |
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Second-order polynomials were fitted to the yearly data to depict better the temporal trends in some European countries (Figure 2): the variation explained by the fitting varies from a minimum in Austria (89.3%) to a maximum in the UK (98.8%). The rate minimum value was estimated to have occurred in 1975 in Sweden, and 5 years later in Italy and Austria. The slope of the linear fitting over the past 10 years shows that the rate of increase declines monotonically from Northern (0.458 in Norway) to Southern European countries (0.175 in Italy).
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Discussion |
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The variation in twinning and multiple birth incidence that occurred in Italy during the second half of the 20th century was consistent with the falling and rising patterns observed in other developed countries (Figures 1 and 2). During the same period, the attitude of Italian women toward family life underwent strong changes that affected the overall population fertility. Higher levels of education and a desire for economic and social success, as well as difficulties in finding a permanent job, played a major role in modifying the reproductive strategies, as is evidenced by delayed parenthood and the reduction in family size (De Sandre et al., 1997; Astolfi et al., 2000
). In the last generation, mean maternal age at delivery increased from about 27 to over 29.5 years, while the number of children per woman of reproductive age fell to 1.20 in 1996, one of the lowest values in Europe.
In the present study, three critical periods in the temporal pattern of twinning rate were chosen, corresponding to the beginning of the decline, the minimum reached, and the recent situation, in order to study its association with the population fertility through the evolution of the relation of twinning with mothers age and parity (Figure 3).
The pattern of twinning rate by mothers age and parity observed in the early 1950s is well known and is assumed to be typical of spontaneous twinning in natural fertility conditions (Figure 3a). A strong decrease in population fertility during the second period is therefore suggested by the abatement of birth order effects, by the low twinning rates in young mothers (aged <25 years), and by the reduction of maternal age influence (Figure 3b). The use of subfertility treatments in recent years is most likely the main cause of the increase in the population twinning rate, and the hypothesis is supported by the strikingly high frequency found among older (35 years) (Figure 3c) and even
40-year primiparae (Table II). The same explanation is also provided by the sharp increase in triple and higher deliveries in Italy, which is generally found in the countries where the use of assisted reproduction has been spreading (Martin and Park, 1999
). Moreover, in spite of the recourse to medical treatments, in Italy the overall rate in 1996 had not reached the twinning value of the 1950s, supporting the notion that spontaneous twinning had not yet recovered from the dip of the late 1970s (Figure 1). It is interesting to note that the increase in twinning rate started earlier and proceeded with higher velocity in northern Europe (Figure 2).
It is significant however that in the young (women aged <25 years) Italian primiparae, who are extremely unlikely to recourse to fertility treatments, the twinning rate has recently reached the natural value observed at the start of the 1950s (Table II). On the assumption that only twinning rates in young mothers should be considered as a monitor of the populations natural fertility and even reproductive health (James, 1998), the present results would suggest that some increase in natural fertility has been taking place in the Italian population.
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Acknowledgements |
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References |
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Submitted on August 9, 2002; accepted on September 25, 2002