This study examines the changes over time of the twinning rate and infant and child mortality across 17 rural villages in the province of Zaragoza (Spain) over a span of 200 years. The aim is to understand how the twinning rate evolved in conjunction with the processes of economic and social modernization, as well as the demographic transition. During the period analyzed, the twinning rate increased by 10%, rising from 1.26 per 1000 births in the second half of the 18th century to 1.38 per 1000 births in the first half of the 20th century. This rate varied due to hereditary issues, biological factors such as the mother’s age and parity, and socioeconomic features like family occupation and the mother’s education level. In terms of child mortality, twins historically faced a significantly higher mortality rate. However, they benefited greatly from the mortality transition, at least in absolute numbers. While nearly 60% of twins did not survive beyond 5 years of age in earlier periods, the mortality rate for twins decreased to 40% by the first half of the 20th century. The excess mortality was particularly severe for girls, driven by a widespread preference for sons, which led to higher mortality rates for girls in the first 5 years of life, especially in the earliest months. These findings help us understand the improvements in uterine and childhood survival rates for contemporary twins, which can be attributed to the socioeconomic and medical advancements of the 20th century.

This study assessed the trends in twin births and their survival in Bangladesh by analyzing over a quarter million live births during 1970–2018, pooled from all eight rounds of the Bangladesh Demographic and Health Survey. In these five decades, the twinning rate increased by 1.5 times, from 5.8 to 8.6 twins per 1000 maternities. The decadal twinning rates varied across maternal age, parity, body mass index, household wealth index, and geographic region. The gap in decadal neonatal, infant, and under-five cumulative survival probability between singleton and multiple births was found to be closing, using Kaplan-Meier curves. Child mortality decreased by 80% and 60% in singleton and multiple births respectively. However, the absolute size of child mortality in multiple births remained six times higher than in singletons and was concentrated in the neonatal period. The share of multiple births surged in all types of child mortality. We predict a further and faster rise in multiple births in the coming decades in the face of upward trends in maternal age overlapping with higher parities, education, career prospects, contraceptive use, and the future demand-supply of assisted reproductive technology. A particular focus on the improvement of perinatal and neonatal care with wider availability is warranted. Otherwise, increased multiple births might raise child mortality and create public health challenges.

In this study, we analyzed the estimated frequency of monozygotic (MZ) and dizygotic (DZ) spontaneous twins in Lombardy during the period 2007−2017. This is a population-based study using the regional healthcare utilization databases of the Lombardy Region. The total number of spontaneous twin deliveries, in separate strata of like and unlike sex, was obtained. Moreover, estimates of DZ and MZ twin births were calculated using Weinberg’s method. The standardized rates (SRs), adjusted for maternal age, of DZ and MZ twin births were computed according to calendar period. The twinning rates were calculated among strata of parity and maternal age. Finally, DZ:MZ ratio was calculated. Among the 734,278 spontaneous deliveries, 9176 (12.5 out of 1000 births) couples of twins were identified. In the three periods considered (i.e. 2007−2010, 2011−2014 and 2015−2017), no trend in the SRs of MZ twins was observed, respectively 0.41 (95% CI [0.40, 0.43]), 0.43 (95% CI [0.42, 0.45]) and 0.43 (95% CI[0.42, 0.45]). Differently, a slightly decreasing trend was observed in DZ twins SRs, respectively 0.87 (95% CI [0.84, 0.89]), 0.81 (95% CI [0.79, 0.83]), and 0.78 (95% CI [0.76, 0.80]). As concerns parity and maternal age, the rate of DZ twin births was consistently higher in nulliparae women aged 35 years or more. In our cohort, despite the increase of maternal age, a decline of spontaneous twin births emerged, especially due to the downward trend of DZ twins.

Over the previous decades, twinning rates worldwide have increased remarkably. This study aimed to describe changes in multiple birth rates and related demographic characteristics, such as maternal age and the level of education of parents in South Korea from 1981 to 2019. This study analyzed birth data obtained from the Korean Statistical Information Service. From 1981 to 2019, the total twinning rate increased from 5 to 22.5 pairs per 1000 births. This rapid increase was due to an increase in dizygotic twin births, which mainly occurred in mothers aged between 30 and 39 years. The average maternal age of multiples increased from 26.06 years in 1981 to 33.98 years in 2019, suggesting that a delay in childbearing contributed to the increase in the twinning rate. The percentage of mothers of multiples who completed a college or higher degree (CHD) increased by 1000% from 1981 to 2019, indicating that a sharp increase in the level of education of females in part explains the delay in childbearing. The percentages of individuals who completed a CHD were higher among parents of multiples than among those of singletons in recent years when public funding arrangements for fertility treatments were available. This result suggested that completion of higher education may be associated with increased use of assisted reproductive technology (ART) independent of the financial status of couples. Taken together, the analysis suggested that increased maternal age, ART and the increased number of females who completed CHD may be responsible for the remarkable increase in the rates of multiple births in South Korea in the last four decades.

In this study, the temporal trends in the rates of multiple maternities in Denmark were analyzed and an association between the rates of multiple maternities and the socioeconomic variations in Denmark was obtained. The findings are compared with the results obtained in studies of Swedish rates. Obtained temporal differences in the twinning rates (TWRs) were similar to the results presented by Fellman and Eriksson (2014). Temporal variation in rates of multiple maternities in Sweden (1751–2000). JP Journal of Biostatistics, 11, 2, 143–156.

Since 1876, statistical data on twin and multiple (TAM) births and deliveries have been registered in Hungary. Although there were some years during this 138-year period when twin births and deliveries were left out of the statistics, the trends can be followed, except for the period of World War II. The trends of twinning over the past four decades are summarized in this article, relying on official statistical data since 1970. Notably, the TAM birth rate rose following the Act CLIV of 1997 on Health. The Act provided state support for assisted reproductive treatments (ART), which spectacularly increased the ratio of twins — and especially of triplets — among live births. The trend has turned around since 2009–2010, and a decreasing era of TAM births seems to have begun. This article discusses some of the main and most interesting findings connected with TAM births, including twinning rates associated with different age groups, educational level, and actual place of residence by mothers, and the number of previous pregnancies of the mother. Ratios of live born males compared with live born females, lengths and weights at birth both among twins and singletons, and infant mortality rates have been calculated from the data. Twinning rates are higher in the western counties, the capital and its suburbs, and lower in the eastern counties.

Using vital statistics, yearly changes in the twinning and triplet rates by zygosity were investigated in the Czech Republic and the Slovak Republic during the period 1972–1995. Monozygotic (MZ) twinning rates in both countries had remained nearly constant (about 3 per 1000 total births) during that period. With a few exceptions, the dizygotic (DZ) twinning rates remained constant from 1972 to 1994, and increased in 1995 for both countries. MZ twinning rates for both countries were the lowest in Europe. As for triplet rates, overall rates increased significantly year by year in the Czech Republic, but not in the Slovak Republic. The triplet rate was significantly higher in 1995 than in the period 1972–1982 for both countries. The MZ triplet rate remained constant during that period in the Czech Republic. The trizygotic (TZ) triplet rates increased 3-fold for the Czech Republic and 4-fold for the Slovak Republic in 1972–1976 and 1992–1995. In the later period, the TZ rate was 1.5-fold higher in the Czech Republic than in the Slovak Republic. The quadruplet rate increased 2.3-fold from 2.9 per million births in 1982–1986 to 6.7 in 1992–1995 in the Czech Republic. The corresponding values were 2.7, 2.20 and 5.9-fold in the Slovak Republic. Both the Czech and the Slovak Republics were not affected by fertility drugs and assisted reproductive techniques until recently.

A study was made of the seasonal variation in all births, and births according to marital status, multiplicity and birth status (live and still) in Switzerland recorded between 1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses are based on rates. When comparing the seasonality in data sets showing markedly different levels, standardised indices were used. Assuming the length of pregnancies with twins to be about one month shorter than for pregnancies with singletons, lagged twinning rates were calculated but, in comparison with actual twinning rates, the general seasonal variation remained. Therefore, this study was based on actual twinning rates. A monotonic increase in the amplitude of the seasonal variation in general births was noted for the period 1876–1930, with strong seasonal variation holding for 1921–1980. After that, a marked decline in the amplitude can be observed. Seasonality of both all births and twin maternities showed very similar pattern for the periods 1876–1930 and 1969–1990, with maxima in the spring (March–May) and troughs in late autumn (October–December). Twin maternities showed a strong seasonality for the period 1876–1930, being about 20% higher in March than in October. The twinning rate in the period 1876–1930 was about 2.6 per thousand units higher than in the period 1969–90. For twin maternities there was also a stronger seasonal variation during the earlier period than during the later one. The pattern of the seasonal variation for extramarital births, showing a maximum in February (conceptions in May–June) and a minimum in August (conceptions in November–December) with a difference of no less than 24% was more marked than for the marital births. It seems likely that this seasonality of extra-marital maternities was due mainly to seasonal variation of coital rates and multiple ovulation in the early summer months coinciding with optima of light, temperature and food supply. A strong reduction in the rate of stillbirths (gestational age more than 29 weeks) was observed during the twentieth century. The stillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 in the 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonal rhythm was noticed throughout the period with high stillbirth rates among births around March and low rates during the summer and autumn. Twin Research (2000) 3, 189–201.

There have been few secular analyses of the seasonal variation in human twinning and the results are conflicting. One reason for this is that the seasonal pattern of twinning varies in different populations and at different periods. Another reason is that the statistical methods used are different. The changing pattern of seasonal variation in twinning rates and total maternities in Denmark was traced for three periods (1855–69, 1870–94, and 1937–84). Two alternative methods of analysis are considered. The method of Walter and Elwood and a trigonometric regression model give closely similar results. The seasonal distribution of twin maternities for the periods in the 19th century showed highly significant departures. For both twin and general maternities, the main peaks can be seen from March to June and a local peak in September. During the spring– summer season the twinning rates were higher than the total birth rates, indicating a stronger seasonal variation for the twin maternities than for the general maternities. For 1937–84, there was a similar, but less accentuated, pattern. Studies of other populations are compared with the Danish results. The more accentuated seasonal variation of twinning in the past indicate that some factors in the past affected women during summer–autumn and around Christmas time, making them more fecund and particularly to be more prone to polyovulation and/or more able to complete a gestation with multiple embryos.

In order to investigate a possible association between maternal smoking during pregnancy and twinning, information on 1 096 330 single births and 12 342 twin births in 1983–95 was obtained from the Swedish Medical Birth Registry (MBR). All odds ratios (OR) were estimated after stratification for year of birth and maternal age, parity, and educational level. Smoking women, compared with non-smoking women, were at increased risk of having dizygotic (DZ) twins, but the risk increase was only evident among multiparas. A strong association between previous involuntary childlessness and dizygotic (DZ) twinning (especially in primiparas) was found. The strongest association between maternal smoking and DZ twinning was found among multiparas without any history of involuntary childlessness (OR: 1.35, 95%CI:1.22–1.49), whereas among women who had experienced involuntary childlessness, the opposite was seen (OR: 0.82, 95%CI:0.66–1.00, no difference between parity strata). Weinberg's differential method was used to estimate the number of monozygotic (MZ) twins, and a method of estimating stratified ORs among mothers of MZ twins was presented. No association was found between MZ twinning and maternal smoking (OR: 0.96, 95%CI:0.86–1.07), and no confounding by parity or previous involuntary childlessness was indicated. Several non-causal explanations to the positive association between DZ twinning and maternal smoking among multiparas were discussed, but homogeneity over strata indicated that maternal smoking may be a true risk factor for double ovulation.

Birth statistics for the Johannesburg Metropolitan Region were collected for 757 151 confinements from 1969 to 1989 (467 513 Black, 194 375 White, 67 250 Coloured and 28 013 Indian confinements). From 1969 to 1978 data on the sexes of twins were also collected for 375 203 of the confinements (203 504 Black, 129 631 White, 28 253 Coloured and 13 815 Indian confinements). A twin confinement was defined as two deliveries during one confinement. Twinning rates (TRs), defined as the number of twin confinements per 1000 total confinements, were calculated per year for each population group and from 1969–1978 estimates of the relative proportions of dizygotic (DZ) and monozygotic (MZ) twins were calculated and thus the relative DZTRs and MZTRs. A significant decline in Black and Coloured TRs was observed between 1969 and 1989. A significant decline in Black DZTR but not Black MZTR was observed between 1969 and 1978, the Coloured twin sample was too small to show significant trends over this period. It is probable that the overall decline in Black twinning may be explained by a decline in the DZTR. An analysis of birth statistics for 159 748 confinements (134 504 Black and 25 244 White confinements) collected as part of a prospective study of TRs in the Johannesburg Metropolitan Region from 1988 to 1990, indicated that the Black TR continued to decline at least until the end of 1990. TRs in the Johannesburg Metropolitan Region calculated from City Health Department birth statistics collected from 1988 to 1990 were: 13.8 and 10.77 for the Black and White populations, respectively. TRs for this period calculated from the combined birth statistics of 14 hospitals, nursing homes and maternity clinics across the region were: 12.4 and 10.88 for the Black and White populations, respectively.

Although many mouflon Ovis gmelini musimon populations have been introduced to continental southern and central Europe, little is known about their reproductive output. Based on post-mortem analysis of 344 harvested females, the variation in reproductive performance was investigated in three wild populations: two located in the French Alps and one in the south of France. Examination of tracts indicated a high pregnancy rate (> 80%) for females ${\ge}1.5$ years old. We found that a significant proportion of female lambs were pregnant in the Alps, c. 1 year earlier than generally reported for first reproduction in the mouflon, whereas female lambs did not conceive in the southern France population. Pregnant female lambs and yearling females always bore a single foetus. Among pregnant females ${\ge}1.5$ years old, twinning occurred less often in southern France (2.5%) than in the Alps (10.8% and 20.7%). The data required to relate body mass and reproductive output of ewes at individual level were lacking, but female lamb–ewe body mass ratio was used to test that the population with high reproductive output should have the highest ratio. As expected, the index was higher in the Alps than in southern France, suggesting between-population differences in the quality of the local environment. Differences in growth pattern and birth timing of lambs, or different tactics of maternal care could also have influenced the female lamb–ewe body mass ratio. Crossings between wild and domestic sheep during the recent history of these populations and differences in environmental conditions might have interplayed to shape age at first reproduction and twinning rates.

Secular changes in twinning and triplet rates were analyzed using vital statistics in Austria, the Czech Republic, the Slovak Republic, England and Wales, Germany, the Netherlands, Switzerland, Denmark, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore during the period from 1972 to 1996. Among those 17 countries, the twinning and triplet rates in the Czech Republic and in the Slovak Republic remained constant from 1972 to 1994, whereas these rates increased significantly year by year in the other 15 countries during the examined period in each country. Twinning rates increased from 1.2-fold in Austria to 2-fold in Denmark from 1972 to 1996. As for triplets, the rate increased from 3-fold in Denmark to 9-fold in Norway during that period. With one exception, that being the Slovak Republic, the triplet rate was highest in the Scandinavian countries, followed by the other European and Asian countries. The rising twinning and triplet rates have been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to IVF.

Finnish mothers with Järvi-Hakola-Nasu disease, progressive dementia with lipomembranous polycystic osteodysplasia (McKusick 221770) have a high rate of twin maternities, 128.2/1000. The exact 99% confidence intervals are 28.7 – 322.2/1000, thus above the average twinning rate in Finland, i.e. 15/1000. This eightfold increase in twinning may be an indication of a disturbed cortico-hypothalmic-hypophyseal axis or an other premorbid hormonal imbalance. It is concluded that even if dizygotic twinning is as a rule an event in itself, not only iatrogenic factors, as ovulation inducers, etc., but also some genetic disorders may be associated with twinning. More studies are needed to elucidate the incidence of twinning in families with these disorders.

A high frequency of twin births has been observed in Linha São Pedro, a small settlement which belongs to the city of Cândido Godói, located 524 km Northwest from Porto Alegre, Rio Grande do Sul, Brazil, in an ethnically homogeneous population of German descent restricted to a small geographic region. From 1990 to 1994, the proportion of twin births in Linha São Pedro was 10%, significantly higher than the 1.8% rate for the state of Rio Grande do Sul as a whole. Genealogical analysis showed a high recurrence of multiple births within families, as well as a high level of inbreeding in the community. Zygosity data indicated that 9 of the 17 pairs of twins studied (53%) were dizygotic. No external environmental factors were detected that could be influencing the appearance of this characteristic. This preliminary investigation confirmed the presumed existence of a high twinning rate in the community. The high familial recurrence and the high inbreeding rate suggests the presence of genetic twinning factors. Complementary studies of twins that have yet to be evaluated and the search for additional risk factors, as well as linkage studies, should contribute to a further understanding of the biological factors related to twin births in the human species.

The twinning incidence from 1984 to 1993 was investigated in Campinas, State of São Paulo, Brazil, and the analyzed data were combined with previous series from the same population. This study has shown that the mean annual incidences and the standard deviations for DZ, MZ and DZ plus MZ twins for the period of 1984 to 1993 are estimated respectively as 4.7 ± 0.92, 4.1 ± 1.11, and 8.8 ± 0.87 per 1,000. In the same period the mean annual incidence of triplets was 0.15 ± 0.16. It was also shown that the incidence of DZ twins is steadily decreasing since 1925, while the incidence of MZ pairs, after a period of decline is increasing since the sixties, due perhaps to the widespread use of oral contraceptives. Concerning the twinning rate as a whole, it has been shown that its declining trend has disappeared over the last few years.

Twin births are known to vary across seasons in several countries. It has been hypothesized that this variation may be due to seasonal changes in luminosity leading to pineal gland-mediated multiovulation among susceptible mothers. To describe seasonal variation of twin births in Washington State, all mothers residing in Washington State who gave birth to both a pair of twins and a singleton baby between 1984-1990 (n = 1168) were identified through linkage of computerized State birth certificates. Using a “matched-on-mother” case-control design, the estimated month of conception of twin gestations (the “case” events) were compared to that for their singleton siblings (the “control” events) to determine their relative occurrence during periods of high vs low sunlight in accordance with local climatological data. For the study population as a whole, there was only a slight tendency for twins to have been conceived during the period of high sunlight compared to their singleton siblings (OR = 1.3, 95% C.I. = 1.0-1.7). When stratified by concordant-sex vs discordant-sex, however, more discordant-sex twin pairs were conceived during the light period than corresponding singletons (OR = 1.7, 95% C.I. = 1.0-2.8), whereas no association was found for concordant-sex twins (OR =1.1, 95% C.I. =0.8-1.6). The presence of an association only among discordant-sex twins, all of whom are dizygotic, is consistent with the hypothesis that exposure to sunlight may stimulate multiple ovulation, and thus increase the incidence of twin gestations among twin-prone mothers.

Total birth records for the Bengalee Muslim population (BMP) and the Bengalee Hindu caste population (BHCP) for the period 1980-1988 were 17,720 and 119,107 respectively. Of these, the number of twin pairs were 363 BMP and 1,229 BHCP. These data were obtained from the registers of the following hospitals: Islamia Hospital, NRS Medical College and Hospitals and RG Kar Medical College and Hospitals, Calcutta and Medinipore Sadar Hospital, West Bengal, India. The twinning rates found were 20.48 and 10.57 per thousand deliveries in the BMP and BHCP respectively. The proportion of twins, 0.02048, in the BMP was significantly higher (Z = 12.38, p ≤ 0.01) than that in the BHCP, 0.01057. This finding of a higher twinning rate in the BMP is corroborated by the fact that available data on the Muslim population of Srinagar in Kammu and Kashmir, and Lucknow and Kanpur in Uttar Pradesh show higher twinning rates than the other populations of India. This increased twinning rate may be due to the greater amount of inbreeding in the BMP.

Based on Dutch twin incidence figures since the beginning of the current century, evidence is provided in support of the idea that the DZ unlike/like sexed ratio has gradually shifted (since 1900) from unity to less than unity. Opposing conclusions with regard to the justification of the use of Weingberg's differential rule are very probably correct in themselves but could depend on country and period of birth of the twin sample used. Furthermore, the fast drop and subsequent rise in DZ twinning rate between about 1963 and 1990 can very likely for the greater part be ascribed to a parallel shift in maternal age.

The aim of this study on all twin births in Sweden between 1973 and 1985 was to evaluate the effect of maternal age, parity, and sex on twin perinatal mortality (PNM), by using national data held at the Medical Birth Registry, National Board of Health and Welfare, Stockholm. Zygosity in this study was estimated by the Hardy-Weinberg formula. In all, 22,374 twins were born; 972 (4.3%) of these died perinatally. The relative risk for perinatal death (after standardization for maternal age, parity, and like sex) decreased significantly during the study period, and reached its lowest value (0.3) in 1983. Maternal age was not associated with PNM, while women having their first delivery had a significantly higher PNM than the rest of the parturients. The MZ twinning rate increased fom 0.36% during 1973-76, to 0.40% during 1977-81 and 0.44% during 1982-85 (trend analysis: χ2 = 40.1, p < 0.001). After stratification for year of delivery and birthweight, like-sexed twins had a significantly higher PNM than unlike-sexed twins (odds ratio, OR = 1.25, 95% confidence interval, CI = 1.02-1.54). Males had a significantly higher PNM than females in like-sexed pairs (OR = 1.39, 95% CI = 1.16-1.66), and in unlike-sexed pairs (OR = 1.46, 95% CI = 1.01-2.10). MZ twins had a significantly higher estimated PNM than DZ ones (OR = 1.31, 95% CI = 1.02-1.67).