Twinning is rare among humans, but there is much variability among populations. Several studies show that certain demographic and socioeconomic factors, such as maternal age, mother’s educational level and income, influence twinning rate. There is no background of analytical studies of twins in Uruguay. To the best of our knowledge, this is the first study that has focused on describing and analyzing Uruguayan twinning rates over a period of 17 years (1999–2015). The birth data were collected from the website of Uruguay’s Ministry of Public Health. Economic data were obtained from Uruguay’s Instituto Nacional de Estadísti’s website for the period 2001–2013, since these variables are defined specifically for that period of time. The statistical software R (The R Project for Statistical Computing) was used. The twinning rate varied from 8.51 to 13 in the studied period. Montevideo has the highest median and the smallest variability in comparison with the other departments. In Uruguay (1999–2015), the highest twinning rate (28.94%) was observed in women aged 45 and older. The analysis also showed a relationship between twin birth rates and the mother’s educational level. In three regions of the country (West, Center and East), twin births show a random pattern but in the other two (North and Metropolitan), there is an increasing trend in the number of twins over time. In conclusion, this study recognizes social, economic and demographic factors that influence in the rate of twin births in Uruguay.

The present study investigates the twinning rates in the city of São Paulo, Brazil, during the years 2003–2014. The data were drawn from the Brazilian Health Department database of Sistema de Informações de Nascidos Vivos de São Paulo—SINASC (Live Births Information System of São Paulo). In general, more information is available on the incidence of twinning in developed countries than in developing ones. A total of 24,589 twin deliveries and 736 multiple deliveries were registered in 140 hospitals of São Paulo out of a total of 2,056,016 deliveries during the studied time period. The overall average rates of singleton, twin, and multiple births per 1,000 maternities (‰) were 987.43, 11.96 (dizygotic (DZ) rate was 7.15 and monozygotic (MZ) 4.42), and 0.36, respectively. We further regressed maternal age and historical time period on percentage of singleton, twin, and multiple birth rates. Our results indicated that maternal age strongly positively predicted twin and multiple birth rates, and negatively predicted singleton birth rates. The historical time period also positively, although weakly, predicted twin birth rates, and had no effect on singleton or multiple birth rates. Further, after applying Weinberg's differential method, we computed regressions separately for the estimated frequencies of DZ and MZ twin rates. DZ twinning was strongly positively predicted by maternal age and, to a smaller degree, by time period, while MZ twinning increased marginally only with higher maternal age. Factors such as increasing body mass index or air pollution can lead to the slight historical increase in DZ twinning rates. Importantly, consistent with previous cross-cultural and historical research, our results support the existence of an age-dependent physiological mechanism that leads to a strong increase in twinning and multiple births, but not singleton births, among mothers of higher age categories. From the ultimate perspective, twinning and multiple births in later age can lead to higher individual reproductive success near the end of the reproductive career of the mother.

We analyzed the effect of total fertility rate (TFR) and crude birth rate (CBR) on the number of males per 100 females at birth, also called the secondary sex ratio (SR), and on the twinning rate (TWR). Earlier studies have noted regional variations in TWR and racial differences in the SR. Statistical analyses have shown that comparisons between SRs demand large data sets because random fluctuations in moderate data are marked. Consequently, reliable results presuppose national birth data. Here, we analyzed historical demographic data and their regional variations between counties in Sweden. We built spatial models for the TFR in 1860 and the CBR in 1751–1870, and as regressors we used geographical coordinates for the provincial capitals of the counties. For both variables, we obtained significant spatial variations, albeit of different patterns and power. The SR among the live-born in 1749–1869 and the TWR in 1751–1860 showed slight spatial variations. The influence of CBR and TFR on the SR and TWR was examined and statistical significant effects were found.

Trends of twinning rates were analyzed using vital statistics in Austria, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore during the period from 1972 to 1996. The twinning rates increased significantly year by year in each country. During these periods, the twinning rate increased by twenty percent in Austria and Canada, and by sixty percent in Norway and Sweden. The twinning rate was 1.6 times higher in Sweden than in Hong Kong in 1972 and by eighty percent in Israel in 1995. Twinning rates were higher in European countries, Canada, and Australia than in Asia. The variations of the twinning rates among countries were not only due to biological factors, but also to assisted reproductive techniques. In Australia, the overall twinning rate was 1.3 times higher in the nuptial births (14.1 per 1000 births) than in the ex-nuptial births (10.7) during the period 1994-1996. As for maternal age, twinning rates in Sweden increased year by year for maternal age groups except the youngest and the oldest age groups. In Sweden, the rising twinning rate has been attributed to the higher proportion of mothers (for the 25-39 year old age groups) treated with ovulation-inducing hormones and attributed to in-vitro fertilization.

The monozygotic (MZ) twinning rate in Japan had remained nearly constant from 1975 (3.74 per 1,000 births) to 1994 (4.23), whereas the dizygotic (DZ) twinning rate had remained nearly constant from 1975 (1.86) to 1986 (2.27), and had gradually increased up to 1994 (3.89). The higher DZ twinning rate since 1987 has been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to in-vitro fertilisation in Japan. As for maternal age, MZ twinning rates have remained nearly constant for maternal age groups except the youngest and the oldest age groups. On the other hand, DZ twinning rates increased up to the 35-39 years of age group and decrease thereafter. In 1994, the twinning rate was higher in DZ twins than in MZ twins for maternal age groups of 30-34 years and of 35-39 years. As for geographical variations in twinning rates, DZ rates statistically significantly increased with the year during the period from 1986 to 1994 in 31 out of 47 prefectures. In 1994, twinning rates in 21 out of 47 prefectures were higher in the DZ than the MZ rate, and the DZ rate is equal to the MZ rate in two prefectures. Geographical variations in twinning rates by zygosity in 1994 drastically changed from those during the period from 1955-1959 and in 1974.

Malawi is a country in South-East Africa with a population of approximately 10 million, over 95% of whom are negro of Bantu origin. The country is divided into 24 administrative districts, each of which provides details of births which are compiled centrally at the Ministry of Health. Using data reported annually by health facilities from 1987-1990, most districts had twinning rates in the range 16 to 24 per 1000 maternities, figures consistent with those of other negro populations in Africa. Two adjoining districts (Rumphi and Mzimba) had rates which were considerably higher, almost 30 per 1000. Although ethnic origin is not recorded in the latest Malawi census, language spoken in the home, which was last recorded in 1966, can be used as a proxy. Rumphi and Mzimba are the only districts with an overwhelming majority of Tumbuka speaking population. These people, who are of both Tumbuka and Ngoni ethnic origin, therefore have an unusually high twinning rate (29.57 per 1000 maternities). The rate for the main ethnic group in Malawi, the Chewa people, was 21.21 per 1000. The district of Thyolo, with a mixed ethnic population, had by far the lowest rate (13.75 per 1000).

Twinning rates in all of Japan for 1951-1968 and 1974-1990 were analyzed using data from vital statistics of Japan. The twinning rate per 1,000 births was 6.43 in 1951 and remained nearly constant until 1968, then decreased to 5.79 in 1974 and gradually increased to 7.00 in 1990. The MZ twinning rate increased slightly up to 1966, but decreased thereafter, whereas the DZ rate declined over the entire period. As for maternal age, the DZ rate increased up to the age group 35-39 and decreased thereafter. The same tendency is seen in the MZ rate, but the maternal age effect is less marked than in DZ twins. The same tendency is also seen in the overall rate for 1975-1985. As for geographical variations, the MZ and DZ rates were computed in each prefecture for 1955-1959 and in 1974. The DZ rate increased from a low level in the southwest of Japan to a high level in the northeast for both periods, whereas the MZ rate was rather constant in 1974 throughout Japan. As for seasonal variation, the highest rates per 1,000 births for MZ(4.56) and DZ(2.20) twins were seen in April, with the lowest rate in September (3.69) for MZ twins and in July (1.71) for DZ twins.

A comparison was made of the stature of women, twinning rates, and breast cancer mortality for 32 countries. As height increased, so did twinning and breast cancer mortality (P < 0.005). Dizygotic twinning and breast cancer increased sharply with the mean height of the female population. With due caution in drawing causal inferences when uncontrolled confounding variables are present, it is suggested that these findings are an evolutionary consequence of the high mortality found in twin pregnancy.

The rise in twinning rates previously reported for U.S. between 1964 and 1987 after adjustment for maternal age and birth order occurred in each of the nine conventional geographic divisions except the Pacific States. Differences and consistencies in rank order among the divisions with respect to crude and adjusted twinning rates and other demographic parameters may hold clues to yet unidentified influences in twinning.

Mean MZ and DZ twinning rates in seven big Greek cities were 3.2 and 4.75 per 1,000 maternities, respectively, during the 1980-1985 period. The seasonal variations in twinning frequencies are not significant. The total twinning rate in Greece shows a decreasing trend from 1956 to 1985.

A secular demographic study on the Iniö Island in SW Finland shows an even stronger decline of twinning rates than in the Åland Islands and the rest of the archipelago. Twinning rates in Iniö declined, in fact from about 30‰ to less than 10‰.

It is suggested that the rise in a population depends not only on the number of females per thousand males, their fecundability, and the survival of offspring (mean live births), but also on the genetic background or inbreeding status of the fetuses. Abortions and stillbirths add to the interference. A hypothetical formula has been devised , where Mb = mean birth rate, G = growth percentage of population sample from preceding Census, F = inbreeding coefficient, Sa = Stillbirths + abortions, and Sr = Sex ratio. It is intriguing that the product, being termed Fraternal Component (FC), is found to be almost equal to the DZ twinning rate of the population sample. Should that be confirmed, one could perphaps estimate the DZ twinning rate by an altogether new approach.

Detailed twin birth rates for the United States are unavailable since 1964. In 1983 the crude twinning rate for women of white race was higher than in 1964, but there had been great changes in maternal age and parity. Indirect standardization for maternal age and birth order provides estimated total twinning rates that can be compared over the entire period. The adjusted rates for whites show a nearly continuous increase except after a 2-year reporting hiatus, 1969-70, when rates dropped back 10%. In blacks the adjusted rate increased between 1966 and 1978, except for the 1968-71 shift. The distributions of rate increases by maternal age and by race argue against effects of medical ovulation stimulants, but a disproportionate increase of triplets argues for such effects. Study is needed of rates specific for maternal age and parity, rather than of total rates.

Linear regression models are used to explain the variations in the twinning rates. Data sets from different countries are analysed and maternal age, parity and marital status are the main regressors. The model building technique is also used in order to study the secular decline in the twinning rate. Linear regression technique makes it possible to compare the effect of different factors but the method requires sufficiently disaggregated data.

The twinning rate remained nearly Constant up to 1968, then decreased in 1974 and gradually increased with the year. The triplet birth rate remained nearly Constant up to 1974, then increased up to 1982, where the rate was 1.8 times higher than in 1968, and decreased thereafter. On the other hand, the quadruplet birth rate remained nearly Constant up to 1968, was eightfold in 1975, then decreased until 1984 and suddenly increased to 8 per million births in 1985. The higher multiple birth rates since 1974 was attributed to the higher proportion of mothers treated with ovulation-inducing hormones in Japan. The stillbirth rates decreased to 2/5 for male twins and to 1/3 for female twins during the 25-year period from 1960, to 2/5 for triplets and to 1/5 for quadruplets during the 34-year period from 1951.

The twinning, triplet and quadruplet rates were found to be 23.8, 0.3 and 0.04 per thousand maternities, respectively in a survey of 71,773 maternities carried out in Ibadan and Igbo Ora, Nigeria. This represents a dramatic fall from the rates found in previous studies. The findings are discussed in the light of changing dietary habits.

A study of 111 mothers of twins in Nottinghamshire, England, in 1981-82 showed that at least 12 had used ovulation stimulants, compared to 2 of 102 mothers of singletons. Thus, the use of ovulation stimulants increased the twinning rate of this population by about 10 percent; this may explain the recently noted levelling off in the secular decline in twinning rates. In contrast to a recent study in France, no association between twinning and prior use of oral contraceptives was seen.

A total of 178 placentas of twins born in Tokyo in 1961-1982 were examined. Only just over one third were dichorionic, 110 being monochorionic and DZ twins were estimated to account for 24.7% of the pairs. The lower frequency of DZ twinning in Japan is attributed to lower gonadotropin values in Japanese women as compared to European, American, or African women. High fetal morbidity was found in monochorionic twins and this is attributed to a frequent occurrence of vascular anastomoses.

The rates of MZ and DZ twin births and of triplet births slightly decreased in Japan in 1955-1967 and 1974, whereas that of trizygotic triplets tripled. The rates of triplet and quadruplet births were comparable to those of Caucasians. Although the sample size is small, Japan seems to be characterized by higher rates of triplets and quadruplets of polyembryonal origin and lower rates of those of polyovulational origin as compared to Caucasian populations. Stillbirth rates in both MZ and DZ twins gradually decreased in 1960-1967 and 1974. The rate was higher in MZ than DZ twins, and higher in males than females. The secular trends of stillbirth rates in like-sexed triplets was similar to that in twins, whereas the rate in unlike-sexed triplets remained almost constant for the entire period. The rate was higher in like-sexed than unlike-sexed triplets.

The dizygotic twinning rate in Western Nigeria is the highest on record (45—50 per mil maternities). It is suggested that, rather than to peculiarities in the population structure or to genetic factors, such a high incidence might be due to the presence in the diet of estrogen-like substances.