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.

Research on twinning in developed countries is well documented, but little is known about twinning in less-developed countries due to a lack of data or data limitations. This study examined the level of, trends in and determinants of twin births and their survival under age five in Jordan, using the 2012 Jordan Population and Family Health Survey (JPFHS) data. The birth history of 11,352 women included in the JPFHS provided information on 9859 live births that occurred within five years of the survey date, and these constituted the study subjects. Descriptive statistics, and bivariate and multivariate multilevel logistic regression models were used for data analysis. About 3.5% of the total live births were twins, which is one of the highest rates in developing countries. The twinning rate showed an increasing trend in Jordan, increasing by 45% from 2.4% in 1990 to 3.5% in 2012. Higher maternal age at child birth, higher parity, poor and richer economic status, contraceptive use status, secondary and above level of education of mothers and non-consanguinity were found to be associated with a higher rate of twin births in Jordan. Twins were observed to have a higher risk of low birth weight, being smaller in physical size at birth and at more than four times higher risk of neonatal death compared with singletons. To reduce the risk adverse outcomes of twin pregnancies and further improve maternal and child health, antenatal, delivery and postnatal care should be tailored towards the special needs of twin pregnancies and twin births in Jordan Twin pregnancies should be detected at the early stage of pregnancy so that perinatal outcomes can be improved by frequent prenatal visits, health education, counselling and proper management of peripartum complications.

Recently, there has been a significant increase in the rate of multiple births in most developed countries. However, few population-based studies have been conducted in China regarding the epidemiology of twin births in recent years. We performed a descriptive analysis of twin births from 1993 to 2005 using data from a population-based perinatal care program in southeast China. The twin birth rate in southeast China was 0.65%, and the twin birth rates from 1993 to 2005 fluctuated between 0.60% and 0.70%. During the three periods of 1993–1996, 1997–2000, and 2001–2005, the twin birth rate increased from 0.57% to 0.71% in urban areas (p = .005) and from 0.59% to 0.68% in mothers who had an education level of high school or higher (p = .046). After 2000, the twin birth rate of primiparae 30 years of age and older significantly increased from 0.72% to greater than 1.20%. We concluded that the twin birth rates in southeast China from 1993 to 2005 stayed constant in the overall population but increased in certain subgroups of women, presumably due to increased use of fertility treatment and the development of assisted reproductive technology.

Stillbirth rates among single and multiple births show markedly decreasing temporal trends. In addition, several studies have demonstrated that the stillbirth rates are dependent on maternal age, in general, showing a U- or J-shaped association with maternal age. In this study, the temporal trends in and the effect of maternal age on the stillbirth rate were considered simultaneously. Our goal was to split the variation into temporal trends and maternal age effects. We applied two-dimensional analysis of variance because no linear association between maternal age and stillbirth rate can be assumed. The temporal trends of stillbirth rates also were not clearly linear. However, the possibility of applying regression analyses based on linear time trends was also considered. Our study is mainly based on official data from England and Wales for the period between 1927 and 2004. These results were compared with registered birth data from Finland between 1937 and 1997. The best fit was obtained when the models were built for the logarithm of the stillbirth rate. Our interpretation of this result is that an association exists between the effects of the factors and the mean stillbirth rate, and consequently, a multiplicative model was applied. Relatively high stillbirth rates were observed among twin births of young mothers and among all births of older mothers.

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 hypothesis of seasonality of twin births was investigated in two important maternity hospitals in the State of Sāo Paulo, Brazil. The study included 1,386 twin births that occurred among 154,699 deliveries from 1984 to 1993. No evidence of seasonality has been detected either for the twin birth rate considered as a whole or for dizygotic twinning rate. The distributions of these rates fitted well sinusoidal regression curves but the cyclic trend did not correspond to any specific season.

The annual variation of the sex ratio (SR) of 1385 twin births was analyzed and compared to that of 85909 singletons. These births referred not only to live births but also to stillbirths in two southeastern Brazilian maternity hospitals from 1984 to 1993. While the annual variation of the SR of singletons was very small, that of twin births was extremely high, due to the significant heterogeneity of the annual data. It is suggested that the large SR annual variation of the twin birth might be a consequence of the variation of male or female monozygotic twins.The hypothesis that twin births show a lower sex ratio than singletons could not be supported by the present data.

Twins and singletons were matched for several confounding factors. The monthly distribution of twin births and that of singleton births were compared with a uniform allocation of births over the year. Opposite seasonal variations emerged that were confirmed by a case/control comparison. Twins occurred more often in winter and early spring while singletons proved to be relatively few; singleton peaked in the fall season when the risk of a twin birth was low. These trends held across maternal age at birth and the time period of birth. Results suggest that the conception of a twin pair is higest in spring and early summer and lowest in winter. The role of sunlight in the twinning liability is discussed along with the role of sexual intercourse. That twin-prone mothers are usually more fecund in spring and early summer is a distinct possibility.