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.

Dichorionic diamniotic (DCDA) twin pregnancies after single blastocyst embryo transfer have been reported recently, although a blastocyst ovum is generally believed to divide into monochorionic twin pregnancy. We investigated the incidence of DCDA twin pregnancy after single blastocyst embryo transfer and their zygosity. This prospective cohort study included 655 consecutive twin pregnancies that were managed from 2006 to 2014 at our institution. Chorionicity and amnionicity were determined using first-trimester ultrasonography and/or placental pathology. Zygosity was analyzed if the cases were DCDA twins after single blastocyst embryo transfer. Among 655 twin pregnancies, there were 348 DCDA cases, 295 monochorionic diamniotic (MCDA) cases and 12 monochorionic monoamniotic cases. Single blastocyst embryo transfer was performed in 43 cases. Six out of the 43 (14%) cases involved DCDA twin pregnancies and the other 37 cases involved MCDA twin pregnancies. Three DCDA twins born after single blastocyst embryo transfer, wherein frozen embryo transfer (FET) was performed in the natural cycle, were dizygotic, and the other three cases, wherein FET with hormone replacement therapy was performed, were monozygotic. DCDA twin pregnancy occurred in 14% (7% for monozygotic and 7% for dizygotic) of twin pregnancies after single blastocyst embryo transfer cases.

In the 19th century, a series of international statistical congresses began that were important for population studies, including twin research. The introduction of common rules for the national demographic registers enabled scientists to contribute to the genesis of statistical research. The congress in St. Petersburg in 1872, in particular, focused on the movements of the population, and how they should be registered. Among the facts to be recorded were in multiple births, the sex and number of children born alive or still-born, whether legitimate or illegitimate, and the age of the mother at the date of the births. During the history of twin research, Hellin's law has played a central role because it is an approximately correct association between the rates of multiple maternities. It has been mathematically proven that Hellin's law does not hold as a general rule. Analyses show divergences from the law that are difficult to explain and/or eliminate. Varying improvements of this law have been proposed. The majority of all studies of Hellin's law are based on empirical rates of multiple maternities, ignoring random errors. Such studies can never confirm the law, but only identify errors with respect to Hellin's law that are too large to be characterized as random. It is of particular interest to note and explain why the rates of higher multiple maternities are sometimes too high or too low when Hellin's law is used as a benchmark. Studies have shown that there were investigators before Hellin who have contributed substantially to Hellin's law. In this article, we re-examine some old data sets and contributions in which Hellin's law has been evaluated and also analyze recent data.

In the 19th century, a series of international statistical congresses introduced common rules for the national demographic registers. This activity contributed to the genesis of statistical research. During the history of twin research, Hellin's law has played a central role because it is an approximately correct association between the rates of multiple maternities. However, it has been mathematically proven that Hellin's law cannot hold exactly. The majority of all studies of Hellin's law are based on empirical rates of multiple maternities. Such studies can never confirm the law, but only identify errors too large to be characterized as random. It is of particular interest to examine why the rates of higher multiple maternities are sometimes too high or too low when Hellin's law is used as a benchmark. However, divergences from the law are often difficult to explain and/or eliminate. Different improvements to the law have been proposed. In this article, we study the seasonality of multiple maternities. We apply Hellin's law to compare the seasonality of twin and triplet rates.

Metabolomic and microbiome profiling are promising tools to identify biomarkers of food intake and health status. The individual's genetic makeup plays a significant role on health, metabolism, gut microbes and diet and twin studies provide unique opportunities to untangle gene–environment effects on complex phenotypes. This brief review discusses the value of twin studies in nutrition research with a particular focus on metabolomics and the gut microbiome. Although, the twin model is a powerful tool to segregate the genetic component, to date, very few studies combine the twin design and metabolomics/microbiome in nutritional sciences. Moreover, since the individual's diet has a strong influence on the microbiome composition and the gut microbiome is modifiable (60 % of microbiome diversity is due to the environment), future studies should target the microbiome via dietary interventions.

Dietary fat intake is correlated with increased insulin resistance (IR). However, it is unknown whether gene–diet interaction modulates the association. This study estimated heritability of IR measures and the related genetic correlations with fat intake, and tested whether dietary fat intake modifies the genetic influence on type 2 diabetes (T2D)-related traits in Chinese child twins. We included 622 twins aged 7–15 years (n 311 pairs, 162 monozygotic (MZ), 149 dizygotic (DZ)) from south-eastern China. Dietary factors were measured using FFQ. Structural equation models were fit using Mx statistical package. The intra-class correlation coefficients for all traits related to T2D were higher for MZ twins than for DZ twins. Dietary fat and fasting serum insulin (additive genetic correlation (rA) 0·20; 95 % CI 0·08, 0·43), glucose (rA 0·12; 95 % CI 0·01, 0·40), homoeostasis model of assessment-insulin resistance (Homa-IR) (rA 0·22; 95 % CI 0·10, 0·50) and the quantitative insulin sensitivity check index (Quicki) (rA −0·22; 95 % CI −0·40, 0·04) showed strong genetic correlations. Heritabilities of dietary fat intake, fasting glucose and insulin were estimated to be 52, 70 and 70 %, respectively. More than 70 % of the phenotypic correlations between dietary fat and insulin, glucose, Homa-IR and the Quicki index appeared to be mediated by shared genetic influence. Dietary fat significantly modified additive genetic effects on these quantitative traits associated with T2D. Analysis of Chinese twins yielded high estimates of heritability of dietary fat intake and IR. Genetic factors appear to contribute to a high proportion of the variance for both insulin sensitivity and IR. Dietary fat intake modifies the genetic influence on blood levels of insulin and glucose, Homa-IR and the Quicki index.

Deciphering the relative importance of genetic and environmental factors, which play a major role in the prevalence of early childhood caries (ECC), can help clinicians with planning a long-term preventive treatment. The objective of the study was to determine the prevalence and heritability of ECC among monozygotic (MZ) and dizygotic (DZ) twins in Chennai, India, in the year 2013. A cross-sectional study was designed to estimate the prevalence of ECC among twins. Zygosity classification for the survey framework was adapted from a highly accurate parental report questionnaire pertaining to the physical similarity between twins. The associated heritability index was estimated. The Decayed, Missing, Filled Surface (DMFS) Index was used as the diagnostic criterion for dental caries. The prevalence of ECC was estimated at 18.7%. The correlation coefficient between the twin pair showed significant correlation. The heritability index for ECC was estimated at 15% higher prevalence of ECC found among children in the age group 25–36 months. The heritability estimate indicated a relatively low genetic influence for early childhood caries among twins. There was no significant difference detected in the concordance rate for the MZ and DZ twins. Further research could be directed toward the prevalence of ECC among higher age group children to explore the role of genetic and environmental factors.

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.

It is unknown whether influences of midlife whole diet on the long-term CHD mortality risk are independent of genetic and common environmental factors or familial predisposition. We addressed this question prospectively using data from the National Heart, Lung, and Blood Institute Twin Study. We included 910 male twins who were middle-aged and had usual diet assessed with nutritionist-administered, cross-checked dietary history interview at baseline (1969–1973). Moderation-quantified healthy diet (MQHD), a dietary pattern, was created to evaluate a whole diet. Primary outcome was time-to-CHD death. Hazard ratios (HR) were estimated using frailty survival model. Known CHD risk factors were controlled. During the follow-up of 40 years through 31 December 2009, 113 CHD deaths, 198 total cardiovascular deaths and 610 all-cause deaths occurred. In the entire cohort, the multivariable-adjusted HR for the overall association (equivalent to a general population association) was 0·76 (95 % CI 0·66, 0·88) per 10-unit increment in the MQHD score for CHD, and the multivariable-adjusted HR for a twin with a MQHD score ten units higher than his co-twin brother was 0·79 (95 % CI 0·64, 0·96, P=0·02) for CHD independent of familial predisposition. Similar results were found for a slightly more food-specified alternative moderation-quantified healthy diet (aMQHD). The between-pair association (reflecting familial influence) was significant for CHD for both MQHD and aMQHD. It is concluded that associations of MQHD and aMQHD with a lower long-term CHD mortality risk are both nutritionally and familially affected, supporting their use for dietary planning to prevent CHD mortality.

There is a lack of evidence pointing to specific dietary elements related to weight gain and obesity prevention in childhood and adulthood. Dietary intake and obesity are both inherited and culturally transmitted, but most prospective studies on the association between diet and weight status do not take genetics into consideration. The objective of this study was to document the association between dietary intake at 9 years and subsequent Body Mass Index (BMI) in adolescent monozygotic boy and girl twin pairs. This research used data from 152 twin pairs. Dietary data were collected from two 24-hour-recall interviews with a parent and the child aged 9 years. Height and weight were obtained when the twins were aged 9, 12, 13, and 14 years. Intrapair variability analysis was performed to identify dietary elements related to BMI changes in subsequent years. BMI-discordant monozygotic twin pairs were also identified to analyze the dietary constituents that may have generated the discordance. After eliminating potential confounding genetic factors, pre-adolescent boys who ate fewer grain products and fruit and consumed more high-fat meat and milk had higher BMIs during adolescence; pre-adolescent girls who consumed more grain products and high-fat meat and milk had higher BMIs during adolescence. Energy intake (EI) at 9 years was not related to BMI in subsequent years. Our study suggests that messages and interventions directed at obesity prevention could take advantage of sex-specific designs and‚ eventually‚ genetic information.

Background: Recent studies have suggested that ovarian stimulation and assisted reproductive techniques (ART) may increase the frequency of monozygotic twins. In this article, we present the analysis of the estimated frequency of twin deliveries following in vitro fertilization (IVF) in Lombardy during the period 2010–2014 for a total of 450,949 pregnancies. Method: This is a population-based study using data from the regional data base of Lombardy, a northern Italian region with a population of about 10 million inhabitants. During the considered period, a total of 461,424 single or multiple births were registered in Lombardy. After exclusion of triplets or more pregnancies, the total number of twin deliveries, in separate strata of like and unlike sex pregnancies twin deliveries, were obtained and the rate of twin deliveries was computed according to spontaneous and non-spontaneous conception and type of ART. Further, estimates of dizygotic or monozygotic twin births were calculated using Weinberg's methods. Results: The frequency of twins deliveries was 1.24/100 deliveries after natural conception and 20.05 after assisted conception. The estimated rates of monozygotic twins was 0.45 and 0.72/100 (95% CI: 0.58–0.91) deliveries after natural and assisted conception, respectively. This difference was statistically significant (p < .05). Conclusion: the present population based study suggests that the risk of monozygotic twin is about 60% higher among assisted than after natural conception.

The aim of the present work was to estimate the risk of monochorionic twin (MCT) pregnancies in in vitro fertilization (IVF) cycles using data from a prenatal diagnosis unit. This was a retrospective cross-sectional study reporting on the frequency of IVF pregnancies among women attending a prenatal diagnosis service specifically dedicated to the management of monochorionic pregnancies. The observed rate was compared with the local regional rate of IVF births (2.2%). A binomial distribution model was used to calculate the 95% CI of proportions. One hundred and forty-five monochorionic pregnancies were selected. Ten of these were achieved with IVF, corresponding to a rate of 6.9% (95% CI: 3.5–11.8), significantly higher than the background rate in the local population of 2.2%. When considering exclusively monochorionic pregnancies achieving delivery of two viable newborns (n = 132), the number of IVF pregnancies was nine (6.8%, 95% CI: 3.7–12.5). We did not detect major differences in pregnancy outcome between IVF and natural monochorionic pregnancies, with the exception of the proportion of newborns with a neonatal birth < 2,500 g (100% vs. 80%, p = .03). In conclusion, data obtained from the perspective of a prenatal diagnosis unit suggest that women undergoing IVF face a 3- to 4-fold increased risk of monochorionic pregnancies.

By definition, monozygotic (MZ) twins carry an identical set of genetic information. The observation of early post-twinning mutational events was shown to cause phenotypic discordance among MZ twin pairs. These mutational events comprise genomic alterations at different scales, ranging from single nucleotide changes to larger copy-number variations (CNVs) of varying sizes, as well as epigenetic changes. Here, we performed whole-exome sequencing (WES) in nine discordant MZ twins to identify somatic mutational events in the affected twin that might exert a dominant negative effect. Five of these MZ twin pairs were discordant for congenital heart defects (CHD), two for endocrine disorders, one for omphalocele, and one for congenital diaphragmatic hernia (CDH). Analysis of WES data from all nine MZ twin pairs using the de novo probability tool DeNovoGear detected only one apparent de novo variation in TMPRSS13 in one of the CHD-affected twins. Analysis of WES data from all nine MZ twin pairs by using standard filter criteria without the de novo probability tool DeNovoGear revealed a total of 6,657 variations in which both the twin pairs differed. After filtering for variations only present in the affected twins and absent in in-house controls, 722 variations remained. Visual inspection for read quality decreased this number to 12, present only in the affected twin. However, Sanger sequencing of the overall 13 variations failed to confirm the variation in the affected twin. These results suggest that somatic mutational events in coding regions do not seem to play a major role in the phenotypic expression of MZ discordant twin pairs.

Over the past decade, epigenetic analyses have made important contributions to our understanding of healthy development and a wide variety of adverse conditions such as cancer and psychopathology. There is increasing evidence that DNA methylation is a mechanism by which environmental factors influence gene transcription and, ultimately, phenotype. However, differentiating the effects of the environment from those of genetics on DNA methylation profiles remains a significant challenge. Monozygotic (MZ) twin study designs are unique in their ability to control for genetic differences because each pair of MZ twins shares essentially the same genetic sequence with the exception of a small number of de novo mutations and copy number variations. Thus, differences within twin pairs in gene expression and phenotype, including behavior, can be attributed in the majority of cases to environmental effects rather than genetic influence. In this article, we review the literature showing how MZ twin designs can be used to study basic epigenetic principles, contributing to understanding the role of early in utero and postnatal environmental factors on the development of psychopathology. We also highlight the importance of initiating longitudinal and experimental studies with MZ twins during pregnancy. This approach is especially important to identify: (1) critical time periods during which the early environment can impact brain and mental health development, and (2) the specific mechanisms through which early environmental effects may be mediated. These studies may inform the optimum timing and design for early preventive interventions aimed at reducing risk for psychopathology.

The human oro-gastrointestinal (GI) tract is a complex system, consisting of oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus, which all together with the accessory digestive organs constitute the digestive system. The function of the digestive system is to break down dietary constituents into small molecules and then absorb these for subsequent distribution throughout the body. Besides digestion and carbohydrate metabolism, the indigenous microbiota has an important influence on host physiological, nutritional and immunological processes, and commensal bacteria are able to modulate the expression of host genes that regulate diverse and fundamental physiological functions. The main external factors that can affect the composition of the microbial community in generally healthy adults include major dietary changes and antibiotic therapy. Changes in some selected bacterial groups have been observed due to controlled changes to the normal diet e.g. high-protein diet, high-fat diet, prebiotics, probiotics and polyphenols. More specifically, changes in the type and quantity of non-digestible carbohydrates in the human diet influence both the metabolic products formed in the lower regions of the GI tract and the bacterial populations detected in faeces. The interactions between dietary factors, gut microbiota and host metabolism are increasingly demonstrated to be important for maintaining homeostasis and health. Therefore the aim of this review is to summarise the effect of diet, and especially dietary interventions, on the human gut microbiota. Furthermore, the most important confounding factors (methodologies used and intrinsic human factors) in relation to gut microbiota analyses are elucidated.

Ageing is a complex multifactorial process, reflecting the progression of all degenerative pathways within an organism. Due to the increase of life expectancy, in recent years, there is a pressing need to identify early-life events and risk factors that determine health outcomes in later life. So far, genetic variation only explains ~20–25 % of the variability of human survival to age 80+. This clearly implies that other factors (environmental, epigenetic and lifestyle) contribute to lifespan and the rate of healthy ageing within an individual. Twin studies in the past two decades proved to be a very powerful tool to discriminate the genetic from the environmental component. The aim of this review is to describe the basic concepts of the twin study design and to report some of the latest studies in which high-throughput technologies (e.g. genome/epigenome-wide assay, next generation sequencing, MS metabolic profiling) combined with the classical twin design have been applied to the analysis of novel ‘omics’ to further understand the molecular mechanisms of human ageing.

It is well established that genotype plays an important role in the ageing process. However, recent studies have suggested that epigenetic mechanisms may also influence the onset of ageing-associated diseases and longevity. Epigenetics is defined as processes that induce heritable changes in gene expression without a change in the DNA nucleotide sequence. The major epigenetic mechanisms are DNA methylation, histone modification and non-coding RNA. Such processes are involved in the regulation of tissue-specific gene expression, cell differentiation and genomic imprinting. However, epigenetic dysregulation is frequently seen with ageing. Relatively little is known about the factors that initiate such changes. However, there is emerging evidence that the early life environment, in particular nutrition, in early life can induce long-term changes in DNA methylation resulting in an altered susceptibility to a range of ageing-associated diseases. In this review, we will focus on the changes in DNA methylation that occur during ageing; their role in the ageing process and how early life nutrition can modulate DNA methylation and influence longevity. Understanding the mechanisms by which diet in early life can influence the epigenome will be crucial for the development of preventative and intervention strategies to increase well-being in later life.

Monozygotic (MZ) twinning is generally considered to be rare in species other than human. We inspected sex ratios in European zoo-bred ring-tailed lemurs (Lemur catta), revealing a significant excess of same-sex twins. Of 94 pairs, 60 (64%) were either both males or both females (p = .004). Application of the Weinberg differential rule argues that 27% of all twins in this species are MZ pairs. In this protected species, where twinning is commonplace (~50% of newborns are twins), the probable existence of frequent MZ twinning has ramifications for breeding programs aimed to maximize genetic diversity, and suggests that twin studies in a species other than human could have potential as a medical research tool.

The associations between social support and depression, and between stress and depression have been the subject of considerable research, and although this has included longitudinal designs, these have rarely controlled for genetic effects that mediate these associations. The sample comprised 7,356 female and 4,882 male participants aged 18–95 from the Australian NHMRC Twin Registry (ATR). Of these, between 100 and 324 female pairs and between 41 and 169 male pairs, depending on the measure, were monozygotic (MZ) pairs discordant for depression. We use the co-twin control design in combination with prospective analyses to explore the association between a composite of predictors (perceived social support, stress, and support × stress) and depression. With familial effects included, both perceived support and stress were antecedents to, and sequelae of, depression, but no stress-buffering occurred. With familial effects controlled, stress was a sequela of a prior depressive episode, and neither lack of support nor stress were antecedents to depression, though their interaction approached significance for males. The male twin who later became depressed had previously reported lower perceived support in the face of multiple stressors compared to his co-twin who did not become depressed. We show that associations commonly observed with prospective designs are partly due to familial factors.

That the zona pellucida (ZP) plays a prominent role in the physiology of some human twinning is an attractive, albeit incompletely proven, medical hypothesis. Indeed, an association has been proposed between manipulation of the ZP and/or native ZP microarchitecture and monozygotic (MZ) twins. Ovulation induction also has been theoretically linked to in vivo ZP alterations facilitating MZ twin development. In vitro fertilization (IVF) relies on necessary (and, in some cases extended) embryo culture techniques potentially creating subtle ZP changes and subsequent MZ twinning. With growing experience in the assisted reproductive technologies and particularly IVF, some preliminary reports have noted an increased frequency of MZ twins after procedures that artificially breach the ZP (i.e., intracytoplasmic sperm injection [ICSI], or ‘assisted hatching’). Such ZP manipulations ostensibly enhance oocyte fertilization or facilitate blastocyst hatching, thus improving pregnancy rates for couples undergoing fertility treatment. Evidence exists both to challenge and support the connection between these phenomena and MZ twins. This report outlines the fundamental embryological processes believed responsible for these conflicting observations; the current literature on the subject of human ZP micro-manipulation and MZ twins is also discussed. Twin Research (2000) 3, 217–223.