Research on twin-singleton differences in externalizing and internalizing problems in childhood is largely cross-sectional and yields contrasting results. The goal of this study was to compare developmental trajectories of externalizing and internalizing problems in 6- to 12-year-old twins and singletons. Child Behavior Checklist (CBCL) maternal reports of externalizing and internalizing problems were obtained for a sample of 9651 twins from the Netherlands Twin Register and for a representative general population sample of 1351 singletons. Latent growth modeling was applied to estimate growth curves for twins and singletons. Twin-singleton differences in the intercepts and slopes of the growth curves were examined. The developmental trajectories of externalizing problems showed a linear decrease over time, and were not significantly different for twins and singletons. Internalizing problems seem to develop similarly for twins and singletons up to age 9. After this age twins' internalizing symptoms start to decrease in comparison to those of singletons, resulting in less internalizing problems than singletons by the age of 12 years. Our findings confirm the generalizability of twin studies to singleton populations with regard to externalizing problems in middle and late childhood. The generalizability of studies on internalizing problems in early adolescence in twin samples should be addressed with care. Twinship may be a protective factor in the development of internalizing problems during early adolescence.

We examined the variation and heritability of DSM-IV nicotine withdrawal (NW) in adult and adolescent male and female twin cigarette smokers (who reported smoking 100 or more cigarettes lifetime). Telephone diagnostic interviews were completed with 3,112 Australian adult male and female smokers (53% women; age: 24–36) and 702 Missouri adolescent male and female smokers (59% girls; age: 15–21). No gender or cohort differences emerged in rates of meeting criteria for NW (44%). Latent class analyses found that NW symptoms were best conceptualized as a severity continuum (three levels in adults and two levels in adolescents). Across all groups, increasing NW severity was associated with difficulty quitting, impairment following cessation, heavy smoking, depression, anxiety, conduct disorder and problems with alcohol use. NW was also associated with seeking smoking cessation treatment and with smoking persistence in adults. The latent class structure of NW was equally heritable across adult and adolescent smokers with additive genetic influences accounting for 49% of the variance and the remaining 51% of variance accounted for by unique environmental influences. Overall, findings suggest remarkable similarity in the pattern and heritability of NW across adult and adolescent smokers, and highlight the important role of NW in psychiatric comorbidity and the process of smoking cessation across both age groups.

Our work assessed the accuracy of the original zygosity classification in the Vietnam Era Twin (VET) Registry using new information from DNA markers on a subset of participants. We then constructed an updated zygosity classification algorithm. The VET Registry includes 7,375 male–male twin pairs who served in the military during the Vietnam era. During the mid-1980s 4,774 twin pairs completed a zygosity questionnaire of 20 items. Additionally, military record information, including blood group, was available. Items from the zygosity questionnaire and blood group were used in the original zygosity classification. Between 1990–2009 DNA was obtained from 612 twin pairs and concordance between co-twins was used to classify zygosity. Next logistic regression was used to construct predicted probabilities of zygosity using items from the zygosity questionnaire with this subsample. All twins were reclassified according to the new zygosity prediction model and compared with the original zygosity assignment. The original and new predicted probabilities of zygosity were highly correlated (r = 0.962) and concordance for the classification of zygosity was similarly high (kappa = 0.936). Errors in the original zygosity assignment were primarily due to monozygotic twins that were misclassified as dizygotic based on military record blood group data. Removing the military record blood group data markedly improved the accuracy of the original classification. Zygosity assignment based on a zygosity questionnaire was highly predictive of DNA-based zygosity. Augmentation of such a zygosity classification from administrative data, military records, or other records, should be done with caution.

The main aim of this study was to develop and optimize a questionnaire-based zygosity determination method in Chinese adolescent twins. Participants were 471 pairs of same-sex twins (345 monozygotic, 126 dizygotic) with a mean age of 14.56 years (SD = 2.62). A second sample was recruited for cross-validation, including 382 pairs of same-sex twins (261 monozygotic, 121 dizygotic) with a mean age of 12.53 years (SD = 2.22). The questionnaire consisted of 12 questions dealing with co-twin similarity or frequency of confusion. Two means were put forward to improve the predictive accuracy of the questionnaire — adding parent-reports to the analysis, and using a 2-point rather than 3-point response format. DNA genotyping was performed on nine short tandem repeat loci, with an estimated zygosity classification accuracy very close to 100%. The validity of all questionnaires was assessed by being compared to the results of DNA analysis. Results of stepwise logistic regression analysis showed that the predictive accuracy of the 3-point self-reported questionnaire is 83.8%. Using parent-reports and 2-point scale led to 3.9% and 4.6% increase in predictive accuracy, respectively. When using the parent-reports and children's self-reports jointly, the predictive accuracy was enhanced to 90.6%. For the cross-validation, the equations and cut-offs derived from the first sample led to an acceptable accuracy (91.3%) in the second sample. In conclusion, the method we developed can be used in future studies among Chinese adolescent twins. Multiple-rater and 2-point response format were suggested for all twin studies for enhancing the predictive accuracy of questionnaires.

Miscarriage is the most common type of pregnancy loss, occurring in up to 15% of clinically recognized pregnancies. Our understanding of the etiology is still limited but is believed to be multifactorial, including endocrine and anatomical abnormalities, immunologic, genetic and lifestyle factors. The aim of this study was to explore whether genetic variability in miscarriage is under any genetic influence. 3234 MZ and DZ female twins completed postal self-completion questionnaires on pregnancies. Rates were adjusted for total number of pregnancies. The relative contribution of genetic and environmental factors to variation in miscarriage was assessed using twin intra-pair correlations and quantified using a variance components model fitting approach. We found 22.7% of our twins reporting having suffered at least one miscarriage. Current age, age at first pregnancy and higher number of pregnancies all had a significant influence on reported miscarriage. The concordance of miscarriage was similar in identical and non-identical twins, 26% and 27%, respectively. Shared environment and predominantly random error and unique environment rather than genetic factors best explained the total variation of miscarriage. To our knowledge, this is the first large twin study exploring heritability of miscarriage which unlike the vast majority of common variable traits, shows no significant genetic influence. In the absence of clear environmental factors, these results suggest the influence of random factors.

A population-based record linkage case cohort of 239,995 births, to 119,214 women, born in Western Australia from 1980 to 2001 inclusive, was used to measure the recording of selected indicators of maternal health (current and prior) during pregnancy. We compared records of women with singleton pregnancies with that in twin pregnancies Mothers of first- and second-born singletons (n = 117,647) were compared with women with a first-born singleton followed by twins (n = 1,567). Binary indicators were used to calculate population prevalence of medical conditions, pregnancy complications and birth outcomes. Infant outcomes included stillbirth, low birthweight, preterm birth and birth defects. Women with twins were significantly older and taller, with similar rates of medical conditions and pregnancy complications during first singleton pregnancies compared with women with two consecutive singletons. However, during their second pregnancy, women with twins had significantly higher rates of essential hypertension, pre-eclampsia, threatened abortion, premature rupture of the membranes and ante partum hemorrhage with abruption than women with singletons. For both groups, maternal conditions in the first pregnancy were underreported in the second pregnancy, including diabetes, epilepsy, asthma, chronic renal dysfunction and essential hypertension. At the second birth, twins were 3 times more likely to be stillborn, 17 times more likely to be low birthweight and 4 times more likely to be delivered preterm compared with singletons. This research demonstrates the importance for epidemiologists and others, of having access to a complete maternal medical history for analyses of risks associated with maternal, infant and childhood morbidity.

We assessed the heritability of head circumference, an approximation of brain size, in twin-sib families of different ages. Data from the youngest participants were collected a few weeks after birth and from the oldest participants around age 50 years. In nearly all age groups the largest part of the variation in head circumference was explained by genetic differences. Heritability estimates were 90% in young infants (4 to 5 months), 85–88% in early childhood, 83–87% in adolescence, 75% in young and mid adulthood. In infants younger than 3 months, heritability was very low or absent. Quantitative sex differences in heritability were observed in 15- and 18-year-olds, but there was no evidence for qualitative sex differences, that is, the same genes were expressed in both males and females. Longitudinal analysis of the data between 5, 7, and 18 years of age showed high genetic stability (.78 > RG > .98). These results indicate that head circumference is a highly heritable biometric trait and a valid target for future GWA studies.

Twin pregnancies are associated with disturbed fetal growth and a higher risk of low birthweight (LBW), which is one of the most important determinants of perinatal morbidity and mortality in Africa. In this study, we compare anthropometric measurements in Sudanese twins and their mothers with singletons. Methods: In 1000 Sudanese mothers with singleton births and 30 mothers with twins, maternal (weight, height, mid-arm circumference) and 11 newborn anthropometric measurements were taken within 24 hours of delivery. Maternal education and socio-economic status were additionally recorded. Results: Mothers of twins had a significantly higher body weight (p = .045) and lean body mass (p = .02) after delivery, and were from higher social classes in general (p = .014). In addition to gestational age, twins displayed a statistically significant reduction in all anthropometric data, compared to singletons, mainly in terms of birth-weight, chest and head circumference, whereas differences in triceps and subscapular skin fold thickness and ponderal index were distinctly lower. The LBW rate in twins was markedly higher than that in singletons (43.3% vs. 8.3%, p < .001). In 20 out of 30 twins (66.7%), Twin A weighed more than Twin B (difference (SD) of 443 (335) g), and in the remaining 10 cases (33.7%), the weight of Twin B was equal to or more than that of twin A (difference (SD) of 211 (240) g, p = .039). In unlike-sex pairs, the mean (SD) difference between Twins A and B in birthweight was 459 (481) g, which was distinctly higher, compared to same-sex pairs (boys, 180 (325) g and girls, 36 (413) g). Conclusions: Sudanese twins displayed significantly reduced anthropometric measurements compared to singletons, but to different degrees. Gender had a higher impact on birthweight in twins than in singletons.

Health is a multidimensional and continual concept. Traditional latent analytic approaches have inherent deficits in capturing the complex nature of the concept; however, the Grade of Membership (GoM) model is well suited for this problem. We applied the GoM method to a set of 31 indicators to construct ideal profiles of health status based on physical, mental and social support items among 848 adult twins from Qingdao, China. Four profiles were identified: healthy individuals (pure type I), individuals with personality disorders (pure type II), individuals with mental impairments (pure type III) and individuals with physical impairments (pure type IV). The most frequently occurring combination in this population was profiles I, II, IV (14.74%), followed by profiles I, II, III, IV (13.44%), and then type I (11.08%). Only 13.56% of subjects fell completely into one single pure type, most individuals exhibited some of the characteristics of two or more pure types. Our results indicated that, compared to conventional statistical methods, the GoM model was more suited to capture the complex concept of health, reflecting its multidimensionality and continuity, while also exhibiting preferable reliability. This study also made an important contribution to research on GoM application in non-independent samples.

The purpose of this study was to examine changes in the contribution of genetic and environmental influences to leisure time physical activity among male and female twins over a 6-year follow-up. At baseline the sample comprised 4,280 monozygotic and 9,276 dizygotic twin individuals, and at follow-up 4,383 monozygotic and 9,439 dizygotic twin individuals. Participants were aged 18–54 years at baseline. Genetic modeling results showed that genetic influences on leisure time physical activity declined from baseline (44%) to follow-up (34%). Most of the genetic influences identified at baseline were present at followup (rg = 0.72). Specific environmental influences increased from baseline (56%) to follow-up (66%) while at follow-up new environmental time-specific influences were observed (re = 0.23). The model with sex differences showed a higher estimate of genetic influences for men than women both at baseline (men 47% vs. women 42%) and at follow-up (men 38% vs. women 31%). The additive genetic correlation for this phenotype was greater for men (rg = 0.79) than women (rg = 0.64). The specific environmental influences were corresponding; at baseline men 53% and women 56% and at follow-up men 62 % and women 69%. The environmental correlations between the two time points were similar for men (re= 0.21) and for women (re= 0.24). In conclusion, in a sample of healthy twins most of the genetic influences on leisure time physical activity expressed at baseline were present at 6 years of follow-up. New specific environmental factors underlying follow-up leisure time physical activity were observed.

Fellman and Eriksson (2010) cited my suggestion that the sex ratio (proportion male) of monozygotic (MZ) twins is lower than that of dizygotic (DZ) twins (James 1975). Here I offer elaborations on and potential explanations for this.

The aim of this study was to present the outcome of bichorionic twin pregnancies complicated by early second trimester rupture of membranes in one sac. Data regarding all cases of ruptured membranes at 13–20 weeks in bichorionic twin pregnancies were collected retrospectively from three fetal medicine units. Patients who have chosen to terminate the pregnancy were excluded from the study. Between January 2003 and July 2009, nine patients met inclusion criteria. Three out of nine couples decided on expectant management, and six preferred selective feticide. With expectant management one fetus died in utero and take home baby rate was 83% (5 of 6 fetuses), delivered at 27–32 weeks. When selective termination was performed, all non-reduced fetuses were born alive at 33–40 weeks. Two survivors of rupture of membranes had limb contractures, none had lung hypoplasia. One patient had clinical signs of amnionitis, which was ruled out later on pathological examination. Her post partum course was uncomplicated. Our data suggest that rupture of membranes in one sac of bichorionic twins at 13–20 weeks has favorable prognosis whether an intervention is preformed or not. Nonetheless, selective termination may have an advantage over expectant management, since gestational age at delivery was higher when selective termination was performed.

Temporal variations in the frequency of multiple maternities in many Western European countries have been described. However, within a single country, regional differences are observed. Urban industrialized regions and rural agricultural areas have experienced in recent decades a distinct decline in multiple deliveries, which in cases have been related to maternal age and parity changes. Research on multiple deliveries in Spain is scarce and none of the studies go back to the beginning of the 20th century or consider regional variation over an extended period of time. The present paper is a yearly study on multiple deliveries in Spain since 1900 including a geographical analysis. Rather than dealing with recent changes in multi-parity, this paper is concerned with Spain's long-term national variation (between 1900 and 2006). The changing pattern of double and triple deliveries was analyzed using data from the Spanish National Statistics Institute (INE). Twinning rates in Spain are low in comparison to those of equivalent periods in other countries, and the minimum rates correspond to the 1980s decade. Results were interpreted by taking into account the influence of age at maternity and reproductive variation up to 1990. A good fit between observed and predicted rates was obtained after the application of models, which besides maternal age and parity, include their interaction. Regarding territorial variability, the values corresponding to southern, northern and insular Spanish provinces are consistent with an earlier reduction of the crude birth rate in the north-east regions and latter in the southern regions and the Canary Islands.

This study reports on the inter- and intrarater reliability of physical features observation. Study subjects were 176 Chinese adult persons, consisting of 89 males and 87 females. Three trained research assistants responded simultaneously and respectively to 12 items regarding the subject's physical features including ‘hair’, ‘Mongoloid folds’, left and right ‘ear lobes’, ‘earwax’, ‘nostril shape’, ‘tongue rolling’, left and right ‘hitchhiker's thumb’, ‘mid-digital hair’ and left and right ‘simian crease’ at the moment of interview. And 14 days later, these subjects received the same observation once again. The results showed that the inter- and intra-observer agreements of ‘hair’, ‘earwax’, ‘tongue rolling’, ‘mid-digital hair’ and ‘simian crease’ were almost perfect with most kappa (κ) coefficients ≥ .80, while ‘Mongoloid fold’ and ‘nostril shape’ showed poor inter-observer agreement and ‘nostril shape’ showed poor intra-observer agreement (κ < .40). Two other physical features, ‘hitchhiker's thumb’ and ‘ear lobes’ showed moderate inter-observer agreement and three features, ‘hitchhiker's thumb’, ‘ear lobes’ and ‘Mongoloid fold’, showed moderate intra-observer agreement (.40≤ κ <.80). In conclusion, this study suggests that as far as reliability is concerned, the five features which were ‘hair’, ‘earwax’, ‘tongue rolling’, ‘mid-digital hair’ and ‘simian crease’ could be considered in zygosity determination of Chinese adult twins, while the two features, ‘Mongoloid fold’ and ‘nostril shape’, should be abandoned.

It has been assumed that sexual activity during pregnancy would lead to an increased risk for miscarriage and other complications of pregnancy. Various studies showed no association of sexual behavior and pregnancy complications in single pregnancies. The aim of our study was to evaluate changes in sexual activity in women with twin pregnancies and whether a higher frequency of sexual intercourse was associated with an increased risk for pregnancy complications. We report on 50 women with twin pregnancies who answered a questionnaire on sexual behavior during pregnancy. At the time of delivery, patients were aged 31.0 ± 4.8 years. All women reported to have had sexual intercourse during pregnancy. In contrast to the first trimester in the vast majority of patients (41/50, 82.0%) the coital frequency decreased in the last month of pregnancy. Patients who had become pregnant after in-vitro fertilization were significantly less likely to have regular sexual intercourse than patients after spontaneous conception during early pregnancy (p = .002). No statistical significances were found when the rates of preterm delivery before the 37th gestational week were compared to the frequencies of sexual intercourse during early pregnancy and during late pregnancy (p >.05). In conclusion, our study demonstrates a decrease in frequency of sexual intercourse from early to late pregnancy in the specific collective of women with twin pregnancies, especially in women after in-vitro fertilization. There was no association between sexual activity and preterm delivery.

The genetic and environmental contributions to determine digital dermatoglyphic traits were investigated by using female dizygotic and monozygotic twin pairs to estimate heritability indexes (h2). The evaluated sample was composed by 20 monozygotic twin pairs and 13 dizygotic twin pairs. A significant heritability (h2 = 0.65 to 0.96) was observed for 12 dermatoglyphic characteristics (delta indexes and ridge counts for right hand, left hand and both hands, and ridge counts for most individual fingers). A negative correlation between the ridge counts and heritability indexes from individual fingers was found for the left hand, which appears to be associated to a higher arch pattern frequency in most left-hand fingers, since this frequency was negatively correlated with ridge counts and positively correlated with heritability indexes. Heritability indexes of right-hand fingers were positively correlated with loop pattern frequency and negatively correlated with whorl pattern frequency. The low heritability of ridge counts from left thumb, ring and little fingers (h2 = 0.11 to 0.32) indicates a higher chance that the chorion type had an influence in the intra-pair variance of monozygotic twins. Results confirmed the predominant genetic influence on the total ridge count. The heritability indexes varied in up to 8 times between different fingers and its association to ridge counts and pattern frequency was very variable between hands, evidencing that the use of dermatoglyphic traits from individual fingers as indicators of genetic influences to other human traits should consider this variability.

The health benefits of breastfeeding are well known. However, less is known about the frequency and duration of breastfeeding in twins. A population-based study of maternal factors associated with the early cessation of breastfeeding in preterm and term twins is reviewed. This is followed by research reviews and news concerning aging in twins, conjoined twin separation and school legislation for multiple birth children. Twins and quadruplets who have distinguished themselves in the areas of education, fashion design and humanitarian efforts are highlighted.

Attempts have been made to identify factors influencing the sex ratio at birth (number of males per 100 females). Statistical analyses have shown that comparisons between sex ratios demand large data sets. The secondary sex ratio has been believed to vary inversely with the frequency of prenatal losses. This hypothesis suggests that the ratio is highest among singletons, medium among twins and lowest among triplets. Birth data in Sweden for the period 1869–2004 showed that among live births the secondary sex ratio was on average 105.9 among singletons, 103.2 among twins and 99.1 among triplets. The secondary sex ratio among stillbirths for both singletons and twins started at a high level, around 130, in the 1860s, but approached live birth values in the 1990s. This trend is associated with the decrease and convergence of stillbirth rates among males and females. For detailed studies, we considered data for Sweden in 1869–1878 and in 1901–1967. Marital status or place of residence (urban or rural) had no marked influence on the secondary sex ratio among twins. For triplets, the sex ratio showed large random fluctuations and was on average low. During the period 1901–1967, 20 quadruplet, two quintuplet and one sextuplet set were registered. The sex ratio was low, around 92.0.

Background: Although the clinical importance of chorionicity in twin pregnancies has been studied widely, the significance of perinatal determination of zygosity using molecular genetic analyses remains controversial. The purpose of this study was to determine the frequency and clinical significance of twin gestations according to zygosity and chorionicity in a Korean population. Methods: We enrolled 569 women who delivered twin newborns (> 24 weeks) at Seoul National University Hospital between 1999 and 2008. Chorionicity was established by histologic examination of placentae. Zygosity was determined with sex of neonates, chorionicity, and DNA analysis of umbilical cord blood. Results: The frequency of dizygotic (DZ) twins was 71.0% (404/569 pairs) based on the opposite sex (238/404 [58.9%]) and DNA analyses (166/404 [41.1%]); that of monozygotic (MZ) twins was 29.0% (165/569), including monochorionic (MC) (72.1% [119/165]) and dichorionic (DC) twins (27.9% [46/165]), which was confirmed by DNA analyses. Among spontaneously conceived twins, the frequency of MZ twins was more than twice that of DZ twins. The risk of low birth weight was 1.8-fold higher among MZDC twins and 1.9-fold higher among MZMC twins than among DZDC twins (p < .05). Bronchopulmonary dysplasia occurred more frequently among MZMC twins than among DZDC twins (adjusted OR 8.42, 95% CI 1.82–39.08, p < .01). However, the frequencies of other neonatal morbidities were not significantly higher in the MZMC group than in the MZDC and DZDC groups. The perinatal mortality rate was 15 per 1000 total births in the DZDC twins, 20 per 1000 total births in the MZDC and 56 per 1000 total births in the MZMC (p < .01). Conclusions: Although monozygosity was shown to be a risk factor for perinatal death and accurate determination of zygosity plays a great role in the future consideration of organ transplantation and twin studies, the value of zygosity determination along with chorionicity in relation to overall neonatal morbidity was not definite.