We applied a noninvasive prenatal test for the determination of fetal gender in multiple pregnancies by using free fetal DNA circulating in maternal blood in order to evaluate whether the quantification of male DNA could distinguish the fetal gender and the number of male and female fetuses in multiple pregnancies. We enrolled consecutively 44 women with twin pregnancies between 11–14 weeks of gestation. Peripheral maternal blood was collected, and genomic DNA was extracted from maternal plasma and analyzed for the multicopy DYS 14 sequence by using real-time PCR to quantify male DNA. Results showed that male DNA concentration was significantly higher in twin pregnancies with at least one male fetus, compared to twin pregnancies with only female fetuses. Comparing male DNA concentration in pregnancies with two male fetuses versus pregnancies with one female fetus and one male fetus, we did not obtain a significant difference between the two groups due to a slight overlapping of the range values. Therefore, our test correctly predicted fetal gender, distinguishing twin pregnancies with at least one male fetus from twin pregnancies with only female fetuses, with a diagnostic accuracy of 100%. For distinguishing pregnancies with two male fetuses from pregnancies with both female and male fetuses, a diagnostic accuracy of 76.1% was achieved.

Birth weight in triplets is, on average, lower than in singletons and twins, and more children are classified as having very low or extremely low birth weight. Still, there is limited research on factors that affect triplet birth weight, and samples under study are often small. Chorionicity and zygosity influence triplet birth weight, but it is unknown whether the effect of zygosity can be entirely ascribed to the effect of chorionicity or whether zygosity has an additional effect on triplet birth weight. This question was investigated in 346 triplets (from 116 trios) registered with the Netherlands Twin Register for whom data on chorionicity were available. ‘Triplet’ refers to one child and the set of three triplets is referred to as ‘trio’. Trios and triplets were classified based on zygosity and chorionicity. With regression analysis, the effects of zygosity and chorionicity on triplet birth weight were examined, while controlling for gestational age, sex, and maternal smoking during pregnancy. In addition, within the dizygotic trios a within-family comparison was made between the birth weight of the triplets that were part of a monozygotic pair (with some pairs sharing a chorion), and the birth weight of the dizygotic triplet. Based on the classification on individual level, monozygotic, monochorionic triplets had a lower mean birth weight than dizygotic, dichorionic triplets. Most remarkably, in dizygotic trios, monozygotic pairs only had a lower mean birth weight than their dizygotic sibling triplet when the pair shared a chorion. We conclude that having shared a chorion, rather than being monozygotic, increases the risk of a low birth weight.

Comorbidity — the clustered occurrence of two traits or disorders — may be studied in genetically informative designs such as the classical twin study, to test whether genetic and/or environmental factors underlying the two disorders are correlated. When a genetic correlation is found, this can be explained by several mechanisms, including pleiotropy (the same genes influencing multiple traits), and causality (one trait causing the other). With a cotwin control design, it can be investigated which scenario is most plausible. In this design, monozygotic twin pairs discordant for the first trait (i.e., one twin is affected, the other is not) are compared in terms of their risk for the second trait: under a causal model, only the twins affected for the first trait will be at increased risk for the second trait. Under genetic pleiotropy, this risk will be increased in both twins because they share the same risk genes. We first discuss the cotwin control design and then illustrate its application with data on migraine and neuroticism that were collected in 5,200 Dutch twins, including 1,648 complete twin pairs (981 monozygotic and 667 dizygotic pairs). There was a significant association between migraine and neuroticism, which could be attributed to genetic and environmental correlations (rG = .27 and rE = .19). In monozygotic and dizygotic twin pairs discordant for neuroticism, the risk of migraine was significantly higher in the twins with a high neuroticism score. This pattern of results is consistent with a causal relationship, suggesting that neuroticism increases the risk of migraine.

The prevalence of the feeling of cold hands and feet (FCHF) is high in the general population but the etiology of FCHF is largely unknown. The aim of the present study was to explore whether the FCHF is heritable. Eight hundred and ninety-four pairs of twins completed a question about FCHF. Tetrachoric correlations for FCHF were .58, .29, .67, .52, and .04 for monozygotic male, dizygotic male, monozygotic female, and dizygotic female twins, respectively. Model-fitting analyses suggested that in the best fitting model, additive genetic and nonshared environmental variance including measurement error were 64% (95% CI: 55%-72%) and 36% (28%-45%), respectively. Sex differences in genetic and environmental influences were not significant.

Monozygotic cotwins are ideal organ donors for one another due to their genetic identity. The present report treats this topic differently, from the perspective of a monozygotic twin who donated her deceased twin sister's organs following her sister's untimely death. This discussion is followed by reviews of recent research concerning increased twinning rates in the United States, availability of informative kinship pedigrees, and twin discordance for physical activities. Noteworthy news items include ethical issues surrounding conjoined twin separation, a genomic discovery for diseased twins, China's One-Child Policy, a striking New Yorker Magazine cartoon, and twin delivery complications.