It has been hypothesized that cerebral palsy of unknown etiology is the result of the death of an unrecognized co-twin — a vanishing twin — in early gestation. We conducted a casecontrol study of vanishing twin as a risk factor for cerebral palsy of unknown etiology in women who had an obstetric ultrasound during pregnancy. Among mothers of cases, one of 86 had evidence of a vanishing twin on ultrasound, as compared to two of 381 control mothers (odds ratio [OR] 2.2, 95% confidence interval [CI] 0.2–24.8; p = 0.5). Bleeding in early pregnancy, which may indicate the loss of a co-twin, was reported by 14 case mothers and 46 control mothers (OR 1.6, 95% CI 0.8–3.0; p = 0.3). On the basis of results presented here, the vanishing twin syndrome is unlikely to account for a high proportion of cases of cerebral palsy, but there is insufficient statistical power to draw firm conclusions.

Fetal growth is influenced by various determinants, with males being heavier than females. It was recently suggested that female birthweight tends to be increased by the presence of a male co-twin. We evaluated this hypothesis in the Israeli population. We reviewed a retrospective population database of the 1993–1998 Israeli Birth Registry. We compared male birthweight in the male-male and malefemale combination sets and female birthweight in the femalefemale and male-female combination sets. We compared the mean birthweight of males and females in three groups of total twin birthweight. The sample of 12,686 sets included 50.6% males. The male-male combination was found in 31.3% sets, female-female in 38.7% and unlike-sex combination in 30.0%. A total twin birthweight of less than 3000g was found in 7.2% sets, of 3001–5000g in 50.7% sets, and of over 5000g in 42.1% sets. The mean male birthweight was 2398 ± 602 g and the mean female birthweight was 2316 ± 566g, p < .0001 (95% CI of 67, 96). The mean birthweight of males in the malemale combination was significantly lower than in the unlike-sex combination, p < .0001, in total birthweights over 3000g. No significant difference was found between the mean birthweight of females in the female-female and in the unlike-sex combinations in all total birthweight strata. Our findings do not substantiate a male-related inter-twin influence on female birthweight. Population differences might underlie the opposing observations in the literature.

The aim of this study was to calculate the risk for aneuploidy in twin pregnancies between 9–14 weeks utilizing maternal age, race and dizygotic twinning rates. Using previously published risks for aneuploidy in singletons and twins at the time of amniocentesis and at term, we calculated new risk estimates for twins at 9–14 weeks gestation or at the time of chorionic villus sampling. Using these tables, the risk for trisomy 21 in at least one fetus of a twin gestation in a 32-year-old at 9–14 weeks is 1/285 for Whites and for African- Americans. This is equivalent to the risk for trisomy 21 (1/265) in a 35-year-old woman with a singleton at the same gestational age. The risks for trisomies 18 and 13 also follow similar trends. In counseling women with twin pregnancies at the time of first trimester nuchal translucency screening or chorionic villus sampling, it should be noted that the maternal age-related risk for aneuploidy for a 32-year-old is equivalent to that of a 35-year-old woman with a singleton gestation.

Perinatal mortality rates (PMRs) in triplets were analyzed using Japanese Vital Statistics during the period of 1980–1998. The total number of perinatal deaths in triplets was 1051. The PMR significantly decreased from 214 per 1000 births in 1980 to 39 in 1998, a reduction of 82%. PMRs in triplets were 11.1-fold higher in 1980 and 6.9-fold higher in 1998 than in singletons, indicating that PMRs improved more in triplets than in singletons during the last two decades in Japan. The PMR was the highest in the third-born, followed by the second- and the first-born triplets in each period. As for maternal age, the PMR was 1.5–3.7 times higher in the < 25 years of age group than the other age groups. Additionally, the PMR was the lowest for birthweight (BW) ≥ 2000g during the entire period. In addition, the PMR decreased with gestational age (GA) of up to 38–39 weeks and increased thereafter. The effects of BW on the PMR were stronger than the effects of GA. The proportion of perinatal deaths in triplets with extremely low BW (< 1000g) was 74% in 1980–1989 and increased to 82% in 1990–1998. The declining PMR was unlikely to be due to the improvement in BW in triplets. It is likely that it was related to the improved medical management of triplets during the perinatal period and the first week of life. Information obtained in the present study may be useful in counseling pregnant woman about triplet births.

The degree of assortative mating for psychological and physical traits in Asian societies is relatively unknown. The present study examined assortative mating for educational level, personality traits, religious affiliation, height, weight, and body mass index in a Korean sample. Age-adjusted spouse correlations were high for educational level (r = .63) and religious affiliation (r = .67), modest for most personality traits (rs = −.01 to .26), and trivial for height (r = .04), weight (r = .05), and body mass index (r = .11). These results were remarkably similar to those found from the western samples. Implications of the present findings in behavior genetic studies and human mating patterns were briefly discussed.

The objective of this study was to evaluate associations between adverse outcomes in twin pregnancies and preterm prelabour rupture of membranes (PPROM). A chart review of 246 consecutive twin pregnancies with confirmed PPROM was conducted. Regression analysis (β [natural log of the odds ratio] and odds ratio [OR]) was performed to identify independent predictors. Two hundred and forty-six twin pregnancies, 492 liveborns, and 20 neonatal deaths. Mean (SD) PPROM gestational age (GA): 31.3 (3.8) wk; delivery GA: 32.0 (3.3) wk. PPROM < 30wk was associated with increased parity (OR: 2.66), and log (admission leukocyte count) (OR: 9.99). Shortened latency was associated with PPROM GA (β = −0.17) and chorioamnionitis (β = 0.95). Neonatal sepsis was predicted by lower delivery GA (OR: 2.04). Adverse perinatal outcomes were protected against by older GA at PPROM (OR 0.53) and shortened latency (OR 0.73). It was concluded that increased leukocytosis and parity implies an infectious aetiology in earlier PPROM. Increased risk for neonatal sepsis at earlier delivery GA is consistent with gestation-dependent fetal immunocompetence. Early PPROM and long latencies were associated with increased adverse perinatal outcomes.

This population based study compares the rates of multiple births in the 1990s in four hospitals of different socioeconomic levels. It is well known that women from higher socioeconomic groups have easiest access to infertility therapies because of greater financial resources. The hospital of lower socioeconomic level presented multiple birth rates of approximately 8 per thousand during the decade, which may be considered as the natural one. The other three hospitals presented increased rates that were positively correlated to socioeconomic level. This increase occurred mainly due to dizygotic twins and to triplets and was as high as 4.8 per thousand in 1999. Maternal age was also positively correlated to socioeconomic level for singletons as well as for twins. However, during the decade the mean maternal age increased only in the two hospitals with better socioeconomic levels. Gestational order decreased as socioeconomic levels increased, mainly for twins and triplets. The percentage of singletons with low birthweight and very low birthweight decreased as socioeconomic level increased. However, twins presented with an equal distribution in the four hospitals, indicating that better socioeconomic level did not affect the incidence of low birthweight and very low birthweight among twins. Fetal death rate decreased as socioeconomic level increased but twin/singleton fetal death ratio is three times greater in the hospital of higher socioeconomic level suggesting that even in ideal conditions of medical and hospital facilities, the mortality of twins continues to be much higher than that of singletons.

The objective of this study was to determine if mothers' retrospective reports about events in their pregnancies with twins are reliable and stable. Six hundred and twenty-four mothers completed psychiatric interviews about their twins. These interviews also contained questions about the mothers' pregnancies, the perinatal period, and the child's early development. The mothers reported first on one twin and then on the other with interviews spaced from 3 days to 2 weeks apart. Thus mothers reported on the same pregnancy twice. Of these mothers, 47 were re-interviewed 6 to 18 months later by raters blind to the results of the initial interview. The twin design allowed us to compare the short-term reliability of the 624 mothers' reports of the same pregnancy. The re-interview of the 47 mothers enabled us to compare the stability of reports over a longer time period. Agreement between the reports was measured with the kappa statistic. Kappas were good to excellent for the short-term reports of pregnancy for each twin for the 624 mothers. Kappas were equally high for the 47 mothers that were re-interviewed 6 to 18 months later. Mothers show good reliability and stability of reporting about events during pregnancy.

The Eating Inventory (EI) is commonly used to measure a range of eating behaviors. It includes three subscales: Cognitive Restraint, Hunger, and Disinhibition. In this study, we decomposed the variance of the three subscales, and evaluated the genetic, common environment and specific environmental effects on each in a sample of female-female twin pairs. Multivariate models were also used to examine whether the EI represented three individual factors, or whether there was extensive covariance among subscales. Heritabilities were estimated at 45% (CI of 32–57%) for Disinhibition, 8% (CI of 0–38%) for Hunger, and 0% (CI of 0–30%) for Restraint. Common environmental influences were estimated at 0% (CI of 0–23%) for Disinhibition, 16% (CI of 0–34%) for Hunger, and 31% (4–42%) for Restraint. Specific environmental influences accounted for the rest of the variance of the subscales. However, multivariate modeling indicated that Disinhibition and Hunger covaried significantly, indicating that these two subscales are influenced by the same set of genetic factors. Furthermore, Restraint appeared to be empirically distinct from Hunger or Disinhibition.

In this issue of Twin Research, we describe different facets of a European Community funded effort, GenomEUtwin, which capitalises on eight of the world's largest and best characterised twin registers and a multi-national population cohort, MORGAM. This international study, reaching beyond the geographical borders of Europe, is based on linkage and association strategies designed to identify genetic contributors to health and disease using integrated expertise of participating groups in genetics, epidemiology and biostatistics. By merging information from numerous epidemiological and genetic databases, GenomEUtwin will create an intellectual and technical infrastructure for future genetic epidemiological studies aiming to define genetic and life style risks for common human diseases.

To compare perinatal outcome of singleton versus twin pregnancies a matched cohort study was performed in Flanders, Belgium. All twins delivered in the region of Flanders during 1998–1999 were compared to singletons, matched for gestational age, fetal sex and maternal parity, resulting in 4384 infants in each group. Above 32 weeks of gestation, birthweight was significantly lower in twins (2095 ± 364 g versus 2315 ± 523 g; p < 0.001, 95% confidence interval 193 to 246 g). Perinatal mortality was also significantly lower in twins (1.98% versus 1.26%; odds ratio for twins 0.63; 95% confidence interval 0.53–0.75; p < 0.001 ), this was mostly due to fetal and not to early neonatal mortality. Congenital malformations occurred less frequently in twins (2.5% versus 3.7%; odds ratio for twins 0.80, 95% confidence interval 0.69–0.92; p = 0.001). From gestational age of 32 weeks on, respiratory distress syndrome was less frequent in twins (6.7% versus 8.0%; odds ratio for twins 0.81; 95% confidence interval 0.68–0.97; p = 0.011 ). No significant differences were noted with regard to intraventricular haemorrhage, neonatal infections and retinopathy of prematurity. Although twins have a lower birthweight, their outcome is more favorable compared to singletons, when matched for gestational age.

Predictive relations between maternal variables and outcome status for 199 pairs of newborn twins were evaluated. The maternal independent variables included pregravid weight, weight at end of pregnancy, weight gain, age, parity, height, socioeconomic status, years of education, and body mass index. The twin dependent variables included birthweight, birth length, 5-min Apgar score, number of days in an isolette, number of days in the hospital, weight at discharge, and chronological age at discharge. For stepwise multiple linear regression analyses, the newborn twins were grouped by term (fullterm/preterm) and sex (female/male). A combination of a larger number of independent variables acted as statistically significant predictors of twin outcome for the preterm twins than for the fullterm twins, and for the male twins than for the female twins. This was noted particularly for the anthropometric variables and for parity. Maternal weight gain, typically considered to be the critical variable related to infant birthweight, consistently had a significant outcome role for the preterm twins and male twins, but not for the fullterm twins or female twins. Demographic variables had a low rate of prediction for newborn outcome. This study has demonstrated that evaluating twins within groups separated by potential risk variables allows for a more accurate description of the influence of maternal variables on twin outcome than evaluating twins across groups as a single population.

When twinning rates are studied, maternal age and parity should be considered. Data on parity are seldom available. We studied information about the mean parity, using the gross reproduction rate, the total fertility rate and the crude birth rate. These are strongly correlated with the mean parity. The crude birth rate is more readily available than the gross reproduction rate or the total fertility rate. Earlier studies have shown that it is difficult to model variations in the twinning rate with data for the macrolevel. In this study these findings are explained by theoretical analyses and illustrated by empirical data. Sweden, having the oldest continuous population statistics and high twinning rates, offers excellent possibilities for analyses of the twinning rate. We considered data for the counties of Gotland, Älvsborg and Stockholm and the city of Stockholm from 1749 to 1960 and for Sweden until 1996. For Älvsborg, the twinning rate was low for the whole period, showing no statistically significant decreasing trend. It is mainly about 11–14 per thousand, which is only 50–60% of the twinning rate in Gotland in the 18th century. In Gotland, in the county of Stockholm, in the city of Stockholm and in Sweden as a whole, the decreasing trends in the twinning rate were statistically significant. The decreasing twinning rates converge towards the low twinning rate of Älvsborg. After standardization of the twinning rate, the differences remained and the low rate in Älvsborg could not be explained by maternal age.

The natural history of 11 cases of twin–twin transfusion syndrome (TTTS) in monochorionic diamniotic (MCDA) twin pregnancies has been reviewed. Seven cases before 28 weeks and four pregnancies after 28 weeks had been followed up without intervention. Eight cases had premature uterine contractions. All seven pregnancies before 28 weeks aborted, leading to a 100% mortality rate. After 28 weeks all mothers delivered live births. The diagnosis of TTTS before 28 weeks, and with premature uterine contraction, seems to be a poor prognostic sign.

The purpose of this study was to clarify the characteristic child-rearing problems for mothers of multiple children who conceived after infertility treatment as compared to mothers of multiple children who conceived spontaneously. The subjects were 990 mothers of multiple children: 359 who conceived after infertility treatment and 631 who conceived spontaneously. Mothers who conceived after infertility treatment were more delighted when informed of a multiple pregnancy than those who conceived spontaneously. In addition, with respect to anxiety during twin pregnancies, mothers of twins who conceived after infertility treatment showed lower rates of anxiety about nursing the infants and economic concerns than those of twins who conceived spontaneously. However, after delivery, mothers who conceived after infertility treatment showed a higher rate of depressive symptoms than those who conceived spontaneously. After adjusting for each associated factor using logistic regression, the risk of depressive symptoms in mothers who conceived after infertility treatment was significantly associated with disabled multiple children and the methods for alleviating stress. The odds ratio indicated that mothers with at least one disabled child were twice as likely to have depressive symptoms as mothers with no disabled children. Furthermore, the odds ratio indicated that mothers who used no methods for alleviating stress were twice as likely to have depressive symptoms than those who did.

We compared the results of different methods for diagnosing zygosity in a sample of 237 same-sex pairs of twins assessed at 5 and 18 months of age. Despite the twins' very young age and early stage of development, physical similarity was concordant with genotyping in 91.9% of cases at 5 months and 93.8% of cases at 18 months, for a subsample of 123 and 113 pairs, respectively. This concordance rate was obtained following a case-by-case assessment of each pair's physical similarity using a shortened version of the Zygosity Questionnaire for Young Twins (Goldsmith, 1991). Taking into account the chorionicity data available from the twins' medical files, we were able to classify correctly 96% of the pairs, an accuracy rate comparable to previously reported rates obtained with older twins. Chorionicity data is especially useful since we found that monochorionic MZ twins are more difficult than dichorionic MZ twins to diagnose by physical similarity at these young ages. The relative cost-benefit of methods based on reported physical similarity and DNA analysis is discussed in light of these results.

With the rapid advances in molecular biology, the near completion of the human genome, the development of appropriate statistical genetic methods and the availability of the necessary computing power, the identification of quantitative trait loci has now become a realistic prospect for quantitative geneticists. We briefly describe the theoretical biometrical foundations underlying quantitative genetics. These theoretical underpinnings are translated into mathematical equations that allow the assessment of the contribution of observed (using DNA samples) and unobserved (using known genetic relationships) genetic variation to population variance in quantitative traits. Several statistical models for quantitative genetic analyses are described, such as models for the classical twin design, multivariate and longitudinal genetic analyses, extended twin analyses, and linkage and association analyses. For each, we show how the theoretical biometrical model can be translated into algebraic equations that may be used to generate scripts for statistical genetic software packages, such as Mx, Lisrel, SOLAR, or MERLIN. For using the former program a web-library (available from http://www.psy.vu.nl/mxbib) has been developed of freely available scripts that can be used to conduct all genetic analyses described in this paper.

This study investigates the validity of retrospective determination of chorion type by asking the question to the mother about the number of placentas. In the “East Flanders Prospective Twin Survey” (EFPTS), accurate information on the placentation and zygosity of the multiples was collected prospectively. The mothers of 231 monozygotic (95 dichorionic and 136 monochorionic) twins and 255 dizygotic twins were asked to fill in a simple questionnaire regarding 1) the zygosity and 2) the number of placentas of their twins. The accuracy of the response to the question on “the number of placentas” was 60% for monozygotic twins and 37% for dizygotic twins. The accuracy of the response to the question on the zygosity of the twins was 93% for monozygotic and 95% for dizygotic twins. If the questionnaire was used for the determination of chorion type, a total of 31 monozygotic twins (13%) should have been assigned as dichorionic on the fact that there were two separate placentas. Of these, 10 (32%) are monochorionic and 12 (39%) were falsely reported as having two placentas. We conclude from these findings that this simple questionnaire method is unreliable for the retrospective determination of the chorion type.

Arecurring criticism of the twin method for quantifying genetic and environmental components of human differences is the necessity of the so-called “equal environments assumption” (EEA) (i.e., that monozygotic and dizygotic twins experience equally correlated environments). It has been proposed to test the EEA by stratifying twin correlations by indices of the amount of shared environment. However, relevant environments may also be influenced by genetic differences. We present a model for the role of genetic factors in niche selection by twins that may account for variation in indices of the shared twin environment (e.g., contact between members of twin pairs). Simulations reveal that stratification of twin correlations by amount of contact can yield spurious evidence of large shared environmental effects in some strata and even give false indications of genotype x environment interaction. The stratification approach to testing the equal environments assumption may be misleading and the results of such tests may actually be consistent with a simpler theory of the role of genetic factors in niche selection.