Perinatal mortality in term and preterm twin and singleton births.

Although, in general, twins have higher perinatal mortality rates than singletons, preterm twins have lower perinatal mortality rates than singletons of the same birth weight or gestational age. This study investigated the hypotheses that this paradoxical twin advantage: 1) is due to gestational age distribution differences between the singleton and twin populations, and 2) is due to increased likelihood of birth having occurred in a tertiary perinatal center. A pre-existing, time-limited data set of all births in the province of Ontario in odd years between 1979 and 1985 was chosen for this study because of the large sample size (n = 618,579). Multivariable logistic regression of the relationship between perinatal mortality and twin status was controlled for mother's age, hospital level and gestational age. Findings confirm the lower mortality of preterm twins. After controlling for level of hospital of birth this difference remained, suggesting that level of hospital of birth was not a major factor responsible for the twin advantage. Analyses in which gestational age was standardized indicate that, for those whose gestational age was less than 2 SD below the mean for their particular group (twin or singleton), twins were actually at higher risk than singletons. These results support hypothesis 1 and do not strongly support hypothesis 2. The results also support earlier authors' suggestions that the definition of term birth should be different for twins and singletons     

Serum levels of macrophage colony-stimulating factor in singleton and twin pregnancy.

Macrophage colony-stimulating factor (M-CSF) is present in the placenta and has been implicated in placental growth and development. M-CSF levels in peripheral blood increase significantly with progression of pregnancy in uncomplicated singleton pregnancy. This study investigated whether serum M-CSF levels in singleton pregnancy differed from those in twin pregnancy. A hundred and four pregnant women, of whom fifty-nine were women carrying single fetuses and forty-five were women with twin fetuses, participated in the study. Their average gestational age at entry was 10, 20, 30, and 38 weeks of gestation in singleton pregnancy and 10, 20, 30, and 35 weeks in twin pregnancy. Peripheral blood was collected and serum was separated after centrifugation and stored at - 20 degrees C. M-CSF levels were determined by ELISA. In both groups, the serum levels of M-CSF increased significantly as pregnancy progressed. M-CSF levels were relatively higher in twin pregnancy than in singleton pregnancy at 10, 20, and 30 weeks. At 35 and 38 weeks, the M-CSF levels were significantly higher in twin pregnancy than in singleton pregnancy. Elevation of serum M-CSF supports M-CSF production in the placenta. This elevation in twin pregnancy may be related to increased demand of M-CSF in twin pregnancy.     

A matched cohort comparison of the outcome of twin versus singleton pregnancies in Flanders, Belgium.

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.     

Premature singleton versus a twin or triplet infant death: parental adjustment studied through a personal interview.

Parental adjustment following the death of a premature singleton or multiple birth infant has hitherto been studied by mailed questionnaires or telephone survey. In the present study, using an in-depth personal interview, grief reactions and adjustment patterns of nine families who lost a singleton premature infant ('Single Group') were compared with those of nine families who lost one of a premature multiple birth cohort ('Multiple Group'). The interview was conducted 1-4 years after the death of the infant and evaluated specific areas or 'scales' of life adjustment, including individual feelings, relationship between husband and wife, and functioning at home and at work. There was no significant difference between the paternal and maternal level of adjustment of the two groups in any of the studied scales. A positive correlation was found between maternal and paternal grief reaction of the same family in the scales of individual feelings (r = 0.65), relationships between husband and wife (r = 0.70), and functioning at home (r = 0.57). Comparing the father's scale with the mother's scale revealed a significant difference only in the area of 'individual feelings'. The gestational age, maternal bonding during hospitalisation of the infant and the parental attendance at the event of death were significantly associated with the process of parental adjustment. The results of this study support previous reports of similar parental reactions following the demise of a premature singleton or multiple birth infant. Since societal environment may not recognise the need for consolation of these families, care, compassion, and sensitivity should be encouraged in dealing with these parents at the time of their infant' death, and for a long time thereafter.     

Hemodynamics of the umbilical artery and conceptus-related measurements in singleton and twin pregnancies in ewes

In sheep, adequate physiological maternal adaptations to twin fetuses are key for their prenatal development. We hypothesized that there is a relationship between maternal circulating steroid hormone concentration, hematocrit, and patterns of conceptus growth and umbilical artery hemodynamics in twin vs singleton pregnancies. Ewes carrying singletons (n = 6) or twins (n = 7) were selected. Hematocrit, progesterone (P₄) and estradiol-17β (E₂) concentrations were analyzed. Blood flow of the umbilical artery (UBF) was assessed through Doppler ultrasonography. Ewes carrying twins had a decreased (p = 0.03) hematocrit. There was a tendency (p = 0.08) for ewes carrying twins to have increased P4 from day 20 to 40. From day 50 to 70, P₄ tended to be increased (p = 0.07) and E₂ was increased (p = 0.01) in ewes carrying twins. From day 90 to 120, ewes carrying twins had greater (p ≤ 0.04) P₄. Ewes carrying twins had increased (p < 0.01) P₄ and tended to have (p = 0.06) increased E₂. From day 60 until 110, placentomes from twins were larger (p ≤ 0.05). On day 50, UBF was greater (p = 0.04; 27.38 vs. 20.95 ± 1.99 ml/min) in twins. It has been suggested that physiological adaptations during early pregnancy can significantly impact fetal development in multiple offspring carrying ewes. Our findings suggest that increased umbilical artery blood flow early in gestation, associated with increased maternal concentrations of P₄ and E₂, and decreased hematocrit (related to plasma volume) could contribute to those adaptations.     

Comparing blood pressure of twins and their singleton siblings: being a twin does not affect adult blood pressure.

The hypothesis was tested that monozygotic (MZ) and dizygotic (DZ) twins, with their lower average birth weight, have higher adult blood pressure than their singleton brothers or sisters. From the Netherlands Twin Registry, 261 twin families were recruited from a young adult and an older adult cohort with mean ages of 26.2 and 50.4 respectively. These families yielded 204 MZ twins with 71 singleton siblings and 271 DZ twins with 103 of their singleton siblings. Anti-hypertensive medication use of these 649 participants was assessed twice with a two-year interval. Resting blood pressure was measured thrice during a standardized laboratory protocol. In spite of a significant difference in birth weight (1036 gram), no differences were found in anti-hypertensive medication use at both time points between twins and singletons nor between their resting laboratory diastolic or systolic blood pressure. These results applied to each gender and to both age cohorts. Limiting the analyses to matched twin-sibling pairs of the same families and taking current weight and height into account did not change the results; no evidence was found for a twin-singleton difference. It was concluded that estimates of genetic and environmental contributions to blood pressure deriving from twin studies do not appear to be biased and may be generalized to singletons. Our results suggest that the lower birth weight in twins does not reflect the intrauterine disadvantage described by the Barker hypothesis.     

Impact of periconceptional nutrition on maternal and fetal leptin and fetal adiposity in singleton and twin pregnancies

It has been proposed that maternal nutrient restriction may alter the functional development of the adipocyte and the synthesis and secretion of the adipocyte-derived hormone, leptin, before birth. We have investigated the effects of restricted periconceptional undernutrition and/or restricted gestational nutrition on fetal plasma leptin concentrations and fetal adiposity in late gestation. There was no effect of either restricted periconceptional or gestational nutrition on maternal or fetal plasma leptin concentrations in singleton or twin pregnancies during late gestation. In ewes carrying twins, but not singletons, maternal plasma leptin concentrations in late gestation were directly related to the change in ewe weight that occurred during the 60 days before mating [maternal leptin = 0.9 (change in ewe weight) + 7.8; r = 0.6, P < 0.05]. In twin, but not singleton, pregnancies, there was also a significant relationship between maternal and fetal leptin concentrations (maternal leptin = 0.5 fetal leptin + 4.2, r = 0.63, P < 0.005). The relative mass of perirenal fat was also significantly increased in twin fetal sheep in the control-restricted group (6.0 +/- 0.5) compared with the other nutritional groups (control-control: 4.1 +/- 0.4; restricted-restricted: 4.4 +/- 0.4; restricted-control: 4.3 +/- 0.3). In conclusion, the impact of maternal undernutrition on maternal plasma leptin concentrations during late gestation is dependent on fetal number. Furthermore, we have found that there is an increased fetal adiposity in the twins of ewes that experienced restricted nutrition throughout gestation, and this may be important in the programming of postnatal adiposity.     

Twin and singleton births in Ghana--a case-control study.

A retrospective study involving 623 twin and 1246 singleton births was conducted to compare the two groups with regard to selected maternal, fetal and labor and delivery characteristics and outcomes. Maternal age and parity were significantly higher for twins. The risks of preterm delivery, arrival in the labor ward in second stage of labor, cesarean births and postpartum haemorrhage were significantly higher in twin than in singleton births. In vaginal deliveries twin mothers were significantly less likely to have had episiotomies or perineal lacerations. There was no difference in the duration of the third stage of labor or in the incidence of retained placentae. Antepartum haemorrhage was a less likely indication for cesarean delivery among twins, while there was no significant difference in the likelihood of severe pre-eclampsia/eclampsia being an indication. Singleton babies were significantly heavier than twins. The incidences of malpresentation, low birth weight, stillbirths and of admission of live births to the neonatal intensive care unit were significantly higher in twins. There was no difference in the rate of instrumental vaginal delivery, or in the route of delivery of fetuses presenting by the breech. There is the need for detailed study of the incidences of antepartum haemorrhage and hypertensive diseases in twin and singleton pregnancies and of the factors determining the mode of delivery when such complications arise. Labor and delivery should also be examined to determine any differences between the two groups, especially in the first and second stages.     

From linked marker to gene.

An area of increasing importance in human genetics research is the strategy of 'reverse genetics' or 'positional cloning', whereby a gene associated with a genetic disease is isolated on the basis of its approximate chromosomal position. This article and the accompanying centrepage diagram describe the main features of this strategy and outline the techniques involved; they are designed to be used for teaching purposes and as a general reference for those from other fields of genetics research.     

From RNA helicases to RNPases.

In eukaryotic cells, all aspects of cellular RNA metabolism require putative RNA helicases of the DEAD and DExH protein families (collectively known as DExD/H families). Based on data from biochemical studies of a few of these RNA helicases, they are generally considered to be involved in the unwinding of duplex RNA molecules. However, recent reports provide evidence indicating that these proteins might also be involved in the active disruption of RNA-protein interactions.     

From protein structure to function.

Several databases of protein structural families now exist-organised according to both evolutionary relationships and common folding arrangements. Although these lag behind sequence databases in size, the prospect of structural genomics initiatives means that they may soon include representatives of many of the sequence families. To some extent, functional information can be derived from structural similarity. For some structural families, their function is highly conserved, whereas, for others, it can only be inherited or derived on the basis of additional information (e.g. sequence patterns, common residue clusters and characteristic surface properties).