The East Flanders Prospective Twin Survey, started in 1964, realised for the first time a population-based register of all multiple births in the Belgian Province of East Flanders, characterized by the systematic examination of the placenta and the accurate determination of zygosity based on blood groups, DNA and structure of fetal membranes. For decades, it has been the only register to include chorionicity in same-sex pairs of twins, an irreplaceable tool allowing almost certain diagnosis of monozygosity and the timing of the division of the zygote.in monozygotic twins. We showed that twins and triplets are far from being an homogenised group and demonstrated differences between these groups important enough to introduce these variables in the standard methodology of twin research. Examples are: Weinberg rule, sex ratio and duration of gestation.

In a normal delivery the second-born twin experiences a slight degree of hypoxia, which, as a rule, has no clinical importance. It is the oxygen debt everybody makes by running 30 metres.

Unlike-sex twins provide a unique natural experiment to investigate the influence of sex on gestation. Our data showed that length of gestation of unlike-sex pairs is similar to that of female same-sex pairs, and significantly (0.4 wks, p = .02) longer than that of male same-sex pairs. Birthweight of female unlike-sex twins was similar to female same-sex twins, but male unlike-sex twins weighed 78 g more than male same-sex twins ( p = .001). These data show that in unlikesex pairs it is the girl that prolongs gestation for her brother, resulting in a higher birthweight than that of same-sex boys.

Twins are not a homogeneous group. According to zygosity and chorionicity essential differences exist. The lower sex proportion at birth is due to the monozygotic twins, especially the monoamnionic variety. X-inactivation patterns in monozygotic twin girls are totally symmetrical in monoamnionic pairs, almost symmetrical in monochorionic diamnionic pairs and can be very asymmetrical in the dichorionic variety.

This article discusses findings of two recent studies conducted in collaboration with the East Flanders Prospective Twin Survey in the field of cognitive ability. The first study examined the effect of chorion type on heritability estimates of intelligence in children. The second study investigated the causes of association between child psychopathology and lower cognitive ability. Findings of these studies are discussed in the light of the current view on cognitive ability (or ‘g’) and recommendations for future research are made.

Alongitudinal study of growth and physical fitness of twins and their parents was designed in 1985. The major aims of this Leuven Longitudinal Twin Study were to quantify the genetic and environmental determination of (1) somatic characteristics, biological maturation and physical performance characteristics during the growth process, (2) the growth and developmental patterns, and (3) the covariation in somatic and performance characteristics.

The focus of research in depression is on negative affect. However, positive affect is under-investigated and plays an important role in resilience against depression by neutralizing the effects of genetic vulnerability to depression.

The East Flanders Prospective Twin Survey comprises data concerning placentation, mode of conception and perinatal outcome. As such, this database is convenient to study differences between spontaneous twins and twins resulting from assisted reproductive technologies (ART). Papers based on this survey demonstrated higher incidences of velamentous cord insertion, single umbilical artery and preterm birth in twins after ART.