Status as a twin is, of course, determined early after conception. If the concept that events very early in pregnancy may determine both pregnancy outcome and postnatal phenotype, then the study of twin pregnancies may provide valuable insights. Twin conceptions have increased dramatically over the past three to four decades (Chauhan et al.
2010; Pinborg
2005), partly as a consequence of increasing use of assisted reproductive technology and partly secondary to increasing maternal age. Twins are born both earlier and smaller than singletons, even when only spontaneous births are considered (Chauhan et al.
2010). Observational studies in humans suggest that fetal reduction of higher order multiples to twins or to singletons may not completely abrogate the increased risk of preterm birth and reduced birthweight associated with multiple pregnancies (Wimalasundera
2010). Pregnancies in sheep also may be either singleton or twin. We have exploited this fact to test the impact of twin conception on gestation length and size at birth experimentally. Random fetal reduction of one twin of a pair in early pregnancy, with the remaining fetus continuing pregnancy as a singleton after having been conceived a twin, does not prevent the shorter gestation length and reduced size at birth seen in twin pregnancies which underwent a sham procedure and in which both fetuses continued to term (Digby et al.
2009). That is, gestation length and size at birth in twins are largely determined in early pregnancy. Although the association between twin conception and later risk of adult disease has been controversial (Phillips et al.
2001), more recent data utilizing within twin-pair statistical techniques suggest that, indeed, twins are at increased risk of diabetes and altered fat deposition in later life (Poulsen et al.
2009; Vaag and Poulsen
2007). A comparison between twins and singletons in the Avon Longitudinal Study of Parents and Children found that language development at age 20 months in twins was approximately 1.7 months behind that of singletons (MacArthur Communicative Development Inventory, by questionnaire), increasing to 3.1 months at 3 years of age (McCarthy Scales of Children’s Abilities, administered by researchers) (Rutter et al.
2003). Twins had lower scores on all subscales of the Preschool Language Scales 3—verbal score, perceptual score, quantitative score, and general cognitive index. The authors concluded that these differences were not due to earlier birth, developmental immaturity, or family factors but were related to status as a twin (Rutter et al.
2003). Within twin pairs there is frequently discordance for language development, with the heritability estimated at between 0.35 and 1.0 (Stromswold
2001). In monozygotic twin pairs, heritability for language development is estimated to be about 0.6 (Stromswold
2001), and antenatal and perinatal environmental factors appear to play a larger role in discordant language development in twins than do postnatal factors (Stromswold
2006). Evidence that monozygotic twins within a twin pair have different levels of gene expression in two different cell types (umbilical vein endothelial cells and blood mononuclear cells) at birth (Gordon et al.
2011) suggests that antenatal environmental factors, in addition to stochastic effects, may alter gene expression in utero, presumably via epigenetic mechanisms. The findings that 19% of monozygotic twins differ with respect to the X chromosome inactivated (Wong et al.
2010; Wong et al.
2011), and that epigenetic differences in twins increase with increasing age (Wong et al.
2010; Fraga et al.
2005) provides further evidence that phenotypic differences within twin pairs are epigenetically determined and that this can occur before birth (Bell and Spector
2011). The differences in X chromosome inactivation patterns suggest that epigenetic differences in twins are likely to occur as early as the wave of epigenetic reprogramming in early embryonic life; however, differences in X chromosome inactivation at this time are likely to be stochastic. Preliminary data from studies in sheep, demonstrating different epigenetic responses to periconceptional undernutrition between singletons and twins (Begum et al.
2011) suggest that environmentally driven epigenetic modifications in early pregnancy may also explain some of the differences in outcomes between singleton and twin pregnancies.