Besides showing the beginnings of behavioral diversity, ultrasounds also allow us to catch a glimpse of the dawning of individual dispositions. Albeit in a very rudimentary, embryonic form these tenuous initial inclinations could be considered the first small signs of later, more complex and structured traits. Individual propensities can be discerned in the variety of repeated fetal movement patterns, of preferred postures and activities. Within general broad similarities, each fetus moves differently and has slightly different rhythms, adjustments and clock times. Some are more active than others, some seem to need more startles before showing signs of activity. Some appear more jerky in their movements, and some react more strongly to co-twin stimuli. Some push their legs more, others go back to sitting in the yoga position as soon as they enter into a new cycle of rest. Some constantly rub their eyes, others touch their legs and feet all the time. One may squat, while another may try to turn upsidedown. A preference for a particular posture could be favoured both by crowding and by purely structural environmental differences. Inequalities in space may leave one twin unable to lie horizontally, while another may be able to rest more easily in a cephalic version. Though spatial constrictions due to crowding are not relevant when fetal motions first begin, even apparently minor initial environmental variations would appear to prompt differential behavior. The placenta may have a lump or a small gorge. One fetus may roost on the edge of this gorge, while another may just dip its arms in. These diversities are always minute if measured against the environmental variety and complexity of behavior encountered in life after birth. Neither space nor behavioral maturation allows more apparent manifestations. When talking about movement, particularly during the first half of pregnancy, we are normally referring to short cycles of activity of the duration of several seconds rather than the several minutes of a mature fetus or neonate. Nevertheless, even in these initial brief periods of activity some minute differences begin to emerge. Time generally shows that this very tenuous first thread progressively takes on the qualities of a slightly more defined path (Piontelli 1995; Piontelli et al. 1999). Defined, however, does not mean definitive. We can speak of

tendencies and inclinations perhaps, but clearly not of well-determined characteristics. Furthermore, the preceding chapters have explained how fraught with perils and variations the intrauterine sojourn of twins is. In life after birth an individual may be changed, hardened, softened, or bent by the fortuitous circumstances of life (Plomin and Dunn 1986; HillGoldsmith et al. 1987; Emde et al. 1992; Kagan 1994). It is not difficult, therefore, to imagine how the risks and hazards that crowd in upon the intrauterine life of twins may change and alter what is only a tenuous, unfolding trend. Nevertheless, all twins, including so-called identical twins, emerge from the troubled time of pregnancy as unique individuals with fairly distinct inclinations and behavioral manifestations. Individuality and uniqueness are shaped during gestation and are evident by the time the twins are born.