Right and Left-Handedness

Dive Deeper

As early as eight weeks after conception, it is possible to determine whether the unborn child is left-handed or right-handed by studying ultrasounds. About 85% of embryos prefer their right hand over their left hand.1 About 85% of adults prefer their right hand, too, and the majority of fetuses do not change their handedness between the womb and age 10.2

Furthermore, by 12 weeks after conception, the fetus makes goal-directed movements towards the uterine wall as well as his eyes and mouth. If he has a twin, he will gently reach out to touch his sibling as well. 3

Already, the embryo will show a preference for either her right or left hand as she moves in the womb. (Image Credit: Priests for Life)
As early as 12 weeks after conception, twins purposefully touch one another in the womb. The fetus moves more gently when reaching for her twin's face. (Image Credit: Science Source)

While the fetus uses both her left and right hand 16 weeks after conception, she will reach for small targets, such as her eyes and mouth, faster and with greater precision when she uses her dominant hand.4

Intriguingly, the brain does not show major left-right differences when hand dominance can first be observed. Therefore, scientists still wonder whether the difference in movement leads to changes in the brain that make some people right-handed versus left-handed, or whether the brain causes the difference in movement which leads one hand to be dominant.