The
Voyage of Life

Right and Left-Handedness

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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.

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Sources
  1. Hepper, Peter G, Glenda R Mccartney, and E. Alyson Shannon. “Lateralised Behaviour in First Trimester Human Foetuses.” Neuropsychologia 36, no. 6 (June 1, 1998): 531–34. https://doi.org/10.1016/S0028-3932(97)00156-5.
  2. Peter G. Hepper, Deborah L. Wells, and Catherine Lynch, “Prenatal Thumb Sucking Is Related to Postnatal Handedness,” Neuropsychologia 43, no. 3 (January 1, 2005): 313–15, https://doi.org/10.1016/j.neuropsychologia.2004.08.009.
  3. Umberto Castiello et al., “Wired to Be Social: The Ontogeny of Human Interaction,” PLOS ONE 5, no. 10 (October 7, 2010): e13199, https://doi.org/10.1371/journal.pone.0013199.
  4. Valentina Parma et al., “The Origin of Human Handedness and Its Role in Pre-Birth Motor Control,” Scientific Reports 7, no. 1 (December 1, 2017): 16804, https://doi.org/10.1038/s41598-017-16827-y.
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All images and illustrations on The Voyage of Life were reviewed by credentialed scientists for accuracy.

Medical art animation was used to provide a schematic representation of fetal development, with some features, including timing, not to scale. The animation was chosen as an ethically uncompromised guide to fetal development. There are many real human images at each developmental age showing the most accurate fetal forms throughout development. A concerted effort was made to use images and material in which the human embryo or fetus did not undergo any known harm.