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Defending life from the moment of conception

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Can you tell if your unborn baby is right or left handed?

31 August 2017


A study in 2004 found that which thumb the baby sucked in the womb is a strong indicator of handedness after birth 

Handedness may be determined by ten weeks gestation.

Which hand is dominant is related to the organisation of cognitive systems in the human brain, and because left-handedness has been associated with a number of developmental disorders, scientists are eager to understand how it comes about.

While it used to be believed that you can't tell which hand is dominant until a child reaches toddler years, it seems that it may be decided very early indeed.

Strong correlation

A study in 2004 found that which thumb the baby sucked in the womb is a strong indicator of handedness after birth, especially right-handedness. This study followed-up 75 individuals who were observed sucking their thumb as fetuses and examined their handedness at 10–12 years of age. Of 60 right-handed unborn babies, all were right-handed postnatally; 10 of 15 left-handed fetuses were left-handed and five right-handed. 

Very early

It has been thought that hand preference comes from the brain. However, the motor cortex isn't linked to the spinal cord until about 15 weeks post conception, whereas co-ordinated hand movements start at just 8 weeks post conception (10 weeks gestational age) when 85% of fetuses exhibit more right arm than left arm movements.

Researchers from the Netherlands and South Africa therefore undertook to research the hypothesis that handedness is determined by mechanisms in the spinal cord, rather than the brain.

Critical period

The study, published in the elife journal, is very dense and academic, but their data did indeed indicate that handedness comes from the spinal cord, not the brain. Furthermore, they concluded: "As week eight after conception represents the onset of coordinated hand movements and behavioral asymmetries of the hands occur first at this time point, we assume that a certain time frame before 10 weeks post-conception represents the critical period for handedness formation." 

So, not only do babies start learning languages in the womb, the hand they will write with for the rest of their life may be knowable before they are 10 weeks old in the womb! 

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