My previous blog post discussed birth risk factors that produce oxygen deprivation to the brain and damage to the left hemisphere during birth.  The left hemisphere damage causes handedness control to switch to the right hemisphere giving rise to left-handedness in the child.  One of the birth risk factors examined by researchers is a mother’s age at the time of the child’s birth. Although mothers over the age of 35 are at higher risk for certain birth complications, there is little research evidence to support increased rates of left-handedness among the children of older mothers.

My second post in honor of Mother’s Day this month deals with other pregnancy factors that may predict later handedness of the child.  One of these is fetal thumb sucking.  Recently, an expectant mother posted an ultrasound image of her fetus on a Facebook page devoted to news of and discussion about left-handedness.  The fetus appeared to be sucking his/her thumb. The mother asked if this behavior could be a predictor of the eventual handedness of the child.  Researchers have studied the side of fetal thumb sucking behavior as a potential predictor of handedness side.  The results indicate that fetuses who suck the right thumb become right-handers while those that suck the left thumb become either right- or left-handed.  Fetal sucking of the right thumb is a better predictor of right-handedness than is fetal sucking of the left thumb a predictor of left-handedness.

Fetus sucking the right thumb.

There is more room for the head of the fetus if it is positioned to the left side of the uterus or to the left of the mother’s body midline.  By the final weeks of pregnancy, the fetus is in a head-down position with the right ear facing out. The majority of births occur with a left-sided presentation and this orientation is a predictor of eventual right-handedness in children.  This connection makes sense since most people are right-handed and most births occur with a left-sided presentation.  The more infrequent right birth presentation is only weakly associated with the eventual development of left-handedness.

Newborns also display a head-turning bias when lying on their backs.  Most newborns (65%) lie on their backs with their heads turned toward the right side.  The remaining number either turn their head to the left (15%) or show both right- and left-sided turning behaviors (20%).  A 2013 study reported that infants prefer to turn their heads toward the side of the thumb sucked as a fetus. Head-turning bias also predicts handedness for reaching during the early months of life.  Right head-turning infants reach for objects with the right hand while left head-turning infants reach with the left hand.

Side biases occur before birth as shown by research on fetal thumb sucking. Fetal thumb sucking occurs as part of a normal pregnancy and it appears to be a more reliable predictor of right- rather than left-handedness.  However, the development of a stable hand preference takes time. Infants show inconsistent hand preferences when reaching for or grasping objects.  Consistent use of either the right or left hand for these movements does not start to appear until around the first year of life.  For this reason, mothers (and fathers) have an opportunity to influence the handedness of the child by encouraging the infant to imitate their own handedness or by placing objects selectively into the infant’s right or left hand.  The fact that infants of left-handed mothers show more left-handed behaviors than infants of right-handed mothers supports the developmental influence on handedness of these parental behaviors.

Read more about parental factors that influence handedness in Chapter 3 of my book Laterality: Exploring the enigma of left-handedness.  The chapter title is ‘Who’s left in the family?’  Also, see my Facebook page at www.facebook.com/4lefthanders.