Development of prenatal lateralization: evidence from fetal mouth movements

Human lateralized behaviors relate to the asymmetric development of the brain. Research of the prenatal origins of laterality is equivocal with some studies suggesting that fetuses exhibit lateralized behavior and other not finding such laterality. Given that by around 22weeks of gestation the left cerebral hemisphere compared to the right is significantly larger in both male and female fetuses we expected that the right side of the fetal face would show more movement with increased gestation. This longitudinal study investigated whether fetuses from 24 to 36weeks of gestation showed increasing lateralized behaviors during mouth opening and whether lateralized mouth movements are related to fetal age, gender and maternal self-reported prenatal stress.

Following ethical approval, fifteen healthy fetuses (8 girls) of primagravid mothers were scanned four times from 24 to 36-gestation. Two types of mouth opening movements - upper lip raiser and mouth stretch - were coded in 60 scans for 10min.

We modeled the proportion of right mouth opening for each fetal scan using a generalized linear mixed model, which takes account of the repeated measures design. There was a significant increase in the proportion of lateralized mouth openings over the period increasing by 11% for each week of gestational age (LRT change in deviance=10.92, 1df; p<0.001). No gender differences were found nor was there any effect of maternally reported stress on fetal lateralized mouth movements. There was also evidence of left lateralization preference in mouth movement, although no evidence of changes in lateralization bias over time. This longitudinal study provides important new insights into the development of lateralized mouth movements from 24 to 36 weeks of gestation.