Sex differences in fetal activity and childhood hyperactivity

Background: Most studies have failed to identify significant sex differences in movement (or activity) during fetal development. However, the sample sizes and lengths of time fetuses were monitored in these studies have been limited. Aim and methods: Using the recollections provided by a sample of 6,546 mothers, this study examines variations in fetal activity levels for every month of pregnancy. Evidence was also sought that fetal activity might beassociated with hyperactivity/ hyperkinesis following birth. Results: By the fourth month of pregnancy, mothers reported that males were significantly more active in the womb than females. Also, fetal activity was positively correlated with hyperactivity following birth, especially for males. Conclusion: Despite numerous prior studies derived from small samples failing to reveal significant sex differences in fetal activity, the present study demonstrates that males are about 10% more active than females during the latter two-thirds of pregnancy and are even more so following birth. Furthermore, even within each gender, fetal activity predicted hyperactivity in childhood, thus indicating that there must be a common biological root for variations in activity levels.