Fetuses begin moving at around 7 weeks of pregnancy, but it's only after around 16 to 18 weeks that expectant mums start to feel any kicking or movement.
Recent studies have suggested that the fetus moves around more in response to maternal stress and also when the mother is happy. More active fetuses also seem to achieve higher scores on a brain maturation test and have better control of body movements after birth.
What is less well known is that these fetal movements may also have an effect on the mother, possibly training her to listen out for her child and even synchronise with its activity patterns in preparation for life after birth.
Janet DiPietro at Johns Hopkins University in Baltimore, Maryland, and her colleagues recently discovered that every time the fetus moves, the mother's heart beats faster and her sympathetic nervous system is stimulated - even when she's not consciously aware of the movements.
The precise significance of these findings remains unclear for now, but people had previously assumed that it was only the mother's behaviour that affected the fetus. It's a bidirectional relationship, says DiPietro. This concludes that the mother and the fetus affect each other.
The sympathetic nervous system controls the fight-or-flight response, as well as playing an important role in maintaining the body's normal internal functions. I think it's teaching them, says DiPietro. It's preparing women to pay attention to their infants.
There maybe some suggestions that an active fetus equals a boisterous child. Although there may be some truth to this, published findings have been inconsistent. In one recent study, Shimon Degani at the Bnai-Zion Medical Centre in Haifa, Israel, and his colleagues studied the activity of 22 pairs of twins between 11 and 14 weeks of pregnancy and found that, in general, the more active twin as measured by ultrasound was more difficult, unpredictable, inadaptable and active at 3 and 6 months after birth, according to assessments by the mother.
However, in a larger and earlier study, DiPietro found that although more active male fetuses at 36 weeks went on to become more active boys, the opposite was true for girls.
Generally though, DiPietro found that more active fetuses became children that were more likely to interact with toys or strangers at the age of 2, regardless of their sex.