I read in my psychology book that babies are more active in the womb if the woman is higher on the economic ladder. Is this true? Can anyone back this up? (I've learned not to believe everything I read or hear )

It's not necessarily true. It's based on a lot of assumptions about nutrition and activities. However, studies have shown that the lower socio-economic status does impact the health of the child.

Basically, it is lifestyle. In general, the lifestyle of a middle-class woman is more condusive to healthy pregnancy, even when adjusting for pre-natal care.