Females do not need hormones to direct prenatal genital development in the same way that males do; all fetuses start off female. However, there is some evidence to suggest that oestrogen might have prenatal effects, such as causing a smaller brain size (Shi et al., 2015). However, oestrogen does play a major role from puberty onwards, as it promotes the development of secondary sexual characteristics (such as breast development) and directs the menstrual cycle.