What hormonal change marks the transition from girlhood to adulthood at puberty?

The hormonal change that marks the transition from girlhood to adulthood at puberty is the increase in oestrogen. During this period, the ovaries begin to produce higher levels of oestrogen, which is crucial for the development of secondary sexual characteristics in females. These characteristics include breast development, the widening of the hips, and the initiation of menstrual cycles. Oestrogen also plays a vital role in regulating the menstrual cycle and preparing the body for potential pregnancy.

While testosterone increases in both males and females during puberty, it is oestrogen that specifically signifies the transition in females. Growth hormone levels fluctuate throughout growth phases but do not mark the specific transition linked to puberty. Progesterone, primarily associated with the menstrual cycle and pregnancy, does not increase significantly until after ovulation occurs, which is later in the progression of puberty. Thus, the increase in oestrogen is the key hormonal change that defines this critical transition into adulthood for females.