Secondary sexual characteristics are those observable traits that occur as a result of primary sexual characteristics, which are directly tied to the necessity of reproduction. In females, enlarged breasts and nipple erection are a direct result of increased estrogen levels. A widened pelvis to assist in the natural childbirth process is also attributable to the presence of estrogen in the female system. Larger thigh muscles on the back of the femur has its roots in estrogen influence, as does more subcutaneous fat deposits in and around the females body. All these characteristics are direct results of estrogen production and proliferation in the blood circulation of females. Estrogen is one of two primarily female hormones, the other being progesterone, that are responsible for cultivating and nurturing the female side of reproduction.