Ductal development of breast is caused by

Estrogen, or oestrogen, is the primary female sex hormone. It is responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens in females that have estrogenic hormonal activity: estrone, estradiol, and estriol Ref: guyton and hall textbook of medical physiology 12 edition page number: 636,637,638