Final answer:
Estrogen and progesterone are the two sex hormones that change the structures of the new mother's brain.
Step-by-step explanation:
The two sex hormones that change the structures of the new mother's brain are estrogen and progesterone. During pregnancy, these hormones are secreted by the corpus luteum and later by the placenta. Estrogen maintains the pregnancy, promotes fetal viability, and stimulates tissue growth, while progesterone prevents new ovarian follicles from developing and suppresses uterine contractility. These hormonal changes during pregnancy and childbirth contribute to the physiological and behavioral adaptations observed in new mothers.