A girl can get pregnant even before her first period. Before age 20 a girl's body is still developing and a pregnancy could be dangerous for her and the baby. Young mothers are more likely to have complications during birth which can cause illness/death. Their spine and pelvic bones can be damaged as they are not yet fully grown. Young mothers' babies have higher risk of being premature and underweight. Young women might want an education/do other things before becoming a mother. Socially and medically it is advised to get pregnant between age 20-35.
Attribution: Straight Talk Foundation