Final answer:
The hormone responsible for increased laxity of the reproductive tract and vaginal prolapse is relaxin, which prepares the body for childbirth by increasing the elasticity of pelvic structures.
Step-by-step explanation:
The hormone responsible for increased laxity of the reproductive tract and vaginal prolapse before and during childbirth is relaxin. Relaxin is secreted by the corpus luteum and then by the placenta, and it plays a crucial role in preparing the body for childbirth by increasing the elasticity of the symphysis pubis joint and pelvic ligaments. This makes room for the growing fetus and allows the expansion of the pelvic outlet for childbirth. Additionally, relaxin helps dilate the cervix during labor.
Progesterone and estrogen do have roles in pregnancy and the changes in the female reproductive system but are not the primary hormones responsible for the increased laxity mentioned in the question. While estrogen can contribute to tissue growth and blood flow, and progesterone is essential for maintaining the pregnancy and for changes in the myometrium, relaxin is the hormone directy associated with laxity of the pelvic ligaments.