Which hormone is primarily responsible for maintaining pregnancy?

Progesterone plays a critical role in maintaining pregnancy. It is primarily produced by the corpus luteum in the early stages of pregnancy and later by the placenta as pregnancy progresses. This hormone is essential for creating an optimal environment in the uterus for implantation of the fertilized egg and for supporting the developing embryo. Progesterone helps to thicken the uterine lining, making it more suitable for implantation, and prevents uterine contractions that could disrupt the pregnancy. Additionally, it modulates the immune response to help protect the developing fetus.

While estradiol is also significant for pregnancy and overall reproductive health, its role is more associated with the development of the uterine lining and secondary sexual characteristics rather than the maintenance of pregnancy. Oxytocin is primarily involved in labor and delivery, as well as in the milk ejection reflex during breastfeeding, rather than in maintaining pregnancy itself. Prolactin is crucial for milk production after childbirth and has some roles in reproductive health, but it is not the primary hormone responsible for sustaining pregnancy. Therefore, when considering the crucial factors in maintaining pregnancy, progesterone is the definitive hormone.