What hormone prevents uterus from initiating labor before the designated delivery time?

The hormone levels are one factor. Estrogen increases contractility of the uterus while progesterone inhibits it. Throughout pregnancy, progesterone levels are high, but just before labor, they fall relative to estrogen levels and contractions start. Another hormone called oxytocin also induces labor. The number of oxytocin receptors increases as pregnancy progresses, and hormone levels rise at labor causing the uterus to contract and begin labor. The uterus also has stretch receptors and will not start contracting unless stretched to a certain degree.