About 266 days after conception, the fetal brain signals the release of this hormone that prepares the fetus for delivery and starts labor

Oxytocin

Explanation:

The release of oxytocin is signaled by the fetal brain about 38 weeks after conception. Oxytocin is also called birth hormone as it stimulates uterine contraction.

The uterine contractions stimulate the release of more oxytocin through a positive feedback loop. Oxytocin stimulates contraction of the smooth muscles in the wall of the uterus which in turn finally leads to childbirth.