The head of a human offspring turns down and prepares to enter the birth canal during
the first trimester.
the second trimester.
the third trimester.
birth.

The head of a human offspring turns down and prepares to enter the birth canal during the third trimester.

During the third trimester, which is the last three months of pregnancy, the baby changes position in the womb with it's head now pointing down towards the birth canal. This is to prepare itself for labor and delivery. In some cases though the baby does not change position and needs assistance in order to turn around sometimes resulting in a cesarean birth rather than a natural one.