Final answer:
Before the baby and placenta are expelled in childbirth, the cervix thins and dilates, the baby is born, and contractions guided by oxytocin occur to facilitate these processes.
Step-by-step explanation:
Before the baby and placenta are expelled, several important events occur. During stage one of labor, the cervix thins and dilates to about 10 cm, preparing the birth canal for the baby to pass through. Stage two is marked by the birth of the baby, where contractions help the baby to be expelled from the uterus. Finally, in stage three, the placenta, which supported the baby with nutrients and oxygen throughout the pregnancy, is expelled after the baby is born.
It is important to note that the hormone oxytocin is primarily responsible for the contractions during labor. Human pregnancy consists of three trimesters, where the first lays down basic body structures, the second continues the development of organs and systems, and the third exhibits the greatest growth, culminating in labor and delivery.