Vigorous contraction of the uterus at the end of pregnancy causes expulsion or delivery of the foetus. This process of delivery of the foetus resulting in childbirth is known as parturition.

Introduction:
The fetal ejection reflex is a physiological response that occurs during the second stage of labor, leading to the expulsion of the fetus from the mother's uterus. This reflex is crucial for the successful delivery of the baby. The correct answer to the given question is option D, which states that the fully developed fetus and placenta induce the fetal ejection reflex.

Explanation:
Several factors contribute to the initiation and progression of the fetal ejection reflex. Let's discuss each of these factors in detail:

1. Fully developed fetus:
The presence of a fully developed fetus is essential for the initiation of the fetal ejection reflex. As the fetus grows and develops, it gains the strength and maturity necessary for the birthing process. It is only when the fetus reaches a certain stage of development that it becomes capable of initiating the reflex.

2. Fully developed placenta:
The placenta plays a crucial role in the fetal ejection reflex. It acts as a communication interface between the fetus and the mother, facilitating the exchange of nutrients, oxygen, and waste products. The placenta also produces hormones, including oxytocin, which is necessary for the initiation of labor contractions. Therefore, the presence of a fully developed placenta is vital for the onset of the fetal ejection reflex.

3. Release of oxytocin from the pituitary gland:
Oxytocin is a hormone released by the pituitary gland in response to various stimuli, including fetal movement, stretching of the cervix, and positive feedback from uterine contractions. Oxytocin plays a significant role in stimulating the uterus to contract during labor. These contractions help push the fetus downward, exerting pressure on the cervix and initiating the fetal ejection reflex.

4. Pressure exerted by amniotic fluid:
Amniotic fluid surrounds the fetus in the amniotic sac. During labor, as the contractions increase in intensity, the amniotic fluid exerts pressure on the cervix and vaginal walls. This pressure helps stimulate the nerve endings in the cervix, triggering the release of oxytocin and initiating the fetal ejection reflex.

Conclusion:
In summary, the fetal ejection reflex in human females is induced by the fully developed fetus and placenta. The presence of a fully developed fetus and placenta is necessary for the initiation and progression of the reflex. Additionally, the release of oxytocin from the pituitary gland and the pressure exerted by amniotic fluid contribute to the onset of the reflex. Understanding the mechanisms behind the fetal ejection reflex is crucial for ensuring a safe and successful delivery.