During birth, prostaglandins initiate the contraction in smooth muscles of uterine wall which makes option D incorrect. The pressure of fetus’s head against the cervix and send signals to make hypothalamus to trigger the release of oxytocin from posterior pituitary. Together, oxytocin (OT) and prostaglandins stimulate strong uterine muscle contraction forcing the fetus downwards. This makes option A correct. Options B and C are incorrect as fetus is expelled out of uterus by strong push of uterine wall contraction; not by reducing the lumen of uterus or by increasing its movement. Correct answer is A.