<prompt> Which of the following probably triggers the onset of labour?ACTH in fetusACTH in motherOxytocinProstaglandin  <response>: Ans: a ACTH in fetus Ref: Ganong, 22nd ed, p. 451This also is a direct pick from self study MCQs Chap 23 Q. 17Mechanism responsible for onset of labour: Circulatory estrogens due to increase in circulatory DHEAS.So gap junction - uterine contractions. GRH by fetal hypothalamus - circulatory ACTH - cortisol leading to maturation of respiratory system.Thus in a sense fetus picks the time to be bom by GRH ACTH secretion 