Ans. (c) 3rd stage of LaborRef High risk pregnancy 4th ed. by James et.al, ch 75- Normal third stage of LaborThere are four stages of labor* The first stage is from the onset of true labor to complete dilation of the cervix.* The second stage is from complete dilation of the cervix to the birth of the baby.* The third stage is from the birth of the baby to delivery of the placenta.* The fourth stage is from delivery of the placenta to stabilization of the patients condition, usually at about 6 hours postpartum.PROSTAGLANDINS* Prostaglandin F (PGF), PGF2a, and oxytocin are the biochemical agents primarily involved in the third stage of labor.* During the first and second stages of labor, only PGF2a and oxytocin are significantly raised in maternal plasma compared with pre-labor concentrations.* At 5 minutes after birth, maternal PGF and PGF2a concentrations peak at about twice the levels found at the commencement of the second stage. A rapid increase in prostaglandin concentrations is also found in umbilical cord venous blood, suggesting that this postpartum prostaglandin surge originates in the placenta.* After placental separation, the concentrations decrease but at rates slower than the metabolic clearance of prostaglandin, indicating that its production continues in the decidua and myometrium.* Plasma oxytocin also drops to prelabor levels within 30 minutes of delivery, unless sustained by exogenous infusion.Also KnowHormonesPeak level at:OxytocinAt birthProlactinLevels decrease during labor but then rise steeply at the end of labor and peak with birth.