Corpus Luteum function and parturition in cattle and pigs

At term pregnancy in cattle and pigs progesterone is from luteal origin. Like in other domestic animal species, parturition in the cow and pig is preceeded by a sudden drop in the matemal plasma progesterone concentration. An increased Output of Prostaglandin from the pregnant uterus is most likely responsible for prepartum luteolysis. The withdrawal of progesterone is not immediately followed by activation ofthe myometrium; rather a period of (relative) myometrial quiescence allows for functional changes to take place in the cervix and myometrium, before parturient uterine contractions develop (between 12 and 14 hours, and between 6 and 9 hours before onset of the expulsive stage, in cows and pigs respectively). These will only subsequently cause a widening ofthe cervix. Maintenance of progesterone dominance, by treatment with exogenous progesterone/ progestagens or by induction of a new set of corpora lutea (pigs), at the time that parturition is initiated, will postpone delivery. However, uterine Prostaglandin output (cows) still increases under these conditions (cows) and many of the calves and piglets are bom dead when (prolonged) expulsion finally takes place after relief from progesteron. This indicates that progesterone is not able to counteract completely the parturition promoting signals generated by the pregnant uterus.