Parturition

Parturition Normal, natural, physiological birth or labor. It is the expulsion of the fetus from the uterus through the maternal passage by natural forces, when it is sufficiently developed to live outside of the dam. The medical terminology for this event is "eutocia". This process is also called: calving in cattle lambing in sheep kidding in goats foaling in mare farrowing in swine whelping in dogs

Preparturition Changes Cows: Relaxation of the pelvic ligaments. Relaxation of the vulva and perineum. Enlargement and edema of the udder. Mucoid discharge from the vagina and dissolution of the cervical seal. Isolation from the herd with restlessness and anorexia may be exhibited in the last few hours before parturition and heifers may show signs of abdominal pain.

Mare: Ewe and Doe: slight relaxation of the sacrosiatic ligaments. The vulva becomes slightly edematous, and the vulva cleft lengthens during the last few weeks prior to foaling. Development of the mammary glands occurs during the last 3 to 6 weeks of gestation, and leaking of colostrum or "waxing" occurs 6 to 48 hours before foaling. Ewe and Doe: In the ewe and nanny, the clinical signs are similar to those of the cow with the exception that udder development is not as great.

leaking of colostrum or "waxing" occurs 6 to 48 hours before foaling

Hormonal control of parturition

Initiation of Parturition During pregnancy myometrial activity is minimal due to the "progesterone block" which inhibits the release of prostaglandin and prevents the transport of calcium ions into the myometrial cells. As parturition approaches, there is activation of the fetal hypothalamic-pituitary-adrenal axis resulting in increased levels of fetal corticosteroids. The mechanism that follows the release of fetal corticosteroids differ among species depending on the source of progesterone maintaining the pregnancy.

Initiation of Parturition fetal stressors such as hypoxia, hypercapnia, changes in blood pressure and blood glucose. It is also postulated that placentally-derived hormones such as oestrogens, progesterone, PGE2 or corticotrophin-releasing factor (CRF) may also act on the hypothalamus. During the last 20–25 days of gestation, there is a dramatic rise in fetal cortisol concentrations, which reach a peak 2–3 days before birth, thereafter declining 7–10 days postpartum. The source of the increase in fetal cortisol is the fetal adrenal, which is due to both an increase in the size of the organ in relation to total body weight, and an increase in its sensitivity to adrenocorticotrophic hormone (ACTH) as a result of accelerated processing of ACTH from pro-opiomelanocortin (POMC); maternal cortisol concentrations only rise around the time of parturition. At the same time, the binding capacity of the fetal plasma increases, thus reducing the amount of free cortisol in the fetal circulation and thereby reducing the negative feedback effect on the secretion of ACTH by the fetal pituitary

Initiation of Parturition 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens Initiation of Parturition also be involved in ACTH secretion since both increase towards the end of gestation. The fetal adrenal becomes more responsive to ACTH stimulation with advancing age. Insulin-like growth factors (IGFs) may have an autocrine and/or paracrine role in regulating ovine fetal adrenal function. Fetal growth hormone, which is elevated from 50–70 days of gestation and then falls until 100 days, before increasing to term, may also modify the response of the fetal adrenal to ACTH. The rise in fetal cortisol stimulates the conversion of placentally-derived progesterone to oestrogen by activating the placental enzyme

Initiation of Parturition 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens Initiation of Parturition

Initiation of Parturition 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens Initiation of Parturition oestrogens have a direct effect upon the myometrium, increasing its responsiveness to oxytocin; secondly, they produce softening of the cervix by altering the structure of collagen fibres; thirdly, they act upon the cotyledon–caruncle complex to stimulate the production and release of prostaglandin F2α (PGF2α). The latter change is induced by the activation of the enzyme phospholipase A2 stimulated by the decline in progesterone and rise in oestrogen. This enzyme stimulates the release of arachidonic acid from phospholipids, so that under the influence of the enzyme prostaglandin synthetase, PGF2α is formed. Prostaglandins play a key role in initiating parturition

Initiation of Parturition 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens Initiation of Parturition Two prostaglandins are produced by the uterus – PGF2α in the endometrium and, during expulsion of the fetus in the myometrium, prostacyclin (PGI2). Prostaglandins have a wide range of actions; they cause smooth muscle contraction, luteolysis, and the softening of cervical collagen as well as stimulating smooth muscle cells to develop special areas of contact called gap junctions, thereby allowing the passage of electrical pulses and ensuring coordinated contractions. PGF2α is considered to be the intrinsic stimulating factor of smooth muscle cells, and thus its release is important in initiating myometrial contractions

Initiation of Parturition 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens 17α-hydroxylase; this hydroxylates progesterone via androstenedione to oestrogen. The consequences of the rise in oestrogens Initiation of Parturition Two prostaglandins are produced by the uterus – PGF2α in the endometrium and, during expulsion of the fetus in the myometrium, prostacyclin (PGI2). Prostaglandins have a wide range of actions; they cause smooth muscle contraction, luteolysis, and the softening of cervical collagen as well as stimulating smooth muscle cells to develop special areas of contact called gap junctions, thereby allowing the passage of electrical pulses and ensuring coordinated contractions. PGF2α is considered to be the intrinsic stimulating factor of smooth muscle cells, and thus its release is important in initiating myometrial contractions

The effect of these contractions is to force the fetal lamb towards the cervix and vagina where it will stimulate sensory receptors and initiate Ferguson’s reflex, with the release of large amounts of oxytocin from the posterior pituitary. Oxytocin will stimulate further myometrial contractions and the release of PGF2α from the myometrium. Hence both these hormones, together with uterine contraction, seem to work as a positive feedback system of increasing magnitude, thus stimulating further uterine contractions and consequent expulsion of the fetus. Maturation of the fetal lamb’s lungs, especially the production of alveolar surfactant, is stimulated by cortisol, as are many other changes in fetal function and structure that enable the lamb to survive after birth.

The role of relaxin: Relaxin, a polypeptide hormone, was shown to be responsible for causing relaxation of the pubic symphysis of guinea pigs. The most potent sources of this hormone are the CLs of the pregnant sow; it is also produced by preovulatory follicles. In the cow, the CL appears to be the main source of the hormone, with values increasing just before calving. In the horse, dog and cat, the main or sole source of the hormone is the placenta. In the mare, concentrations start to rise from about 80 days of gestation, although there is considerable breed variation. In the bitch, relaxin stimulates the growth of the cervix during late pregnancy as well as causing relaxation before parturition. The latter changes, which are also influenced by the oestrogen: progesterone ratio, involve changes at a biochemical level by influencing the glycosaminoglycans: collagen ratio and histological structure. Relaxin plays an important role in cervical relaxation at term. Studies have shown that when highly purified porcine relaxin was placed directly on the external os of the cervix at 276–278 days of gestation, cervical relaxation occurred 8–12 hours later. Similar results have been obtained when parturition has been induced with dexamethasone.

In sheep: cortisol stimulates the placenta to convert progesterone to estrogen. The elevated levels of estrogen stimulate the secretion of prostaglandin and the development of oxytocin receptors. In CL dependent species: cortisol in addition to the synthesis of estrogen causes a release of prostaglandin from the endometrium. As a result of alterations in steroid patterns, increased uterine sensitivity to oxytocin and production of prostaglandin myometrial contractility comes more coordinated and intensifies as parturition approaches.

In conjunction with myometrial activity, a softening of the cervix, relaxation of pelvic ligaments and a generalized expansion of the birth canal become evident 1 to 2 days prior to parturition. The ovarian hormone, relaxin, has been implicated in regulating this process, as have placental estrogens and uterine prostaglandin. The distension of the cervix and vagina by the conceptus initiates the neurohormonal reflex (Ferguson's reflex) which produces the expulsive force of abdominal muscular contractions (straining) and the release of high levels of oxytocin which in turn stimulates myometrial contractions.

The essential components of the birth process are: The expulsive forces. The expulsive forces combine the contraction of the myometrium and abdominal muscles (uterine 90%, abdominal contraction 10%). The fetus. The birth canal.

Stages of Parturition The preparatory stage or stage of cervical dilation. 2. The stage of expulsion of the fetus. 3. The stage of expulsion of the fetal membranes and involution of the uterus.

The preparatory stage or stage of cervical dilation (2-6 hours) This includes: a. Relaxation of the soft birth canal b. Relaxation of the bony birth canal c. Lubrication of the birth canal d. Dilation of the cervix and begins of uterine contraction e. Arrangement of the fetus

The cervix dilates and rhythmic contractions of the uterus begin The cervix dilates and rhythmic contractions of the uterus begin. Initially, contractions occur at approximately 15-minute intervals. As labor progresses, they become more frequent until they occur every few minutes. At this time a portion of the placenta (water sac) is forced into the pelvis and increase the dilation of the cervix. Arrangement of the fetus: Just prior to labor, the fetus rotates to an upright position with its forelegs and head pointed toward the birth canal.

Relaxation of the birth canal is primarily under the influence of hormones. Estrogen, produces relaxation of the vagina, vulva and perineum, including muscles. Relaxation of the sacro-iliac junction allows dorsal and lateral expansion of the bony pelvic inlets. This process is also under the influence of hormones, particularly estrogen. Lubrication of the birth canal by increased activity of the cervical glands results in lubrication of the birth canal. Maximum lubrication is attained at the time of parturition following rupture of the allantoic and amniotic sac and the release of their contents.

The stage of expulsion of the fetus (one hour or less) This stage begins when the fetus enters the birth canal, and usually the cow is lying down. Uterine contractions are now about every two minutes and are accompanied by voluntary contractions of the diaphragm and abdominal muscles. Surrounded by membranes, the calf's forelegs and nose now protrude from the vulva. The stage completed by expulsion of calf. Upon passage through the vulva, the umbilical cord generally breaks, and the lungs become functional. Delivery is normally completed in one hour or less. Special assistance is required if this stage goes beyond two to three hours.

Stage of expulsion of the fetal membranes and uterine involution Following expulsion of the fetus contractions of the uterus continues for 48 hours, this is necessary to prevent hemorrhage, as well as, to aid in the expulsion of the fetal membranes and involution of the uterus. Expulsion of the fetal membranes usually takes place between l/2 to 8 hours, but occasionally it may be delayed for 12 hours without being considered abnormal. After the fetal membranes are expelled, the uterus still continues to contract resulting in the expulsion of lochia. In the cow involution of the uterus occurss on an average of about 47 days after calving.