Table of Contents

Initiation of Parturition

The initiation of parturition is a complex physiological process controlled by fetal and maternal hormonal signals.

The initiation of parturition is explained by various theories and mechanisms, primarily involving hormonal and fetal-maternal interactions.

The fetus exerts an overriding control on the length of gestation and that the mother can influence the time of birth within narrow limits. Other words the fetus decides the day of birth and the dam decides the time of birth.

The uterine musculature is the key component of labour and the essential physiological change between gestation and birth is liberation of the contractile potential of the myometrium. The factors involved in this transformation are neural, humoral and mechanical.

The mechanisms that are responsible for the initiation of parturition vary slightly between species. The mechanisms are fairly well defined in sheep, cattle, goats and pigs but in horses it is not well understood. In dog and cat little information is currently available.

Ewes

Stress caused by:

The lack of space in the uterus.
Requirement of increased level of nutrient.
Increased responsiveness of the foetal hypothalamus, pituitary and adrenal gland which initiates the parturition.

Before 120 days of gestation much of the cortisol present in the fetal circulation is derived from the ewe via transplacental transfer. During the last 20-25 days of gestation there is a dramatic rise in fetal cortisol concentration which reaches 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. In fact, maternal cortisol concentration 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 adrenocorticotrophic hormone by the fetal pituitary.

There is an increase in corticotrophin releasing hormone in the fetal hypothalamus during the last 10 days of gestation and the placenta of the sheep can also secrete cortiotrophic releasing hormone. Endogenous opioids may also stimulate ACTH secretion via the effect on hypothalamus rather than the pituitary.

The rise in fetal cortisol stimulates the conversion of placental–derived progesterone to oestrogen by activating the placental enzymes 17 α hydroxylase; this hydroxylates progesterone via androstenedione to estrogen. So the rise of estrogen in the peripheral circulation is three folds.

Functions of Estrogens

Estrogens have a direct effect upon the myometrium and hence increasing its responsiveness to oxytocin.

They produce softening of the cervix by altering the structure of collagen fibres.

They act upon the cotyledon-caruncle complex to stimulate the production or release of PGF2α. This change is induced by the activation of the enzyme phospholipase A₂ which is stimulated by the decline in progesterone and rise in estrogen. This enzyme stimulates the release of arachidonic acid from phospholipids, and then the influence of the enzyme prostaglandin synthatase, the PGF2α is formed.

Further stimulation of synthesis and release of PGF2α from the myometrium can also be induced by the action of oxytocin and mechanical stimulation of vagina which readily pass from cell to cell via cell membrane or between cells in the extra cellular fluid.

Two prostaglandins are produced by the uterus:

PGF_2α secreted from the endometrium
PGI_2α secreted from the myometrium during the expulsion of the fetus

Functions of PGF_2α

Smooth muscle contraction
Luteolysis
Softening of cervical collagen
Stimulate smooth muscle cells to develop special areas of contact called gap junctions, thereby allowing the passage of electrical pulses and ensuring coordinated contractions

The effect of PGF2α on myometrial contraction is to force the fetal lamb towards the cervix and vagina. Where it stimulates sensory receptors and initiates 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. These 2 hormones together stimulate uterine contractions and cause expulsion of the fetus. Other important change is maturation of fetal lungs especially by the production of alveolar surfactant which is stimulated by cortisol.

Cow

Mechanism similar to that of ewe.

The placenta assumes the main role of progesterone production between 150 and 250 days of gestation. If the corpus luteum is removed during this stage, the pregnancy will continue, but in the ovariectomized cows parturition is frequently abnormal because the regression plays an important role in the endocrine changes which are necessary for the initiation of parturition. Other mechanisms are direct effect of glucocorticoids, placental estrogen and PGF2α which are responsible for the luteal regression and the latter one is released as a result of the effect of placental estrogen.

Goat and Sow

Similar to ewe.

Mare

Not well understood. The fetal adrenal undergoes rapid hypertrophy immediately before parturition. Fetal plasma cortisol elevated 10 folds during the last 8 days before foaling. Progesterone remain low from the middle of gestation until the last 2-3 months of gestation then increases during last 20 days, reach peak about 48 hrs before parturition.

Then they decreased rapidly to low levels at the time of parturition. Plasma E2 concentration decline during the last 100 days of gestation rather than increase as in other species reaching low levels at parturition although this is largely a reflection of the decline in oestrone, equilin and equilenin, since concentrations of oestradiol – 17b remain fairly constant.

Bitch and cat

Peripheral cortisol attains peak 8-24 hrs prepartum. Progesterone concentration start to decline 30th day of gestation and fall low level between 12-40 hrs before the birth of the first puppy. Oestrogen remains at a fairly constant level throughout the pregnancy and starts to decline about 2 days prepartum, reaching non-pregnant values at the time of parturition.

Prolactin increases as progesterone concentration decreases 1-2 days before whelping and not known in any role in parturition. The decreased level of progesterone and estrogen are due to the release of PGF_2α during the 48 hrs before whelping.

Queen Cat

Progesterone declines and estrogen tends to increases towards the end of parturition.