Parturition in the goat: plasma concentrations of prostaglandin F and steroid hormones and uterine activity during late pregnancy and parturition.

Changes in plasma concentrations of hormones and uterine activity associated with spontaneous parturition in the goat were examined. No change in oestradiol-17beta concentration was detected during the experimental period, but oestradiol-17alpha consistently increased in concentration 3-4 days before parturition. This was followed by an increase in prostaglandin F two days later, while a pre-partum decline in progesterone concentration occurred 18-20 h after the significant increase in prostaglandin F. The onset of uterine contractions coincided with a significant decline in progesterone concentration and a period of continually rising prostaglandin F and oestradiol-17alpha concentrations. Uterine activity expressed in Montevideo units did not change until 10-8 h before foetal expulsion. These results support the hypothesis that in the goat the elevated prepartum concentration of prostaglandin F brings about luteolysis and a subsequent decline in progesterone concentration.     

Relaxin on induction of parturition in beef heifers

Purified porcine relaxin (3000 U/mg) was administered into the cervical os of primiparous beef heifers on day 278 of gestation (approximately 5 days before parturition normally occurs) to determine its effects on the induction of parturition, changes in progesterone, estrone (E1), 17 beta-estradiol (17 beta-E2), cervical dilation, and pelvic relaxation. Heifers were assigned randomly to 1 of 3 treatments: relaxin-double (two infusions of 3000 U, 12 h apart; n = 17), relaxin-single (3000 U; n = 14), and PBS-gel vehicle (n = 16). Relaxin induced marked earlier calving (P less than 0.002) than PBS-gel vehicle. The intervals between the administration of relaxin or the PBS-gel vehicle and calving were 2.0, 2.5, and 5.3 days for heifers given relaxin-double, relaxin-single, and PBS-gel vehicle, respectively. The duration of gestation was significantly reduced (P less than 0.002) in relaxin-treated heifers compared with that in control heifers. A precipitous decrease in progesterone (7.1 ng/ml) occurred in peripheral blood plasma within 24 h after relaxin treatment. Coincident with a decline in levels of progesterone, E1 and 17 beta-E2 increased by 1700 and 400 pg/ml, respectively, an increase of 35% compared with the 12% increase in these steroids in control heifers. Mean deviations of cervical dilation increased 643%, 526%, and 11% in heifers given relaxin-double, relaxin-single, and PBS-gel vehicle, respectively. Relaxin induced maximum pelvic opening between 12-36 h after treatment. Although relaxin induced significantly earlier calving, there was no incidence (0 of 31 heifers) of retained placenta. We conclude from this study that purified relaxin administered intracervically to primiparous beef heifers during late pregnancy induced premature parturition. Marked shifts of progesterone, E1, 17 beta-E2, pelvic canal expansion, and cervical relaxation reflect the premature parturition induced by relaxin.     

Radioimmunoassay of plasma relaxin levels throughout pregnancy and at parturition in the pig.

Relaxin concentrations in peripheral plasma have been measured throughout pregnancy and immediately following parturition in the pig by radioimmunoassay. Gestation ranged from 114-117 days. Relaxin concentrations remained below 2 ng/ml during the first 100 days of pregnancy and then rose gradually to a mean of 11.9 ng/ml by 3 days before parturition. At this time, relaxin levels began to rise more rapidly and over the next 2 days increased to a mean concentration of 44.0 ng/ml by 30 h before parturition. Relaxin concentrations then increased sharply to a mean of 145.6 ng/ml by 14 h before parturition. This maximum was followed be a rapid decrease to a mean of 42.4 ng/ml by 2 h before parturition. At one day following parturition, relaxin levels were less than 1 ng/ml.     

Daily hormonal changes in the maternal, fetal, and amniotic fluid compartments before parturition in a primate species

The daily hormonal fluctuations that occur simultaneously in the fetus, mother, and amniotic fluid during late gestation and before preterm parturition were studied in long term catheterized rhesus macaques. Blood and amniotic fluid samples were collected twice daily and analyzed by RIA for estrone, estradiol, dehydroepiandrosterone sulfate (DHEAS), progesterone, cortisol, and prostaglandin F2 alpha metabolite (PGFM). Vaginal delivery in monkeys with live fetuses was preceded by rising concentrations of DHEAS in fetal, but not maternal, blood. Parallel increases in fetal plasma estrone, maternal plasma estrone and estradiol, and amniotic fluid estrone preceded the rise in amniotic fluid PGFM (P less than 0.005, by analysis of variance). Cortisol levels remained stable in maternal blood and amniotic fluid, but increased before delivery in fetal blood. Nocturnal progesterone peaks in both fetal and maternal blood increased progressively in magnitude in fetuses before parturition. Rising concentrations of fetal DHEAS, estrone, and progesterone indicated an increase in adrenal activity before parturition in the rhesus fetus. PG production, reflected in amniotic fluid PGFM concentrations, was temporally related to increasing amniotic fluid concentrations of estrone. Although progesterone withdrawal may occur at a local tissue level, parturition occurred without an apparent decrease in circulating maternal, circulating fetal, or amniotic fluid progesterone concentrations.     

Antiprogesterone, RU 486, facilitates parturition in cattle.

RU 486, a potent progesterone antagonist with high affinity for progesterone receptor, was used alone or in combination with relaxin in late pregnant cattle to determine its effect on induction of parturition. Cross-bred beef cattle were bred by artificial insemination. An indwelling cannula was inserted into a jugular vein on day 269 (expected term = day 283) for repeated blood sample collection. On day 277, the cattle were assigned randomly to three groups (n = 6 each): group 1 received RU 486 (2 mg/kg BW, im) at 0800 h on days 277 and 278; group 2 received the same RU 486 treatment plus 3000 U relaxin, injected sc at 0800 h on day 278; and group 3 served as controls and received vehicle injection. Parturition occurred 55 h after treatment in group 1 and 53 h after treatment in group 2 compared with 210 h in the controls (P less than 0.01). The calves from treated groups were vigorous at birth, and their birth weights (32 and 33 kg in groups 1 and 2) were less than those of control calves (38 kg; P less than 0.01). There was no incidence of difficult birth (dystocia) with RU 486 treatment compared with that in the controls. Placenta delivery averaged 6.5 h after birth in both RU 486-treated groups and did not differ from the control value (5 h). Plasma progesterone concentrations averaged 8.2 ng/ml during the pretreatment period for all animals. Progesterone started to decrease markedly by 1200 h on day 278, dropped to about 4 ng/ml by 2400 h that same day, and was at basal levels on day 279, the day of calving, in two hormone-treated groups. In sharp contrast, progesterone was maintained at about 6 ng/ml in placebo-treated controls during this period and did not decrease to basal levels until 2 days before parturition on day 286 (P less than 0.01). Peak RU 486 in plasma was 7.2 ng/ml after the first injection and 14.3 ng/ml after the second injection, and averaged 7.9 ng/ml on the day of induced calving (day 279). Peak relaxin was 4.1 ng/ml after hormone injection. The results indicate that RU 486 alone or combined with relaxin precisely controlled the time of parturition in cattle in late pregnancy. Such treatment can be used to facilitate parturition and increase survival rates of neonatal calves without detrimental effects of dystocia, retention of placenta, and delayed postpartum fertility.     

Endocrinology of human parturition

The mechanisms involved in human pregnancy maintenance and parturition are highly complex and involve mother, fetus and placenta. The “final common pathway” to delivery is composed by inflammatory and endocrine interactive paths that tip the balance in favor of coordinated uterine contractility and cervical dilation. These mechanisms involve a shift from progesterone to estrogen dominance, CRH action, increased sensitivity to oxytocin, gap junction formation, and increased prostaglandins activity. Complementary changes in the cervix involve a decrease in progesterone dominance and the actions of prostaglandins and relaxin, via connective tissue alterations, leading to cervical softening and dilation. Neuronal, hormonal, inflammatory and immune pathways participate in initiation of labor and the utero-placental unit plays a major role in the synthesis and release of parturition mediators.     

Mammary gland lactose, plasma progesterone and lactogenesis in the marsupial Macropus eugenii

Mammary gland lactose concentrations in pregnant tammar wallabies remained low at 115 +/- 24 (S.E.M.) micrograms/g wet weight of tissue until immediately before parturition, then increased to 1274 +/- 262 micrograms/g after birth. Concentrations in non-pregnant cyclic animals were generally low (143 +/- 36 micrograms/g), but were raised in three animals around the time of oestrus. Removal of the corpus luteum on day 18 of pregnancy or the oestrous cycle caused an increase in lactose concentrations in both lutectomized and sham-operated animals. This occurred despite a significant lowering of peripheral plasma progesterone concentration in only the lutectomized group. Plasma cortisol concentrations were high in some of these animals, but showed no consistent relationships with the raised lactose concentrations. The increased peripartum lactose concentration normally coincides with a sharp fall in peripheral plasma progesterone concentration, but artificial maintenance of high progesterone levels had no effect on the increase of mammary gland lactose at parturition. Mammary gland lactose concentrations in tammar wallabies are therefore a useful indicator of biosynthetic activity and as an index of lactogenesis but the role, if any, of progesterone withdrawal in lactogenesis remains unclear.     

Prostaglandin in the peripheral plasma of tammar wallabies during parturition

Tammar wallabies (Macropus eugenii) were observed for 7 days, 24 h/day, at the expected time of birth in two consecutive breeding seasons. Blood was collected from the lateral tail vein 1-2 days before birth, then at 10- to 20-min intervals in the peripartum period and less frequently to 30-h post partum. Plasma was assayed for the prostaglandin metabolite 13,14-dihydro-15-oxo-prostaglandin F2 alpha (PGFM), progesterone and LH. An assay for PGFM was validated which allows direct measurement in 100 microliter unextracted plasma with a sensitivity of 0.14 nmol/l (50 pg/ml). There was a short-lived peak of PGFM immediately before or at birth (7.15 +/- 2.52 nmol/l; 2536 +/- 892 pg/ml) which declined to less than 0.28 nmol/l (100 pg/ml) within 2-h post partum. Progesterone concentrations declined about the time of birth, coincident with the peak of PGFM, and reached levels observed in lactationally quiescent animals by 16-h post partum, which was also the time of the LH peak. The transient prostaglandin pulse was detected only by frequent sampling and suggests that, as in other mammals, prostaglandin is important in parturition.     

Initiation of parturition in the goat: evidence for control by foetal glucocorticoid through activation of placental C21-steroid 17alpha-hydroxylase.

Infusion of dexamethasone into chronically catheterized foetal kids induced delivery in 41--65 h. Changes in the concentrations of placental and ovarian steroids in the maternal circulation at dexamethasone-induced delivery mimicked those preceding spontaneous kidding at term; in both instances the peripheral concentration of progesterone fell and the concentration of oestradiol-17beta rose. The concentration of cortisol in the foetus was low at dexamethasone-induced delivery. Metabolism of pregnenolone, progesterone, 17alpha-hydroxyprogesterone and androst-4-ene-3,17-dione by extracts of foetal placenta was investigated in late pregnancy, after premature parturition induced with dexamethasone or prostaglandins and after spontaneous parturition at term. In placenta obtained before the onset of labour (or from animals induced to kid by administration of prostaglandins), the main product of progesterone metabolism was a 5beta-pregnane-3,20-diol. In contrast, placentae from animals in which the foetal level of glucocorticoid had been raised (after spontaneous parturition or by administration of dexamethasone to the foetus) were able to 17alpha-hydroxylate and progesterone was metabolized to 5beta-pregnane-3alpha/3beta,17alpha,20alpha-triols and 17alpha,20alpha-dihydroxypregn-4-en-3-one. The appearance of placental 17alpha-hydroxylase was correlated with raised maternal concentrations of 17alpha,20alpha-dihydroxypregn-4-en-3-one and androstenedione. The induction or activation of placental 17alpha-hydroxylase may represent the mechanism by which foetal glucocorticoid controls the onset of labour in the goat.     

Control of corpus luteum function in the pregnant rabbit: role of estrogen and lack of a direct luteotropic role of the placenta

The corpus luteum is essential for pregnancy maintenance in the rabbit and appears to require two luteotropins: estrogen from ovarian follicles and a placental luteotropic factor. We have investigated the role of the placental luteotropic factor in maintaining corpus luteum function in the pregnant rabbit in the absence of estrogen. In Exp 1, follicular estrogen was withdrawn on day 21 of pregnancy by ovulating follicles with 10 IU hCG. In Exp 2, estrogen was withdrawn in hypophysectomized pregnant rabbits on day 21 by removing an estradiol (E2) implant. In the presence of this estrogen implant, luteal function and pregnancy are maintained after hypophysectomy, performed on day 4 of pregnancy. In both experiments, fetoplacental viability was ensured by treating the rabbits with medroxyprogesterone acetate (MPA). In both Exp 1 and 2, withdrawal of estrogen on day 21 of pregnancy caused a dramatic decline in serum progesterone concentrations by day 22. Serum progesterone concentrations remained low, and corpora lutea regressed, although viable fetuses were maintained with MPA. In animals not receiving MPA, estrogen withdrawal caused the loss of luteal function, followed by abortion on days 23-24. In contrast, estrogen replacement (via E2 implant) on day 22 in Exp 1 was fully capable of restoring serum progesterone concentrations to pretreatment values on days 24-27 in MPA-treated rabbits. In rabbits not receiving MPA, estrogen replacement also restored serum progesterone concentrations and prevented abortion. These results provide further evidence that estrogen is essential for normal luteal function in the pregnant rabbit. In the absence of estrogen, the rabbit placenta maintained by the progestagen MPA has no direct luteotropic activity.     

Dynamic changes in the cervical epithelial tight junction complex and differentiation occur during cervical ripening and parturition

Cervical epithelia have numerous functions that include proliferation, differentiation, maintenance of fluid balance, protection from environmental hazards, and paracellular transport of solutes via tight junctions (TJs). Epithelial functions must be tightly regulated during pregnancy and parturition as the cervix undergoes extensive growth and remodeling. This study evaluated TJ proteins, as well as markers of epithelial cell differentiation in normal and cervical ripening defective mice to gain insights into how the permeability barrier is regulated during pregnancy and parturition. Although numerous TJ proteins are expressed in the nonpregnant cervix, claudins 1 and 2 are temporally regulated in pregnancy. Claudin 1 mRNA expression is increased, whereas claudin 2 expression declines. The cellular localization of claudin 1 shifts at the end of pregnancy (gestation d 18.75) to the plasma membrane in a lattice pattern, consistent with TJs in the apical cells. The timing of claudin 1-enriched TJs coincides with initiation of terminal differentiation of cervical squamous epithelia as evidenced by the increased expression of genes by differentiated epithelia late on gestation d 18. The cervical ripening defective steroid 5alpha-reductase type 1 deficient mouse, which has an elevated local progesterone concentration, also has aberrant claudin 1 and 2 expressions, fails to form claudin 1-enriched TJs, and lacks normal expression of genes involved in epithelial terminal differentiation. These data suggest that changes in permeability barrier properties during cervical ripening are, in part, negatively regulated by progesterone, and that dynamic changes in barrier properties of the cervix occur during pregnancy and parturition.     

Effect of inhibition of prostaglandin synthesis on cervical softening and uterine activity during ovine parturition resulting from progesterone withdrawal induced by epostane

The effects of an inhibitor of 3 beta-hydroxysteroid dehydrogenase (epostane) on uterine activity and cervical softening have been studied in eight sheep during late pregnancy. Treatment with epostane led to a rapid decline in the concentration of progesterone measured in utero-ovarian venous plasma, to less than 10% of the pretreatment value within 30 min of bolus injection. This was followed by a significant (P less than 0.02) increase in the concentrations of metabolites of prostaglandins E and F in utero-ovarian venous plasma and uterine activity similar to that seen in the final stages of normal labour. Measurements of cervical tissue extensibility made ex vivo showed the cervix to have softened considerably. These changes occurred without any significant change in the concentration of oestradiol-17 beta in utero-ovarian venous plasma. Infusion of mefenamic acid, an inhibitor of prostaglandin synthesis, prevented the changes in uterine activity and cervical softening that occurred after injection of epostane alone. Mefenamic acid also reduced the increase in concentrations of metabolites of prostaglandins E and F in plasma, although the concentration of progesterone in these animals showed the same abrupt fall which occurred in sheep after injection of epostane alone. These results suggest that progesterone withdrawal, in the absence of any subsequent rise in circulating oestrogen concentrations, is sufficient stimulus to induce cervical softening in the ewe. Cervical softening following progesterone withdrawal can be prevented by inhibition of prostaglandin synthesis.     

On the role of prostaglandins in parturition in the rat.

Treatment of both intact and hypophysectomized pregnant rats on days 19 and 20 of pregnancy with PGF2alpha results in a rapid fall in plasma progesterone concentrations by day 20 followed by premature delivery on day 21. Administration of depoprovera did not prevent the PGF2alpha-induced fall in progesterone levels but premature littering was adverted. Indomethacin, an inhibitor of prostaglandin biosynthesis, postponed the onset of parturition by 20 h when administered in late pregnancy but rats treated with both indomethacin and PGF2alpha delivered at the normal time. Treatment of both intact and hypophysectomized animals with indomethacin delayed the fall in plasma progesterone concentrations which normally occurs between days 21 and 22 of pregnancy. Concomitant PGF2alpha treatment of rats receiving indomethacin caused plasma progesterone levels to fall in a pattern similar to controls. These findings indicate that endogenous PGF2alpha induces a fall in blood progesterone concentrations which is followed by increased myometrial activity.     

Changes in total and free concentrations of steroid hormones in the plasma of women throughout pregnancy: effects of medroxyprogesterone acetate in the first trimester

The total (protein-bound plus free) and free concentrations of progesterone, oestradiol-17 beta and cortisol were measured serially throughout pregnancy in the plasma of two groups of women whose pregnancies went to term. Group A (n = 53) experienced an uneventful low-risk pregnancy with a spontaneous abortion rate of 8.6%. Women in group B (n = 22) were treated orally with medroxyprogesterone acetate (MPA; 80-120 mg daily) until 18 weeks gestation for threatened abortion within the first 6 weeks of pregnancy. In both groups of women the proportion of each hormone circulating in the free or unbound form remained constant despite the overall increases with occurred in total circulating hormone concentrations as pregnancy progressed. The steroid hormonal profiles in the first half of pregnancy were similar in both groups of women. However, from weeks 20 to 40 total and free progesterone concentrations were significantly (P less than 0.05 in each case) higher in group B compared with group A. Conversely, total and free oestradiol-17 beta concentrations were lower (P less than 0.005 and P less than 0.01 respectively) in group B. At this stage it is not known if these differences were attributable to the administration of MPA to women in group B or to altered placental steroidogenesis as a result of earlier uterine bleeding.     

Hormones and parturition in primates

In primates, the endocrine signals which correlate with the end of gestation, i.e. account for fetal maturity, and initiate the parturition, i.e. trigger the myometrium contractility, remain unknown. Direct and indirect evidence supports the view that, as with domestic mammals, progesterone (or the estrogen/-progesterone ratio) plays a prominent role in inhibiting the contractility of the pregnant uterus. In the past few years an increasing number of endocrine factors have been identified in the placenta. They may contribute to the control of local or systemic steroid production but their effects are extraordinarily intermingled and it is impossible today to state whether any of them are relevant to the mechanism of parturition. The trophoblast and the myometrium establish close contact in the primate pregnancy. This is evidenced by histological studies and also by the influence of the proximity of the placenta on tissue steroid concentrations and the mechanisms of hormone coupling in the myometrium. Specific types or subtypes of myometrium hormone receptors are now well identified (e.g. to oxytocin, to catecholamines) and this now permits a better understanding of the role of their endogenous agonists in the course of parturition. However, such data are still lacking for other factors (e.g. prostanoids, VIP, relaxin...) involved to varying degrees in this process.     

Plasma oxytocin and steroid concentrations during late pregnancy, parturition and lactation in the miniature pig.

Plasma oxytocin concentrations were measured during late pregnancy, parturition and lactation in the miniature pig. Measurements were made of plasma oestradiol, oestrone and progesterone to determine whether there was any relationship between the concentrations of oxytocin and these steroids in the circulation. Plasma oxytocin concentrations were low or undetectable in late pregnancy. Rises of up to 68.8 mum./ml were seen at the time of delivery of the foetuses and at the expulsion of the placenta. The only steroid that seemed to relat to oxytocin release was progesterone. Oxytocin release was consistently seen when progesterone concentrations had fallen to below 10 ng/ml but no increase in concentration was observed while oestrone and oestradiol increased to their maximum concentrations of 3.86--11.6 and 0.43--0.70 ng/ml respectively. During lactation, when both oestrogen and progesterone concentrations were low, suckling caused the levels of oxytocin to increase to 7.4 muu./ml. These increases were greater during the first 2 weeks of lactation than later.     

Relaxin concentrations in pig plasma following the administration of prostaglandin F2alpha during late pregnancy.

Concentrations of relaxin in peripheral plasma were measured following the injection of 10 mg of PGF2alpha to pregnant sows on day 112 of gestation. The mean concentrations of relaxin before injection of PGF2alpha were less than 13 ng/ml. Relaxin concentrations increased sharply to a mean of 104 ng/ml by 45 min following injection of PGF2alpha and then fell rapidly to a mean of 40 ng/ml 4 h later. Following this initial peak of relaxin, there was a gradual rise and fall of relaxin concentrations during the day preceding parturition. The levels of relaxin during this period were also influenced by PGF2alpha administration since the mean levels of relaxin were significantly lower from the 22nd through the 10th h before parturition in pigs given PGF2alpha, than in pigs given saline.     

Influence of light-dark cycle on antepartum serum relaxin and progesterone immunoactivity levels and on birth in the rat

Relaxin and progesterone are produced by the corpora lutea in the pregnant rat. Relaxin immunoactivity levels are elevated in peripheral sera during the last 12 days of pregnancy. In rats maintained under a conventional photoperiod of 14 h of light (0600-2000 h) and 10 h of darkness (2000-0600 h), there is an antepartum elevation in serum relaxin to maximal levels coincident with the rapid decline in serum progesterone to basal levels during the light phase of the photoperiod 1 day before birth. Therefore, we postulated that this maximal elevation in serum relaxin levels may be temporally associated with functional luteolysis and linked to the photoperiod. In the present study the photoperiod was advanced near midpregnancy in order to examine further the relationship of the antepartum elevation in serum relaxin levels with both functional luteolysis and the photoperiod. Three groups of rats were maintained under a conventional photoperiod of 14 h of light (0500-1900 h) and 10 h of darkness until days 7 and 8 of pregnancy when the photoperiod was advanced 8 h in group 2 (G2) and advanced 18 h in G3 relative to the conventional photoperiod that was maintained in G1. Serum relaxin and progesterone levels were determined in blood samples obtained at 4-h intervals from 2000 h on day 19 of pregnancy until birth. The times of occurrence of birth, maximal relaxin levels, and decline of progesterone to basal levels in G2 and G3 were generaly advanced 50-60% of the advancement of the photoperiod. There was a close temporal association between the attainment of maximal relaxin levels and basal progesterone levels; they both occurred during the light phase of the photoperiod, approximately 24 h before birth in all three groups. We conclude that the antepartum elevation of serum relaxin to maximal levels may be associated with functional luteolysis and that its time of occurrence is influenced by the photoperiod. This study also provides evidence that the antepartum elevation of relaxin levels consists of two phases which occur at a 24-h interval. It is proposed that these two phases in the elevation of relaxin levels may be indicative of an increasingly effective endogenous circadian luteolytic process whose time of occurrence is influenced by the light-dark schedule.     

The Endocrinology of equine parturition

Delivery of viable young requires co-ordination of fetal maturation with the onset of labour at term. In turn, this depends on a cascade of fetal and maternal endocrine events. The sequence of these events is broadly similar in most mammals but there are differences in placental hormone synthesis and in the timing and magnitude of key prepartum endocrine changes between species. In most farm animals, maternal progesterone (P4) levels decline and oestrogen levels increase in the last 5 - 10 days before delivery in response to activation of the fetal hypothalamicpituitary-adrenal (HPA) axis and increased fetal cortisol concentrations. This cortisol surge is also responsible for fetal maturation and increasing uteroplacental prostaglandin (PG) synthesis. In the mare, there is little, if any, P4 in the maternal plasma during late gestation and both progestagens and oestrogens are produced by a feto-placental unit which uses precursors supplied by the fetus to synthesise a range of C21 and C18 steroids, many of which are unique to the horse. Regulation of uterine quiescence and activation is, therefore, complex in the mare near term. Indeed, total progestagen concentrations rise and total oestrogen levels fall in the mare during the last 20 - 30 days of gestation and only show the changes typical of impending parturition in other species in the last 24 - 48 h before delivery. Fetal cortisol concentrations also rise late in gestation in the horse compared to other species. In common with other species, the prepartum endocrine cascade appears to begin in the fetal horse with activation of the fetal HPA axis but, initially, the primary product of the fetal equine adrenal appears to be pregnenolone (P5) and not cortisol. This leads to increased progestagen production by the uteroplacental tissues, which maintains uterine quiescence in the face of increasing uterine stretch caused by the rapidly growing fetus. Very close to term in association with increasing fetal ACTH levels, the fetal equine adrenals appear to switch to producing cortisol. This late cortisol surge induces a period of rapid fetal maturation and may also contribute to increased uteroplacental oestradiol-17 beta and PG production. The fall in P5 availability may reduce uteroplacental progestagen production and lift the block on myometrial contractility. Finally, increased PG secretion activates myometrial contractions, which stimulate oxytocin release via a neuroendocrine reflex. The endocrine regulation of equine parturition, therefore, involves progestagens, oestrogens, PGs and oxytocin as in other species. However, further studies are required to establish the causes and consequences of the rise and fall in maternal progestagens and the extent to which initiation of equine labour depends on the fetal HPA axis.     

Epitestosterone in the plasma of the goat during pregnancy and at parturition.

Epitestosterone, a product of the metabolism of androstenedione by the caprine placenta in vitro, is present in the plasma of the pregnant goat. The maternal concentrations of both epitestosterone and unconjugated oestrogens (mostly oestradiol-17 alpha) in the blood increased before parturition and dropped post partum. Measurement of arteriovenous differences at term indicated that epitestosterone was secreted by the uterus; its production was not dependent on the presence of corpora lutea. It is suggested that the concentration of epitestosterone (+ androstenedione + oestrogens) in maternal plasma may be used as an indicator of placental C-17,20 lyase activity; the slight rise in the concentration of these compounds prepartum suggests a relatively small increase in flow through this enzyme.