Progestagens Monitoring in 147 Mares in Order to Induce Foaling With Oxytocin

ABSTRACT

A total of 147 mares of different breeds (110 were sport or race horses, often very valuable) have been monitored over 5 years from 2012 to 2016. When at or near term, the mares were monitored in order to induce foaling, by checking the mammary development by palpation, analysing the colour and consistency of the precolostrum, estimating the calcium and antibodies concentrations in the precolostrum and by measuring the plasmatic progestagens concentration every day at 6 p.m.

At the end of gestation, it has been observed that the progestagens rate levelled off (the top concentration varies from mare to mare: it can be as low as 35 ng/mL or up to 260 ng/mL) before falling sharply. Parturition was induced when the progestagens concentration fell of about 50% within 24 to 48 h. Induction was performed at 9 p.m. in 93 mares that followed the rule of the drop of progestagens (to respect the nycthemeral cycle) by a single IM oxytocin injection from 10 to 50 UI according to the weight of the mare (120 to 900 kg). Following the oxytocin injection, 90% (n = 84) of the foals were born within 20 to 50 min, 7.5% after 60 min (usually due to an excess of volume or dystocia), all alive and viable. Thirty to 60 min after birth, a manual delivery was performed for 71% (n = 66) of the mares. Among the 54 mares with spontaneous parturition (not induced because of insufficient proof of imminent birth), all foals were alive except for 2 twins that were found dead. A manual delivery was necessary for 17% (n = 9) of them. A 50% drop of the progestagens concentration is a good indication of the imminence of foaling: It is then safe to induce foaling, which usually results in a higher probability for a healthy foal and a reduced risk of genital lesions to the mare.

Parturition was induced when the progestagens concentration fell of about 50% within 24 to 48 h. Induction was performed at 9 p.m. in 93 mares that followed the rule of the drop of progestagens, by a single IM oxytocin injection from 10 to 50 UI according to the weight of the mare (120 to 900 kg). Following the oxytocin injection, 90% (n = 84) of the foals were born within 20 to 50 min, all alive and viable.

1. Introduction

Systematic induction of parturition in mares is controversial due to the difficulty to predict the term of pregnancy. Gestational length in mares varies from 320 to 362 days (Howell and Rollins 1951), the risks of a non‐viable premature foal are therefore high (Ousey 2003).

Induction of birth is not practised routinely in equine veterinary medicine. The difficulty is that the period of foetal maturation is relatively short in horses: the foetus is only mature during the last 2 or 3 days of gestation whereas in lambs for instance, the foetus is mature 7 to 10 days before birth (Fowden and Silver 1995). Induction is however recommended in pathological gestations to try to save the life of the mare and foal, in particular in case of abdominal hernia or prepubic tendon rupture (Macpherson and Paccamonti 2011), or more generally, when dystocia is expected. Indeed, once the amniotic membrane has been visible for 30 min, any additional 10 min delay increases the risk of a dead foal by 10%, rapidly reaching 50% (Mc Cue 2012). Induction of parturition has been proposed for student teaching purposes: the risk associated with induction of delivery can however outweigh the apparent benefits.

A prerequisite for inducing labour in mares is determining whether the foetus is capable of surviving extra uterine life. Purvis (Purvis 1977) has described three criteria for inducing parturition in mares: the gestational length, the mammary development and precolostrum production, and the cervical softening. The importance of cervical relaxation is a controversial point. However, mares with a spontaneously dilated cervix (assessed by digital examination per vagina) prior to induction usually deliver their foals quicker than those with a closed cervix (Fowden and Silver 1995).

Mammary development and colostrum production in mares are considered as the most reliable signs of imminent parturition as well as foetal maturity (Ousey et al. 1984; Peaker et al. 1979). However, many primiparous mares do give birth without producing any precolostrum and sometimes without any udder development. An additional difficulty is that some mares may refuse to be milked. Determination of precolostrum pH daily in the evening enabled prediction of parturition in almost 80% of mares (Berthelon and Tournut 1956). Salivary cortisol concentration starts to increase one to two days before parturition (Hillman 1975).

In 1956, Berthelon (Hillman 1975) induced birth in mares using oxytocin to obtain a live foal and to keep the mare healthy and able to reproduce further, provided that the induced birth is triggered at the right time. The mare was considered at term when white milk droplets would appear at the end of each teat. However, a few mares would never produce any wax or precolostrum before foaling. On his first attempt, he injected oxytocin intravenously (IV). When injected intravenously, oxytocin can trigger severe colics that may endanger the life of mare and foal. When the same dose (20 IU) was injected intramuscularly (IM), it was well tolerated and very efficient, probably because of slower resorption. Berthelon immediately understood the danger of IV and adapted the method : when the mare was considered at term, he would inject a first dose of oxytocin (20 IU) IM, and if after 30 min the delivery has not started yet, he would inject an additional dose (10 IU) IM (Hillman 1975). He would usually start the procedure at 8 a.m. in order to have the whole day to monitor the mare. He would usually repeat the injection at 11 am. Induction of parturition in mares using oxytocin has been evaluated with different dosages and protocols (Purvis 1977; Pashen 1980, Macpherson et al. 1997; Camillo et al. 2000; Chavatte‐Palmer et al. 2001; Villani and Romano 2008; Rossdale et al. 1976). In 2000, to make sure that the foal was born fully at term and therefore able to survive, Camillo (Villani and Romano 2008) injected a low dose of oxytocin (2.5 IU) IV between 2 p.m. and 6 p.m. If the foal were ready, the birth would take place within 90 min following the injection. If the foal were not fully at term, the birth would not take place. The injection was then renewed every afternoon until the birth of the foal.

Prostaglandins have also been recommended to induce foaling in mares (Macpherson et al. 1997; Ousey et al. 1984; Tainturier 1985; Leadon and Jeffcott 1984).

The oxytocin method has been improved by measuring antibodies with a refractometer and by analysing daily (at 6 p.m.) the calcium rate in the precolostrum (Peaker et al. 1979; Bain and Howey 1975) in order to induce the parturition at the right time with a single dose of 30 IU of oxytocin IM (at around 9 p.m.). The birth usually occurs within an hour after the injection. Injection in the evening is considered more efficient because respecting the nycthemeral birth cycle: 80% of natural foalings take place during the night (Mc Cue 2012; Chritensen 2011). The foetus determines the day of the parturition, but the mare determines the hour (Cash et al. 1985).

Our hypothesis was that following the progestagens serum levels allows the prediction of the exact night of foaling, safe induction of parturition and birth of a healthy foal. Birth can then be induced within 3 h of obtaining the last result, enabling the required persons to be present during labour and therefore avoiding many parturition accidents. This procedure allows a much better survival rate for the foal than any other monitoring method.

2. Materials and Methods

Following the French legislation, no ethical committee approval was needed for a clinical study but the mandatory informed consent from the owner was obtained following the same legislation.

2.1. Animals

One hundred and forty seven mares have been monitored over 5 years between 2012 and 2016 (Table 1) : 3 Shetlands, 30 ponies (of French or foreign origins),110 saddle mares (70 French Saddle, 15 Thoroughbreds, 10 French Trotters usually carrying a transplanted embryo, 15 from diverse origins), 3 draft horses, 1 unknown breed. All mares in this study belong to private owners. One owner has provided 39 mares, another 17, a third 11, a fourth 6 and the others 1 or 2. Most of the provided mares had previously encountered foaling problems: dystocia by excess of volume or death of the foal in the following hours/days following birth. Towards the end of the study, it was mainly mares carrying a high value foal (i.e. an embryo from a famous stallion) or mares with a high risk of dystocia by excess of volume.

TABLE 1.

Foaling monitoring in 147 mares between 2012 and 2016 with spontaneous (group S) or induced parturition (group I).

	Year	Group S number of mares		Group I number of mares		% of induced parturition
Batch 1	2012	11	35	10	37	51%
	2013	11		12
	2014	13		15
Batch 2	2015	9	19	30	56	75%
	2016	10		26

Fifty‐four mares gave birth spontaneously (group S), whereas 93 were induced using different IM doses of oxytocin according to their weight: 10 IU for Shetlands, 20 IU for ponies (250–350 kg) 30 IU for Saddle horses (450–600 kg) and 40 to 50 IU for draft horses (700–900 kg) (group I).

A French Saddle riding mare has been induced four times (over four consecutive years), three others three times (two French Saddles and a pony), six others twice (five French Saddles and one Thoroughbred).

Mares from the group S were planned to be induced but they did foal naturally before any oxytocin injection, or they were not induced because their progestagens concentration was too imprecise or not dosed at the right moment to detect the drop.

2.2. Follow‐Up of the Prodromes of Parturition

As soon as the mare arrived, a 5 mL blood sample was taken from the jugular vein, using a vacutainer on a lithium‐heparinate tube, without any disinfectant in order to collect her standard concentration of progestagens (any alcohol sucked by the needle of the vacutainer may distort the result). Plasma was obtained by centrifugation of the blood tube for 10 min at 3000 rpm. A simple decantation is not sufficient, as it can give false results if the progestagens is not properly separated from its carrier molecule.

Progestagens concentration determination was performed from 200 µL of plasma using a compact automatic analyser (Mini‐Vidas; BioMerieux, Marcy l'Etoile, France), designed initially for human medicine and based on the E.L.F.A. (Enzyme Linked Fluorescent Assay) principle, with immunofluorescence reading at 450 nm. It enables the measurement of progesterone concentration from 0.25 ng/mL to 80 ng/mL in 45 min. In mares, the anti‐progesterone antibodies also enable the detection of the progestagens produced in high quantity by the foetal‐placental unit at the end of the gestation, via a crossed reaction. The principle of the measurement combines the method by competition and the final detection of fluorescent.

Because the progestagens plasma concentration was higher than the limit of 80 ng/mL measured by the analyser in several mares, it has been decided to dilute the plasma when the initial progestagens concentration value was higher than 80 ng/mL after 2015. Several diluters have been tested: the most reliable has been obtained with the plasma of a gelding, provided that it was castrated several months ago. A 1:2 dilution was not always sufficient as the progestagens concentration could be higher than 160 ng/mL. A further dilution of 1:3 would allow dosage up to 240 ng/mL and a dilution of 1:4 would enable to measure a progestagens concentration up to 320 ng/mL. Successive dilutions were needed to found the real concentration of progestagens.

If the udder was sufficiently developed, a sample of precolostrum was also taken in order to analyse its content in calcium and antibodies.

The precolostrum was diluted at 1/7 in distilled water and the calcium rate can then be measured with reactive paper strips, which enables the evaluation of water hardness in a semi‐quantitative manner. Each strip is made up four reactive zones characterised by a growing sensitivity to the calcium and magnesium rate. They change colour according to the concentration of these elements (Peaker et al. 1979; Allen and McGladdery 1991; Hodge et al. 1982). Antibodies were dosed with a refractometer.

According to the obtained results, the date of arrival relative to the term, the development of the udder, the colour of the colostrum and the predicted month of the term, the next samples of blood and precolostrum would be taken the following day, in case that the mare is not ready, the samples would be taken in 2–3 days or the following week.

Each further sampling was performed at 6 p.m. and was systematically preceded by a palpation of the udder to estimate its volume and consistency and a check of the precolostrum colour. If the udder was flabby, not sufficiently developed or if the precolostrum was still transparent, the mare was not about to foal so the sampling was not performed.

2.3. Induction of Parturition

To induce parturition, the progestagens concentration must fall by 50% within 24 h or 48 h. If any doubt, a new sample would be taken within 3 h to confirm or disconfirm the fall.

Before the 15 May, an additional criteria to induce parturition was the date of the term (the term = 11 months after ovulation). If the date of the term was not reached, the mare was not induced. Before the 15 May, a short gestation can be the sign of a infection by Equine Herpesvirus (EHV‐1), a placenta detachment or the death of a twin. After the 15 May, the gestational length naturally tends to shorten so parturition could be induced a few days before the term.

The decision to induce would also take into account the usual criteria such as the mammary development, the colour and consistency of the precolostrum, as well as the high concentration of calcium and antibodies in the precolostrum.

A distinction was however made between primiparas and pluriparas: For pluriparas, the udder grows regularly and can double in volume the day before foaling. The precolostrum, which is first transparent, turns white and sticky, due to its high content in lactose. The end of the teats is usually covered by a yellow wax (sometimes as early as 3 to 4 days before foaling) or a few drops of white milk may be dripping. The calcium content of the precolostrum is then maximal (4 crosses on the test strips), the antibodies rate is also very high although a very high rate may sometimes slightly decrease as if diluted by the milky secretion. For primiparas, the udder does not always grow and the precolostrum may be totally absent. Therefore, if the udder was not fully developed with hardly any milky secretion for a primipara, the progestagens concentration drop was considered as decisive and parturition was induced (Pashen et al. 1979). If any doubt (colour of the colostrum only light yellow, calcium rate at 3 crosses instead of 4, low and stable concentration of antibodies in the precolostrum), parturition was not induced. The mare would be closely monitored during the whole night. These cases were rare and the monitoring never exceeded one night. When the decision to induce was taken, the oxytocin injection was performed at 9 p.m.

2.4. Parturition

As soon as the injection was performed, the mare tail was bound with sticky plaster (N.D.) and the following material prepared: 3 obstetrical straps and 3 tractions sticks (kept in a bucket with bleached water), some lubricant, a pair of scissors, a scalpel (in case of episiotomy) and a pair of clean gloves. Six or seven persons (often including the owner) would watch over the parturient. When the foal limbs would start to come out at the vulva, the presentation and position of the foal would be checked before pulling the limbs of the foal, with one hand on each cannon. Only when necessary, a traction stick would be placed above each fetlock in order to help with the pulling. If the dilatation of the vulva was not sufficient, in particular in mares which had vulvoplasty, an episiotomy was carried out.

During parturition, blood samples were taken in umbilical artery and vein in six foals and progestagens were measured following the procedure described above.

3. Results

3.1. Follow‐Up of the Prodromes of the Part

3.1.1. Batch 1: Breeding Seasons 2012–2013–2014

During the first 3 breeding seasons, 37 mares out of 72 (51%) were induced. They all had a well‐developed mammary, the colostrum was opaque and white, rich in calcium (4 crosses on the testing strips) and with an average antibodies rate of 67 mg/mL (extreme values: 18–110 mg/mL). During the 24 h that preceded the last measurement, the antibodies rate had increased for 15 mares, decreased for 11 and was stable for the last 11. In all cases, a sharp fall of the plasmatic progestagens concentration was measured (average: 52%, extreme values: 31%–65%) (Table 2). The average progestagens concentration at which induction was decided was 34.5 ng/mL (extreme values: 17–52 ng/mL) (Figure 1, Table 3).

TABLE 2.

Progestagens concentration drop (%) in 93 mares with induced parturition between 2012 and 2016.

Year	Progestagens concentration drop (%)
	Min	Max	Average
2012	36	76	54
2013	31	65	52
2014	35	65	50
2015	33	80	51
2016	37	63	50

FIGURE 1.

Plasma progestagens concentration at 6 p.m. (3 h before induction of parturition) in 37 mares with induced parturition in 2012 (n = 10), 2013 (n = 12) and 2014 (n = 15).

TABLE 3.

Progestagens concentration (ng/mL) at which induction was decided for 93 mares between 2012 and 2016.

Year	Progestagens concentration (ng/mL) at which induction was decided
	Min	Max	Average
2012	19	48	31
2013	17	50	33
2014	25	52	37
2015	17	157	53
2016	16	108	50

Thirty‐five mares were not induced; 31 gave birth with an average plasmatic progestagens concentration of 28 ng/mL (extreme values: 11–58.5 ng/mL). The plasmatic progestagens concentration of the last 4 mares is unknown as they gave birth before any dosage could be made. Among the 31 mares whose progestagens concentration was monitored (Figure 2), 3 mares were planned to be induced at 9 p.m., but gave birth just before, 3 mares had too irregular progestagens concentrations, and 18 mares had a progestagens concentration well above 80 ng/mL and the drop was therefore missed (or seems too moderate). For the last seven mares, a sharp drop was recorded but the induction was not triggered because the mammary was still under developed or the colostrum still transparent with a low calcium content, or the date of the term was not reached. For one of them, the date of the term was actually incorrect (there had been confusion between the date of the embryo transfer and the date of the artificial insemination). The first three mares had an average progestagens fall of 39% in 24 h and of 75% in 31 h. The average progestagens fall of the other four mares was of 34% in 48 h and 55% in 58 h.

FIGURE 2.

Plasma progestagens concentration at 6 p.m. before the foaling night in 31 mares with spontaneous parturition in 2012 (n = 10/11), 2013 (n = 8/11) and 2014 (n = 13/13).

3.1.2. Batch 2: Breeding Seasons 2015–2016

Among the 75 mares, 56 (75%) were induced: their progestagens concentration was between 16 and 157 ng/mL (with an average of 52 ng/mL) (Figure 3, Table 3). A saddle mare did reach the record concentration of 263 ng/mL before dropping to 151 ng/mL. Thanks to the dilution method, the mare was induced successfully and gave birth to a healthy female foal.

FIGURE 3.

Plasma progestagens concentration at 6 p.m. (3 h before induction of parturition) in 56 mares with induced parturition in 2015 (n = 30) and 2016 (n = 26).

For the 19 group S mares, the progestagens concentration was between 17 and 153 ng/mL the evening before delivery (Figure 4).

FIGURE 4.

Plasma progestagens concentration at 6 p.m. before the foaling night in 16 mares with spontaneous parturition in 2015 (n = 7/9), 2016 (n = 9/10).

3.2. Induction of Parturition

After the oxytocin injection, 90% of the foals were born within 20 to 55 min (Figure 5). If the amniotic membrane was still not visible out of the vulva of the mare 1 h after the injection, a vaginal exploration was performed. The origin of the delay was either an excess of volume of the foal (the dilatation of the upper reproductive tract is then delayed) or a dystocia caused by the head or the members of the foal.

FIGURE 5.

Delay between the oxytocin intramuscular injection and foaling in 93 mares.

3.3. Parturition

3.3.1. Batch 1 – Group I

All deliveries were presented by the front, except for one case that presented by the rear. All 93 foals were alive and none was premature. They all behaved normally. For example (observations were not recorded for all foals), the foals got up within 45 min of their birth (n = 43), the females being slighter faster than the males (42 min in average for the females (n = 23) vs. 49 min in average for the males (n = 20)). They suckled 87 min in average after being born (n = 22), the females being once again quicker than the males (70 min in average for the females (n = 11) vs. 104 min for the males (n = 11)). The sex ratio was 41 females for 52 males (56%) (Table 4). Two foals had severe deformations. The foal that delivered by the rear (presentation by the hocks) did not have any left eye, its hind legs were not straight and plantigrade (the mother had been treated for colic during the gestation). It was euthanised at the age of 48 h because without any economic value. Another foal was suffering from arthrogryposis and was not able to stand up. Despite intensive physiotherapy, he died at the age of 40 h. A third foal died 4 h after its birth because it was stepped on by his mother although the box stall was properly lit. The foal had broken ribs, a perforated heart and a perforated right lung. A fourth foal was born one‐eyed (the mother had a treatment during the gestation) but it later became a champion in jumping.

TABLE 4.

Sex of the 147 foals.

	Female	Male
Spontaneous parturition	22	33
Induced parturition	41	52
Total	63	85

3.3.2. Batch 2 – Group S

A heavy draft mare gave birth to twins that were found dead at dawn (the mare was not properly monitored because of a progestagens concentration higher than 80 ng/mL, but this was before the use of the dilution method). Another mare gave birth to a blind foal that was sucking properly and could move within the box stall without touching the walls. It was however euthanised after 48 h. The sex ratio was 22 females for 32 males (59%) (Table 4). In both groups combined, it must be pointed that out of the 148 foals (including the twins), 3 had ocular anomalies.

3.3.3. Progestagens in Umbilical Vessels

 Mean progesterone concentration was 201 ng/mL (120 to 310 ng/mL) in umbilical vein and 83 ng/mL (46 to 129 ng/mL) in umbilical artery. Concentration in umbilical artery was similar to progestagens blood concentration in the mare with blood sample taken at the jugular vein.

3.4. Delivery

3.4.1. Group I

The mare has to deliver within 6 h following foaling to prevent any complications such as metritis or laminitis. To avoid checking in the middle of the night at 3 a.m., the delivery was systematically performed 30 to 60 min. After birth, a slow torsion coupled to a gentle pulling was applied to the placenta which is hanging out of the vulva, without touching the upper genital parts of the mare (to avoid any bacteria contamination of the uterine cavity) and without any new injection of oxytocin (the oxytocin effects upon the smooth uterine fibres last for about 3 to 6 h). Over 5 years, 66 out of 93 deliveries were performed manually (71%).

3.4.2. Group S

The delivery by manual torsion of the placenta was only necessary for nine mares out of 54 (17%).

3.5. Gestational Length

Gestational length was between 326 and 363 days. Data have been collected for 134 mares. The mares can be divided into two groups: those that have their foal before the 15 May (75 mares) and those that gave birth after the 15 May (59 mares). In the first group, 4 gave birth before the term (5%) and 38 gave birth more than 10 days after the term (51%). In the second group, 15 gave birth before the term (25%) and only 8 gave birth more than 10 days after the term (13%) (Figure 6). The term is the date at which the foal is due, that is, 11 months after the insemination day, at the moment of ovulation.

FIGURE 6.

Interval between foaling and predicted term date (2012–2016) in 134 mares.

4. Discussion

By the end of gestation, the progestagens concentration is reasonably constant. It then falls by about 50% just before foaling. To be able to take the decision for inducing parturition at the right time, it is necessary to monitor the progestagens concentration a few days before term in order to know its reference concentration. It is not possible to decide upon a single dosage, unless the very first dosage is extremely low (15–20 ng/mL) with all prodromes indicating an imminent foaling.

In practice, the first dosage was performed when the mare arrived at the maternity facility in order to have a first reference value (after dilution of the plasma if necessary). The next progestagens measurement was performed the next day, or after 2/3 days, or the following week according to the udder prodromes. For pluriparous mares, foaling is usually preceded by a regular development of the udder and a secretion of precolostrum, whose colour and composition evolve with time. For primiparous mares, given that the udder may not change and precolostrum may never be produced, the frequency of the sampling should only depend upon the results of the progestagens concentration and the measurement should soon be performed daily.

At the beginning of the breeding season (before the 15 May), parturition was never induced before the date of the term. Mares with abnormal placenta (placentitis, death of one twin) usually show precocious mammary gland development including modification of the secretion electrolyte, most commonly a precocious rise in calcium (Hess‐Dudan 2006). After the 15 May, given than 25% of mares give birth before term, the monitoring should start 10 days before the term. Gestational length is highly variable; it varies from 320 to 362 days in light horse mare (Howell and Rollins 1951), which is in line with the gestational length values obtained during this study (from 326 to 363 days). Observations also confirmed that mares that foal during the shorter days of the beginning of the breeding season have a longer gestation, while mares that foal during the longer days of the end of the breeding season have a shorter gestation (Howell and Rollins 1951; Holtan et al. 1991) Significant improvements have been obtained by regularly analysing the progestagens concentration, together with the prodromes of the part, starting from the date of the term at the beginning of the breeding season (gestation of 11 months) and a few days before the term at the end of the breeding season (at the end of the breeding season, the gestation is often shorter).

In batch 2, 75% of the mares were induced (56 out of 75) versus 51% in batch 1 (37 out of 72). This difference is easily explained by the fact that the dilution method was only used for batch 2. When the progestagens concentration was higher than 80 ng/mL in batch 2, the plasma was diluted in order to measure the exact rate. This method has enabled the monitoring of high progestagens concentration and in particular the detection of the fall of this rate, including when it would remain above 80 ng/mL. In batch 1, 18 mares had a progestagens concentration above 80 ng/mL; the fall could not be observed or was too moderate to induce the birth. If the dilution method had been used, a similar proportion of mares would have been induced (37+18 = 55 out of 72 = 76%).

All foals behave normally after birth (except for 2 that were deformed): they got up and suckled within the usual delays. Mc Cue (2012), who has studied 1047 foalings, has shown that a foal gets up within 49 min in average after birth. The females get up faster than the males (44 min vs. 54 min). The first suckle takes place 112 min after birth (103 min for the females and 119 min for the males). The figures obtained in this study are slightly better: the foals get up after 43 min in average and suckle after 87 min, the females being indeed quicker than the males. Our number of studied foals is however smaller and the students have a tendency to help the foals to get up and suck. What matter is that none of the foals born from an induced parturition behaved as a premature foal. The 50% fall of progestagens concentration may not allow the prediction of the exact night of foaling, but it is a good indicator that birth can be induced without any risk of prematurity.

The udder palpation should be performed by an experienced person. The imminence of foaling can be detected by a sudden increase of the volume of the udder or a turgidity of the teats, by the fact that the mare is more willing to be milked and by more abundant precolostrum. When the udder palpation is performed by different students, who may not be familiar with milking, the changes may not always be detected and therefore the progestagens analysis may not be performed as often as it should have been. In practice, about 25% of the mares did not fulfil all criteria. They were therefore not induced but closely monitored. For the 75% that did meet all criteria and were therefore induced, the foal was born within 20 to 55 min following the oxytocin injection. The main advantage of induction being that several persons can be present during foaling and intervene quickly if required, in particular in the event of dystocia, giving more chances of survival to the foal and reducing the risk of genital lesions to the mare.

In mares, progesterone is produced by the ovaries (by the primary and then secondary corpus luteum). Corpora lutea slowly decrease until disappearing on day 150 of the gestation, reducing the progesterone concentration (Rosales et al. 2017; Blanchard and Varner 1993). After day 150, the main source of progesterone is the placenta (Ousey 2003), but the production stops by the end of the gestation (Frazer 2003). The equine foetus synthesises then pregnenolone from cholesterol in its adrenal glands. The pregnenolone is then metabolised into various progestagens produced by the foeto‐placental unit in high concentration (Frazer 2003). Maternal progestagens concentration increases in late gestation and only declines 24–48 h before parturition (Frazer 2003). The rapid fall of the progestagens concentration is a criterion to induce foaling. The production of these progestagens requires a healthy foal and depends upon the functional surface of the placenta. The progesterone antibodies used in the ELFA technique enables the exact dosing of progesterone during heats and parturition in bitches, or during the oestral cycle and calving in cows (personal unpublished data). The progesterone antibodies used in the ELFA technique do not interfere with estrone (cross‐reactivity 0.01%), oestradiol, estriol (<0.01%), 16α or 17α or 20α or 6β hydroxyprogesteron (cross‐reactivity 0.03% and 1.18%) but they have not been tested with the progestagens produced by the foeto‐placental unit of the mother at the end of the gestation. It is therefore possible that the anti‐progesterone antibodies enable the dosing of these progestagens, which is particularly interesting in mares at the end of the gestation. One hypothesis may be that the average 50% fall of the progestagens concentration 24 to 48 h before foaling, may be the result of a degeneration of the placenta that does not let the progestagens produced by the foeto‐placental unit through the maternal blood anymore. At birth, the progestagens concentration of the newborn foal is twice as high as the progestagens concentration of the mare. Samples taken at the umbilical vein (from the placenta) before the rupture of the umbilical cord, and at the umbilical artery (from the foetus) show that the progestagens concentration is twice as high in the vein than in the artery. These results confirmed that the progestagens measured in this study were synthesised by the placenta. The placenta degeneration indicates and precedes the parturition of a few dozens of hours. It corresponds to the period of foetal maturation that is relatively short in horses, as it is only within the last 2 or 3 days of gestation (Fowden and Silver 1995).

The delivery must be performed within 6 h after foaling, in order to avoid complications such as severe metritis or laminitis. Delivery occurs within 4 h for 95% of the mares according to Rosales and al. who performed a study over 969 mares in New Zealand (Platt et al. 2019).

In the induced group, a manual delivery was performed 30 to 60 min after foaling for 65 mares out of 93 (71%). In the group S, the manual delivery was only necessary for 9 mares out of 54 (17%). The difference is not a problem of inadequate maturation of the placenta in the induced group. The main reason for the manual delivery was to avoid checking the mare in the middle of the night at 3 or 4 a.m. for potential placenta retention. (The induction was always performed at about 9 p.m.) In the group S, foaling usually occurred between midnight and 7 a.m. In the event of placenta retention problem, the delivery could be manually made at the latest 6 h later, early in the morning. When the natural birth occurred before midnight, the mare would be manually delivered 1 h later (if required) to avoid getting up in the middle of the night. This explains the difference of manual delivery occurrence of 17% versus 4% for Rosales (Platt et al. 2019), who never intervene in the next 4 h following birth. Manual delivery was recommended for a long time, but this is not the case anymore. Manual delivery was performed by separating the placenta from the endometrium using the flat of one hand as a cutter, while pulling over the envelopes with the other hand. This method has been since highly criticised for tearing the microvilli of the placenta that were trapped within the endometrial crypts or for causing diffuse haemorrhage or bruises to the endometrium. The current recommended method consists of injecting 12 L of warm saline solution into the chorioallantoic cavity. The dilatation of the uterine cavity triggers a discharge of endogenous oxytocin and a separation of the chorionic villi from the endometrial crypts (Blanchard and Varner 1993; Frazer 2003). Actually, the discussion whether a manual delivery is more recommended that a medical treatment is not fully decided, because no fertility difference has been noticed between the mares that have been manually delivered and the others (Platt et al. 2019). Our method is different from the classical manual delivery, as it consists of a simple torsion associated to a gentle pull of the placenta, outside the vulva, for mares that have received (for the induced group) an injection of oxytocin 1 h 30 min to 2 h earlier.

5. Conclusion

In practice, the measurement of progestagens concentration should be performed every evening during several consecutive days when the mare is at term. A 50% drop of the progestagens concentration within 24 to 48 h indicates that the foal is viable. The udder development, the colour and quantity of the precolostrum as well as its content in calcium and antibodies should be monitored in parallel. When all criteria are met (75% of the mares), parturition can be induced safely.

Author Contributions

Daniel Tainturier: conceptualisation, data curation, formal analysis, investigation, methodology, resources, supervision, validation, visualisation, writing – original draft, writing – review & editing. Benoit Tainturier: formal analysis, investigation, methodology. Sandrine Michaud: formal analysis. Lamia Briand‐Amirat: formal analysis, writing – review & editing. Emmanuel Topie: formal analysis, writing – review & editing.

Funding

The authors have nothing to report.

Conflicts of Interest

None of the authors have any conflicts of interest to declare.

Tainturier, D. , Tainturier B., Michaud S., Briand‐Amirat L., Topie E., and Moreno D.. 2026. “Progestagens Monitoring in 147 Mares in Order to Induce Foaling With Oxytocin.” Veterinary Medicine and Science 12, no. 1: e70697. 10.1002/vms3.70697

Data Availability Statement

Data are available on request due to privacy/ethical restrictions.