Researchers tracked 147 pregnant mares and found that when blood progestagen levels fell about half within a day, a single injection of oxytocin (10‑50 IU) reliably started labor, with most foals born in 20‑50 minutes. This protocol works for horses but doesn’t translate to human health or performance goals.

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.