Do I give Regumate™ to prevent pregnancy failure?

Introduction

Progesterone and progestagens are imperative for equine pregnancy maintenance and fetal development. During the first trimester the equine pregnancy depends solely on ovarian progesterone by the primary corpus luteum (CL) and accessory corpora lutea. By 120 days the placenta is fully established and takes over the progestagen production. Ovarian progesterone is no longer required [1] and the placenta becomes the sole source of progestagens until parturition. In daily stud practice mares are often supplemented with exogenous progestin (Regumate™) as a result of suspicions of inadequate progesterone production or a clinician’s frustration when facing mares that repeatedly fail to conceive or carry a pregnancy to term [2]. However, what is the clinical evidence for the use of Regumate™ to prevent pregnancy failure?

Data acquisition

PubMed was searched using multiple combinations of the terms ‘altrenogest’, ‘progesterone’, ‘early embryonic death’ and ‘equine pregnancy’. Cross-referencing from studies yielded additional references.

Pregnancy loss during early pregnancy

Although 60% of pregnancy losses in Thoroughbred mares occur between 15 and 45 days after ovulation [3], early embryonic death due to progesterone insufficiency is rare. Only one out of 17 cases (6%) of early embryonic death could be attributed to low progesterone (<4 ng/ml) in a retrospective study on 287 Standardbred mares [4]. The literature search revealed 2 individual case reports of luteal insufficiency which resulted in the birth of a healthy foal: one case without adjunctive hormone therapy [5] and one case with supplementation of Regumate™ for the first 150 days of pregnancy [2].  […]