A long gestation period makes mares vulnerable to complications. | Photos.comOne of the defining features of pregnancy in horses is the lengthy gestation period, ranging from 320 to 360 days and averaging around 340 days. Generally speaking, if pregnancy loss in horses occurs after hormone-producing endometrial cups form in the uterus between 30 and 45 days of gestation, she cannot conceive again until the following year. Because the timing is so delicate and the stakes are often high, horse owners and veterinarians focus heavily on identifying and avoiding potential abortion triggers in mares.Lauren Pasch, DVM, Dipl. ACT, CVA, of Rhinebeck Equine, in New York, reviewed the key risk factors for equine pregnancy loss at the 2025 Cavalcade Education Equine Reproduction Seminar, held on Jan. 8 in Red Hook, New York.What Makes Equine Pregnancies VulnerableA long gestation period isnt the only thing that makes mares vulnerable to complications. Pasch listed three other unique elements as innate risk factors for abortion:1. Long pre-implementation period. Horses have a unique type of placentacalled a diffuse, epitheliochorial placentaand the embryo doesnt fully implant until Days 40 to 42. This prolonged pre-implantation period makes the early stages of pregnancy particularly vulnerable.2. Delicate maternal recognition process. The embryo moving around the uterus signals the mares uterus not to release prostaglandin, which would terminate the pregnancy. Progesterone, produced by the corpus luteum (CL) initially and then by the fetal-placental unit, is essential for maintaining the pregnancy. Progesterone insufficiencies can lead to abortions.3. Placental Inefficiency. The horses placenta is remarkably thick and inefficient, with six layers of separation between the fetus and maternal blood supplies, Pasch explained. This makes the placenta highly susceptible to disruptions and insufficiencies.Risk Factors for Early-Term Pregnancy Loss in MaresMost equine abortions occur before Day 42, when the primary CL remains the sole source of progesterone. Significant risk factors for pregnancy loss include:Previous abortion;Advanced maternal age;Systemic illness or pathology; andChromosomal abnormalities.One area of growing interest is aneuploidy (an abnormal number of chromosomes) and chromosomal misalignment, Pasch explained. These chromosomal abnormalities are almost always lethal to the fetus.Risk Factors for Late-Term Pregnancy Loss in HorsesOnce past the early, somewhat precarious stages of pregnancy, late-term abortions can still occur due to many complications, including:Umbilical cord torsions, which cut off blood flow to the fetus;Various congenital (present at birth) anomalies; andPlacental insufficiency, often attributed to placentitis. Ascending bacterial placentitiscaused by infections moving up through the reproductive tractaccounts for most cases. Evaluating a mare for placentitis involves rectal ultrasonography to measure the combined thickness of the uterus and placenta (CTUP), Pasch said. If elevated, the CTUP indicates inflammation because the placenta is thicker than normal.Infectious Causes of Pregnancy Loss in MaresLastbut certainly not the least of broodmare owners concernsare infectious causes of abortion. Equine herpesvirus (EHV) and equine arteritis virus (EVA) are two significant ones, Pasch said. Biosecurity measures and strategic immunization remain crucial for controlling the spread of these viruses.Take-Home MessageInvoluntary abortion in mares can be complicated, with many potential causes linked to the long, intricate nature of their gestation, and a range of external factors. By understanding, avoiding, and mitigating underlying risk factors, veterinarians and horse owners can minimize the devastating impact of equine pregnancy loss.