Vaccinations a month in advance of foaling are important to the foal with respect to his own immune system but the vaccination of the mare to prevent her own illness during pregnancy is also extremely important. Fetal development is very sensitive to temperature. Abnormalities can occur following even short periods of temperature elevation in the mare. Some conditions such as equine herpes 1 and perhaps equine herpes 4, can cause abortion. However, not all foals will abort due to an ailment. The neonate may actually hit the ground looking healthy and then in a rapid decline, succumb to the effects of an infection acquired in utero.

     Many toxins and organisms that are released into the mare’s blood stream during an illness can also victimize the fetus. A mare afflicted with a reproductive tract infection is at high-risk of producing an sick foal. The fetus may be exposed in utero or even more commonly, the organism infects the neonate during parturition by exposure in the birth canal, with the foal becoming ill shortly after delivery. Even a case of colic can put the neonate at risk. A mare suffering from any inflammatory bowl disorder that results in endotoxemia, poisons the fetus with toxins as well.

     If the mare does become ill or is injured and must receive excessive amounts of medication during her pregnancy the foal’s health is again at stake. The tissues of the fetus are developmentally sensitive to outside agents (teratogens), due to the fetus’s accelerated cellular process. Any medication or pharmacologic agent that acts systemically or is distributed by blood can cross the placental barrier and harm the fetus. This concern is greatest in early pregnancy when the fetal organs and related systems are being developed. Damage usually will result in malformed organs. Often these foals are aborted or stillborn. During later gestation, teratogens will more likely affect the function of an organ rather than structural integrity. Foals will generally be born with a normal appearance, eventually showing signs of system failure over a period of a few hours to days to weeks, depending on the severity of the impairment.

     High quality fetal development requires that the mare receives a balanced diet with sufficient nutrients. A poorly nourished mare is in a high-risk condition. She will not be able to supply the needs of the foal and in severe cases may produce a dysmature foal. Poor attachment due to scar tissue can also prevent the fetus from acquiring the needed nutrition.

     Knowing the gestational age of the fetus in advance of the due date by means of an ultrasound, will aid the owner in providing the special care the endangered newborn requires. Likewise, twin fetuses may compete for the limited supply of placental nutrition, rarely terminating in the birth of even one healthy foal. A mare that has a history of twinning should be examined at fifteen to twenty-five days following breeding. If a twin conception is evident the ultrasound will aid in a reduction of one of the twin embryos.

     Acute stress during pregnancy can endanger the fetus as well. Anything from trauma and bleeding to surgery and its associated anesthetic can precipitate a stress-induced inability to fight disease. The inhibition of the normal immune response system (immunosuppression) can even follow a stressful transport of the mare. When her immune system is down she becomes vulnerable to infection, risking the fetus’s health. If the system is down at the time of "making bag," it is doubtful she will produce enough antibodies in her colostrum for the foal.

     Mares who are unable to produce milk (agalactia) or those that drip milk well in advance of parturition endanger the health of their newborns. Colostrum or "first milk" contains vital antibodies essential for the protection of the foal from infectious diseases. A successful transfer of immunity requires adequate production of colostrum and ingestion of the colostrum by the newborn within the first twelve hours of life. In either of these cases they may not fill the foal’s needs for colostrum.

     Any mare who has previously produced a foal with neonatal isoerythrolysis may risk the health of her subsequent foals. This blood-incompatibility disease, also known as yellow foal or hemolytic disease of the newborn, is caused when the mare’s immune system becomes sensitized to the foal’s red blood cell type. This can happen through a blood transfusion but more often during a previous traumatic birthing in which some of the earlier foal’s blood leaks into the mare’s circulation. The mare forms antibodies directed against that foal’s red blood cells and can later transmit the antibodies to her subsequent offspring. Should the offspring be sired by a stallion having the same red blood cell antigens to which the mare has become sensitized to and the newborn ingests the mare’s antibody-rich colostrum, the foal will become anemic as the antibodies destroy the neonate’s own red blood cells. Screening of the mare for increased levels of antibodies directed against red blood cells should be done before foaling. If detected, steps can be taken to insure the foal’s well-being.

     Some conditions are more apt to carry a high risk of dystocia (difficult birth). Mares that are reluctant to move due to chronic lameness or incoordination, are more likely to experience a potentially perilous birth. Such impediments may prevent the mare from properly positioning the foal for delivery. The smooth transit of the foal during birth can also be hindered by anatomic abnormalities of the pelvis such as a tumor, a past pelvic fracture or poor development of the mare during her growth years.