How sad to get your mare in foal, be looking forward to the foal's arrival, and then have the mare lose her foal before full term.
In Victoria the equine breeding season - those months when mares normally foal and are re-bred - is considered to stretch from August to February. The "off season" is thus considered to be March to July in this part of the world. During these months owners of pregnant mares must still be vigilant to make sure that the pregnancy is going smoothly and the mare remains healthy. Ideally mares should be checked at least once a day, with a brief inspection to include a glance at the udder and under the tail.
By the way, these months are not the natural breeding season for horses, which is actually centred around the longest dayof the year - 22nd of December. This is because of all the environmental factors which affect the breeding behaviour of mares and stallions, day length is the one that has the strongest influence. This is no doubt the reason why we always find mares are easiest to get in foal, and tend to have less trouble foaling, if they are served between mid-October andmid-January.
Normally a mare will carry her foal for 330-340 days, and up to 380 days is still considered normal. At any stage of the pregnancy abortion may occur, but most likely will happen before 50 days of pregnancy ("early embryonic death") or between the 7th and 10th month of the pregnancy.
A mare that loses her foal early in gestation may not show any signs prior to the abortion. Later in gestation, the two most common signs of impending abortion are premature udder development and vaginal discharge.
In a normal pregnancy the mare will start developing an udder 2-3 weeks before she gives birth, so if she starts bagging up much earlier it is important to have her checked by your veterinarian. Vaginal discharge also warrants a call. Fever, colic and unusual behaviour are other possible indicators of an upcoming abortion.
Owners and breeders must also be aware that some mares can abort foals without any warning signs, and foxes and crows may remove the dead foal quite quickly. This underlines the importance of regular checks on pregnant mares.
In the event of warning signs, a veterinarian will perform an ultrasonic exam on the mare and the foal, investigating the cervix and uterus, the placenta (thickness, smoothness, degree of separation), amniotic and allantoic fluid (clarity, amount of debris), and the foal (size, heart rate, movements). Depending on the results of the exam the veterinarian may prescribe some medication such as anti-inflammatories, hormones, and antibiotics.
If a mare does abort her foal, her own health may also be at risk
Generally the foetus will die before the mare aborts and therefore the foetus cannot position itself properly in the birth canal to enable a normal delivery. Some signs that the foetus has died include a fluid discharge, a bloody tail of the mare, and a string of tissue hanging from the mare's vulva. In this case the mare will need assistance with the delivery - best given by your veterinarian.
Once the foetus has been delivered, you must ensure that the entire placenta is passed in a timely manner (preferably within 3 hours, never longer than 6 hours). The mare can develop a uterine infection if all of the foetal membranes have not been passed. This type of infection can cause subfertility, and also has the potential to produce a severe condition called endotoxaemie that in turn can lead to founder and even death of the mare.
This placenta from an aborted pregnancy clearly shows the line of diseased tissue which has ascended with the infection from the cervix. This is termed "ascending placentitis"
When a mare loses her foal it is important to make sure the abortion does not cause more problems, especially with other pregnant mares. Which means disinfecting all surfaces that came incontact with the aborted material and isolating the mare from other pregnant mares.
An autopsy of the foetus and the placenta, optionally including laboratory tests, may reveal the cause of the abortion. Possible causes include placentitis, viruses, twinning, poisonous plants, caterpillars, insufficient placentation, defects of the umbilical cord, and many more. For your veterinarian to carry out an effective examination post-abortion you will need to provide the dead foal and all the membranes, in a bucket with a lid or a strong plastic bag, as freshly as possible after the abortion has taken place.
Also useful is a comprehensive reproductive history of the mare, and details of mare management such as paddock size, companion horses, types of gestation, and usual feed.
Here is a picture of a foetus at approximately 8 months, euthanased along with its mother who had inoperable cancer. It demonstrates that the aborted foal at this stage is fully formed, and gives you an idea what size to expect if your mare is unfortunate enough to abort in the off-season