So, you have anxiously awaited that 340 day mark for your mare... you have judiciously made sure that you removed her from fescue grass, and have been slowly increasing her grain, and have groomed her, loved her, and made sure she is good with you touching her udder... YOU ARE READY!!! But is she? IF ONLY! Day 340 came, and went... and nada... nothing... udder development is unimpressive, and she is slowly getting bigger and bigger. You watch and wait until day 350... OMG!!! Is she ever going to POP! Do I worry? She seems fine, but how long is too long? I was induced with my second chilid because he got too big... man- don't want to remember that... Can you induce a mare? SHOULD you do anything?
Short answer is "TAKE A BREATH and RELAX." Your mare is nothing like you... Humans and horses are very different species, and are managed very differently. Inducing horses is typically not advised- even when significantly overdue. Why? Horses' placenta begins to detach when the water breaks. If the foal isn't in proper position and the water is broken (as in a human), you could actually be causing a dangerous situation which could kill a mare and/or her foal. If there are some extenuating circumstances and your veterinarian determines that she should be induced, this should be done only under close veterinarian supervision.
Some facts: The Average gestation on a mare is 340 days. Mares can go as early as 320 days and the longest horse gestation with a healthy foal was 400 days. As I write this, I have one mare who is turning 364 days tomorrow. It is nerve-wracking, to be certain. But there are some things you can check on to be certain that your mare is still doing well. Check temperature, pulse, and respiration. Normal temperature is 99-101.5F (37.5-38.5C), normal pulse is 28-48 beats per minute, and normal respiration is 10-24 breaths per minute. If mare's vitals are within normal limits, and she does not appear in distress, if mare is eating and drinking normally (perhaps a reduced appetite due to decreasing abdominal cavity space), and that alien inside her is still moving around some, then all signs point to "Baby just isn't ready yet". If your girl is clearly in distress, or any of her vitals are beyond the normal limits (up or down); if she has a discharge from her vulva or you don't see the baby moving anymore- these are "red alert signs"! Call your veterinarian immediately- DO NOT WAIT. Besides the "red alert signs" for overdue mares, if all signs point to normal- just taking too long; then sit back and relax- grab a pizza and a good book. These girls don't have a handbook, and she will have her baby at the least convenient time for you.... just to say she did! If you watch her-she will wait, if the temperature isn't perfect-she will wait, if the dog keeps barking or the chickens keep cock-a-doodle- doooing- she will wait... BUT... good news is that baby won't stay in there forever- he or she will have to bless this world with their presence, and all your patience, waiting, and fretting will see a new grandbaby into the world! Happy foaling, relax and breathe with your overdue mares, and best of luck- the best blessings are worth waiting for!!