You've potentially spent thousands on service fees, shipping and vet fees and the mare is hopefully still pregnant. What happens now?
Well there are different places in the uterus where the embryo can attach. Ideally, you want the attachment to occur in one of the horns. This is common and gives the best outcome for the foal. Attachment within the body of the uterus isn't great and often leads to spontaneous embryo loss. Attachment near the cervix is disastrous and will definitely result in embryo loss.
But let's assume everything has gone according to plan (we'll go over various problems later).
The average gestation length is 340 days. "Normal" ranges from 320 - 360 days, but this varies depending on who you talk to. The longest recorded gestation is 432 days (as far as we know), so don't get too hung up worry that your mare is overdue. It's not bad, dangerous or abnormal - we'll explain more later.
From around day 24* the embryo is about 1/2 inch in size and a heartbeat can be visualised on an ultrasound - assuming your vet has the fancy kind.
Around day 40* the embryo is about 3/4 inches in size and starts to resemble a foal shaped blob inside the embryonic vesicle.
Between days 60-70 and 110-140* the gender of the foetus can be determined. However, in our experience, the average vet is only right about 50% of the time. 😂
Around day 150 the foetus is about the size of a rabbit and has started growing hair.
Day 270 the foetus looks like a foal! It has a complete set of fur, mane, tail, fully formed limbs, organs even eyelashes.
As the foal matures, the finishing touches are put on. Some studies have shown that the longer a foal stays in the womb, the more white develops. That's pretty cool!
The final 10 days of the pregnancy are extremely critical. This is when the lungs are matured to a point where they can function. Without this final 10 days maturation the foal stands little chance of survival but the problem is you don't know WHEN the final 10 days are supposed to occur. Is it between 330-340 days? Is it 360-370? Is it 420- 430 days? You just don't know.
This is why you NEVER induce a mare. It doesn't matter if you think the mare is overdue or it's been too long. The foal will be born when it's ready to be born. Rushing them along is almost always a bad idea. The only time you would induce a mare is when her life is in danger or something has gone terribly wrong with the foal/uterus.
In the 24 - 48 hours before birth there is a spike of adrenaline and the foal completes the final maturation. The mare’s udder should be full by now and might even start dripping or waxing.
It's also good to note that you should be careful not to over feed a pregnant mare, and do not under feed a mare with foal at foot.
MCKINNON, A. O. (2011). Equine reproduction. Chichester, West Sussex, U.K., Wiley-Blackwell.