When a mare has her first foal, it can be difficult to predict her foaling date. It is not uncommon for an inexperienced mare to foal later than expected. She may also give misleading signals, such as premature udder development.
Luckily, if the mare is pregnant again, it will be much easier to figure out when she is due to foal! This is because each mare will follow the same pattern of behaviour every time she foals. The gestation length tends to be the same for every foal apart from the first one.