I'm in the situation of not knowing when my mares were bred (four horses for the price of two, people!) and so I don't have anything better to go on than the vet's estimation of 8-9 months for one, and 5-6 months for the other (and this was on the 22nd of February). If we assume the more-pregnant-one was only 8 months on that date, she'd now be at around 348 days... and the other could be as little as 258 days! Argh! And they're in Chile, while I'm in Congo.
So basically I'm just hanging around waiting. Sigh.