Do you know if she has ever had a foal before? Maidens may show a little differently than a mare who has foaled before. If she was bred in September, she should be showing signs that she will be foaling soon. Is she showing signs of bagging up? That is her udder getting bigger? With a maiden, she may bag up then have it go down. If it is, it's a pretty good bet that she is pregnant. A mare may start bagging up a few weeks before foaling or a few hours before foaling.
If she is bagged up, has wax on the ends of her nipples, the muscles around her tail head are soft and loose and if she is showing colic like signs, stomping her feet, looking at her sides and rolling, expect a foal very soon. Until you can get the vet out, make sure the area she is on is clean, a large clean stall or a grassy pasture, in the event she foals before the vet comes. Make sure the stall or pasture is secure so the foal can't get out. The vet will be able to tell you for sure if she is in foal or not.