How long can I wait before I have to purchase my wedding dress?

I’m on a serious weight loss regimen, because my (surprisingly realistic) hope is to drop 100lbs before my wedding next October. Obviously, this seriously impacts the wedding dress purchase. I wanted to have the weight lost by the end of August 2013, well in time for a late October wedding. But everyone seems adamant that I should order my dress no later than March. March-August could make a huge difference in the fit/ability to alter the dress. MoH suggested ordering the dress a size or two smaller, which makes me nervous… but she (and a few bridesmaids) insisted that if I don’t get the dress by March, I won’t get it in time.

What do I do?