Should I or shouldn't I?

Saw this divine dress today, gorgeous leaf green floral print - empire line, linen blend. Just what I've been looking for: trans-seasonal (could wear it as is in summer, with a merino and knee high boots right now!), some colour for my wardrobe, something pretty to wear to S's 2nd birthday party. Cons: doesn't quite fit across the boobs and is just a little baggy across the small of my back.....it was on sale for $69 reduced from $499 and I totally love the fabric.....not sure if I should buy it and hope that the weight I lose in between now and the 2nd of September will make it fit just perfectly or if I would be wasting my money.....I think I'll sit on the idea until Wednesday - if it's still there then I'll decide.