Glad its not only me Westwinger!!! 
I weighed myself this morning & I've put on another 1lb!!! I wouldn't mind, except I've not been eating HUGE amounts & its all healthy ish. I mean, a semi-skimmed choc milk (Asda reduced sugar nesquik stuff), 2 slices of wholemeal toast, a small pain au chocolat at break, a chicken sandwich & fruit at lunch, maybe a biscuit/cookie afternoon & a normal dinner. Perhaps I can eat more fruit & less pastries/biscuits/cakes, but it isn't that different to how I'd normally eat. I can't understand it. Maybe it is twins??!!! 
And Passme I agree with Westwinger. I think you could get away with a normal frock. I've got a party this weekend, & admittedly its wraparound in style & I'm only 13wks then, but I'm wearing a normal size 10 dress. I'm about the same size & height as you & think I wore size 12 before going into maternity clothes by about 20wks (certainly I was in my normal jeans until then, maybe tailored work clothes I switched up earlier). I was almost back to my pre-pg size & weight (except the boobs) by 7wks after, as I was a bridesmaid for my best friend. They ordered me a size 16 to allow it to be altered, and I was 26wks at the fitting, & it was adequate!! I could get it on... Then inches were taken off post-DD. What I'm trying to say is that, if I could get into a (ungenerous, fitted style) size 16 bridesmaid dress at 6mths, then you will probably find a size 10-12 (depending on style) would be fine at 18wks. :)
Just looking at my pg diary to work out my size & dates for the above comment & I've seen I was already 66kg at 18wks. So if I put on the weight at the same rate, I'll only be a lb or 2 heavier than with DD. I suppose that's not too bad... I'm just clearly a heffalump during pg!!! 
And sorry for self-indulgent whinge. Honestly, I'm not normally this self-conscious or bothered about size/weight. I'm hoping its hormones. I'm taking a while to adjust to giving up my body again or something!!! 