I had this problem a couple of weeks ago. I have my sister's wedding in 2 weeks and have been a size 18 for nearly 5 weeks now. I had hoped to be in a 16 for the wedding and in a few things I am.
I played it safe and have bought a dress in both sizes so I can wait right until the last minute before choosing.
I'd really suggest doing this to be on the safe side.
I really hope you do make it into a 16.
I think i depends where you carry your weight hun. I am very pear shaped so can get away with 14 tops and dresses (sometimes even skirts as my waist is small.........ish) but need a 15 in jeans at the moment lol. I am easing my 14 jeans slowly over my ample arse and then they button up fine. Go for the 16.
Order the 18 now, delivery in a few days - try it and see the fit - then if you feel like if you might be better with the 16 (only you know how your body has adjusted itself over a 3 week period) you will have plenty of time to send it back, get the refund and order the 16.