Congrats, Teresa! Glad you got positive news on the testing for Down's. And, my word of caution title is not related to anything about testing, but rather about buying too far ahead on clothes for you LO. First off, every manufacturer sizes their clothes differently (some run true to size, a few run large, and many run way small!). Second, there is no way of knowing whether you LO will be average, small or large for his/her age, which significantly impacts how the manufacturer sizing will work for you LO. My DS was born in March and was a little larger than average at birth - 7 lbs 11 oz and 21 1/4 inches. He is now in the 95th percentile for height and is still average for weight. He is about to grow out of 6 mo clothing for most manufacturers and is already in 9 mo clothing for others - and he is just over 4 mos!
I did find that by the time it really started to get warm this Spring, many stores already had their summer clothing on sale. So, I was able to buy a little bit ahead for his summer clothing needs and return as needed! His growth has been a little more than I (and the ped) expected recently, so I am scrambling now to make sure he will have some hot weather things to carry him through September since I am not sure how much more he will grow between now and then. I dare not think about Fall clothing at the moment, as I have no idea where DS will be by then. I will likely get a few things in late Aug and wait for the Fall clothing to go on sale before buying a lot.
Congrats again! Also, you are smart to go ahead and buy baby infrastructure ahead of time. That stuff really takes a hit on your wallet and it is nice to have it done ahead of the holiday rush.