Thanks a lot for all the responses, I am now officially out of panic mode

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I did ring up a few places around Sterling and indeed no mention of waitlists, most places reckon they were likely to have availability around then and prices quoted between $1200 and $1700 a month, priciest being for a Montessori center. So now we are not going to bother about going on waiting lists, at 150$ a pop we feel it's daylight robbery... Also not knowing the place we might end up securing a place somewhere we don't like or very impractictal locationwise.
So we'll investigate once we have found where we want to live and if we don't find anything right around where we live, we can put her somewhere around Sterling, where my husband will work, until we get something closer.
Nearly making me wish we could live in Sterling as everything seems a lot more affordable, but I'll be working in DC and I think it'd be a nightmare of a commute. Although I hope the Arlington - Sterling/Ashburn commute won't be too dreadful for my husband, I've looked online a bit, but it's hard to find an accurate estimate. If anyone has any ideas, I'm all ears

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Thanks again for the very useful advice !!
Marion