Well you can take it now, as soon as you miss your period, but you should take it with the first morning urine (its the most conentrated), but its best to take it at least a week after your missed pririod with first morning urine, and even then you still need to keep in mind that you could get a false negative because every body is different
but I guess teh waiting a week thing is to see if you actualy do get your period, just in case its late, as much as its for letting the hormonme build up (thats my guess at least)
~alicia~