300 Million

The Census Bureau says that the US will reach 300 million people at 7:46 am Eastern time on Tuesday. How can they know that? And will the parents of the 300 millionth person know about it when the baby is born? I'm assuming this means that the 300 millionth person will be born; but could it be someone moving here, or gaining citizenship, or illegally crossing the border?

CNN is making a big deal out of this. I suppose it should be a big deal- 300 million is quite the landmark. But their headline is "At 300 million, US enters uncharted territory." Every baby born or citizenship granted is "uncharted territory." There won't suddenly be a host of new needs the country has once that one baby is born.