Here is the breakdown of the three core counties (plus St. Louis City) on the Missouri side:
Click to expand
If the table is too small, or if you'd like to see more counties, please visit the Census website.
As you can see, the Census estimates that St. Louis City declined every so slightly from its July 1, 2008 population--a loss of 143 people. The current figure--356,587--is still significantly above the 2000 population of 348,189. This is probably a good sign in a city that has been shedding people for a half century.
If I were to guess what the April 1, 2010 Census base estimate for St. Louis will be, I'd go with a number right around the 2009 figure. With the recession and the difficulty moving residential units during the economic crisis, I doubt St. Louis has seen much more growth. Look for a figure of 357,000 at the highest. For what it's worth.
Continued, though slowed, growth in St. Charles County likely means that the 2010 Census will show that county overtake St. Louis City as the region's second most populous "county".