I was just wondering if there were any percentages of woman who are on some form of birth control? Say, women between 16-40 or so. Pills, IUDs, shots, etc.
Are you interested only in the forms of birth control that are used independently of actual immediate need, like the pills, IUDs, and shots that you mention? Or are you also interested in forms of birth control that are only used when sexual intercourse is imminent, like diaphragms, and foams/jellies? The three forms that you mention are by prescription only. Do you want to know about only those methods that require medical intervention, or about all methods? What about condoms? What about permanent forms of birth control like tubal ligation? What about the “morning after” pill?
In other words, maybe you could clarify your question.
Yes. Sorry I was vague. Non-premanant, ongoing birth control.
This FAQ from Planned Parenthood doesn’t have the exact answer you are looking for, but it might get you started in the right direction:
FWIW, here’s an interesting article on the history of birth control that I found on the site:
Here are some stats from a 1995 US national survey. These same stats also show up in the faststats of CDC’s National Center for Health Statistics as well as the Statistical Abstract of the United States, 2001 (Census Bureau), which makes me wonder if there hasn’t been a more recent comprehensive survey.
Here’s some hard stats from the CDC:
http://www.cdc.gov/nchs/datawh/statab/pubd/2319_41.htm
And a page that might have some other useful links for you:
I’d guess at most half.
I’m sorry, sometimes I just can’t help myself… 
Eric