A useful figure to know

BBC - Future - Health - Sex: What are the chances?

I missed this article from earlier in the year, but I reckon it has a good figure that is handy to know for anyone who needs to talk to teenagers about consequences:

 The bottom line is that a single act of intercourse between a young couple has on average a one in 20 chance of pregnancy – this assumes the opportunity presented itself on a random day, as these things tend do when you are young.

 However, the article then goes on to talk about the effectiveness of various forms of contraception, but only seems to quote figures if they are used correctly.   This seems a bit of an oversight.