Scarleteen

If a woman has a 34 days cycle, it means she ovulates at day 20 (33-14).

Now, let' say she has sex on day 7, wouldn't it be very safe?
Why is there claim like "there is no such thing as 'safe days'"?

I was wondering could actually go wrong? What's the chance?

Thanks in advance.

Thing is, it doesn't mean that. People's fertility cycles are much more diverse than that, and ovulation can't be determined accurately based on the length of the menstrual cycle.

You're asking for information on natural family planning. We've got a good primer on that here, which answers these questions well: Get With the Flow: All About FAM. Why don't you take a look then pop back here if you've still questions?