How can the pill affect chances of getting pregnant?

Decreases it.: If you mean later, once you are off of the pill - it does not decrease ones future fertility potential. Other factors do - if someone is older, say 39 years or older - then it can be harder to conceive; other medical conditions, obesity can all negatively impact fertility. When you are actively taking the pill with 'perfect' use, the chance of getting pregnant is only 1-2%.