Do your chances of getting pregnant increase the longer you have an IUD in?

Getting pregnant on the pill is unfortunately not that difficult. Miss a pill, or be on a pill that does not contain a high enough dose of hormones for your body and it's about like taking a sugar pill everyday.

Except in some drug interactions, the amount of hormone is always strong enough to be used as contraception. Otherwise, sometimes the amount of hormones isn't appropriated and you experiment some side effects. In those cases, changing for a different pill would be preferable (we might say "this pill wasn't strong enough for you"... but that NEVER means that you were at risk for getting pregnant). The most common factor is when you miss a pill.

IUD's work by not allowing a fertilized egg to implant in the uterine wall. The fact you got pregnant on the pill is not actually going to have an effect on how an IUD works. However I strongly suggest that you discuss all the risks of IUD's with your doctor because they have in some cases cause permanent damage to woman's uterus. If you intend to have children in the future this could be complicated.