FionaS,
when you start IVF they give you a blood test on day 2 or 3 of your cycle to measure your FSH that is folicle stimulating hormone. raised levels of this means your body is working harder to produce an egg to release each month which inidctes your egg reserve is diminishing. that is the only real way of measuring declining fertility.
i think your OB sounds right with the cycle length. I was always a regular 30 days but after I had my DD at 40, it slipped back to 28 and now it is about 26 with some even shorter ones thrown in. we have been TTC number 2 for 5 years and my cycle length backs up that I am no longer fertile. But my FSH is still OK funnily enough. i think if you are charting and find you are Oing regularly around day 14 , you should be fine. good luck