I do not have "irregular periods" (that's when you have periods that come at random--skip months etc)--I do have long cycles, though that average between 33-52 days (52 was the longest ever) and this is most often due to a hormonal imbalance. Other things that can affect cycle length are diet, stress, post-partum issues, breastfeeding (or having breastfed in the previous year) as well as underlying illness.
The culprit is often progesterone. Many times doctors prescribe birth control pills in this case--which is not a very wise idea as the progesterone level that makes your cycle longer is only in play toward the end of your cycle (after ovulation) and if you are taking birth control pills all the time it just masks symptoms but does not get to the root of the problem. In your case, because you are TTC birth control is not even an option.
I recommend looking into natural family planning. A very standardized, concise, clinical and researched method that I have been using is the Creighton Method which is also used to study and diagnose womens' health issues.
http://www.creightonmodel.com/
For specifics dealing with hormonal imbalance:
http://www.naprotechnology.com/infertility.htm &
An OPK can help as it can predict ovulation but there are other factors in a woman's cycle that those trained to look for them can see that can further pinpoint why she cannot conceive. By charting your cycle you can gain immense insight into the overall health of your own body as reflected in your cycle and as we all know, that overall health affects conception.
For example, it is possible for a woman to be conceiving every cycle because the first phase of her cycle (from her period until ovulation) is fine because the high levels of estrogen that affect that phase are fine but at the same time she can be repeatedly losing pregnancies by spontaneous abortion (which she may not even be aware of) because the egg cannot implant properly in the uterine lining. In this case, the culprit of her infertility is not her ability to conceive, it is probably the fact that her progesterone levels (which control the thickness and health of the uterine lining which directly affects implantation of the egg) may be off kilter. Progesterone is the key component then from ovulation until the end of the cycle.
Until the underlying hormonal problem is addressed she could be actually getting pregnant every cycle yet having a period as she loses the pregnancy--even before the HCG level in her body is high enough to read positive on a home pregnancy test. This is just one example.
NFP usually not only can help pinpoint the reason, but offer comprehensive, holistic treatment and help her achieve a healthy pregnancy without invasive, expensive (often ineffective treatment). Most fertility treatments report about a 25% rate of success in helping a couple conceive whereas NFP usually reports around an 85% rate of success--it's low-cost and worth a try if you haven't looked at it.
I know for me, that this method has helped a ton at addressing my female health problems I have had for over a decade that no one else could treat.
Hope this might be helpful. :0)
