You can conceive until your monthly cycles end completely (menopause). Perimenopause (the time before menopause and more likely what you may be experiencing) is still a very fertile time, though you may have some annovulatory (no ovulation) cycles making it difficult to conceive when *you* are ready. So you may have a regularly fertile month, then the next month things on the surface or the same, but you may not ovulate making conception impossible. This is common stuff during perimenopause becuase your body may have shifting hormone levels, making it more challenging for your body to produce an egg each cycle. But rest assured, most doctors say it isn't over until it's actually over - you are no longer having any period at all for 6 month or longer.
There is no way to know if this is happening unless you chart your daily temperatures (using the natural family planning method) or you have a series of blood tests done to check your homone levels (a doctor will draw blood several times during one or more cycles to measure hormone levels).
So in short, it may be a little more difficult to get pregnant during the perimenopause years, it is not impossible. It just becomes more challenging getting the timing down as cycles may become more unpredictible. So if you want to conceive, continue taking prenatal vitamins, refrain from smoking and drinking, exercise and eat right so you will always be at your optimum health.
There are many great books out there on the topic of fertility and life stages. Read any books by Shelia Kipley, or Toni Weschler (sp?). They will help ease your mind and may even help with conception.
I highly recommend "Take charge of your fertility" by Toni Weschler and "Fertility, Cycles, and Nutrition" by Marilyn Shannon. "The Art of Natural Family Planning" by Shelia Kipley.
It is little known fact, that using Natural Family Planning can greatly improve your ability to conceive despite age. You may want to find a OB who understands and uses the Creighton Method of Natural Family Planning. Do a google search, and you should find one in your area. Otherwise, go to www.ccli.org and contact a clinic in your area. They should be able to refer you to an OB in your area that can help you.