A woman’s baby-making potential typically starts to decline in her mid to late 20s and undergoes a steep drop around her 35th birthday. That said, many, many women conceive easily in their late 30s and even early 40s. To gauge whether or not you'll have fertility issues, ask yourself: Do you have irregular or very painful menstrual cycles? Do you suffer from a chronic illness such as diabetes, thyroid disease or PCOS (ovarian cysts)? Did your mother go through menopause early? A "yes" to any of these indicates that you might have trouble conceiving. But the best way to assess your chances is to start trying as soon as possible.
If you still aren't pregnant after six months, see a reproductive endocrinologist (aka a fertility doc) to get tested for specific hormonal issues that impede pregnancy, rule out your partner as a cause of infertility, and discuss your options. You can call in a specialist even sooner if you suspect you're not ovulating or that something just isn't right. Many fertility roadblocks are easily fixable, and modern medicine can do a lot to help the process along.