My husband and I are trying to conceive our second child. I've always had very regular cycles before (28-29 days), but my last two periods both came a week early. What could be causing this? Will it continue? Help, I'm going crazy.

L.J.

Age, weight, thyroid status, prolactin level, insulin resistance, stress, hypothalamic dysfunction, exercise and dietary changes all can result in disturbances in the normal menstrual cycle. Also, as a woman approaches menopause, the menstrual cycle length often shortens.

In a normal 28-day cycle, counting the first day of menstrual bleeding as day 1, ovulation typically occurs around day 14. I would recommend you monitor your basal body temperatures and cervical mucus pattern and/or test your urinary LH levels with a home kit to predict the time of ovulation. (A good guidebook for this is Toni Weschler's Take Charge of Your Fertility.) Once you know when you are ovulating, you can determine whether the shortening is occurring in the follicular phase (the first half of your cycle, from the start of your period to ovulation) or in the luteal phase (the second half, from ovulation to the start of the next period).

A shortened luteal phase is probably not a cause for infertility. However, a shortening of the time interval before ovulation is often seen in women withdiminished ovarian reserve. If ovarian reserve testing is normal, then ovulation induction (regulating ovulation through the use of medication) can often correct the problem and enhance fertility. Without a thorough medical history, physical examination, ultrasound examination and laboratory testing to determine the cause of your cycle changes, I cannot predict whether your situation will reverse itself or become progressively more abnormal.