Moodiness? Hot flashes? Sleep problems? It may be peri-menopause.

At this stage in my life — moving into my late 30s, with many friends rounding the corner into their 40s — I’m starting to hear rumblings, rumors and some early ranting about peri-menopause, and I don’t particularly like it.

Although it means a winding up of your reproductive years and seems to signal getting, um, old, there’s no real reason for dread or alarm, says gynecologist Margery Gass, executive director of the nonprofit North American Menopause Society. “Peri-menopause is a normal and natural phase of a woman’s life — it’s not a deficiency state and it’s not a disease,” she says, noting that peri-menopause ends — and menopause begins — when a woman hasn’t had a period in a full year.

While there’s a huge amount of variability in the timing of peri-menopause, it may be much earlier than you think. “It’s amazing to me the number of women who come into my office in their late 40s completely freaked out because their periods have changed and they didn’t think of this as a possibility,” says Ellen Whitaker, a gynecologist at MedStar Washington Hospital Center.

She explains that while the median age of menopause is 51, there is a “very wide range of normal” when it comes to peri-menopause, usually starting in the late 30s and occasionally stretching into the late 50s. Whitaker says that factors such heredity, diet, smoking and exercise play a role, and that there is no definitive blood or hormone test for peri-menopause. Instead, a diagnosis is generally made by “looking at your age, menstrual pattern, other symptoms and saying, ‘Yeah, it looks like you are peri-menopausal,’” she says. “It’s not particularly precise.”

So what, exactly, happens during peri-menopause? It starts with age-related hormonal fluctuations that indicate that the ovaries are “starting to sputter,” says reproductive endocrinologist Michelle Warren, director of the Center for Menopause, Hormonal Disorders and Women’s Health in New York. There is an extreme variability in women’s experiences. One of the first signs is a shift in your menstrual pattern, even if it is simply that your period arrives more than a week early or a week late.

“That’s usually the first thing, and then you could go to skipping cycles and then finally stopping completely, but in between everything is possible,” adds Gass. “Some women have normal cycles until one last one and that’s it, while some go through a more protracted stage of irregularity.”

The rise and fall of these hormones can cause other delightful symptoms, such as hot flashes, night sweats, sleep disturbances, mood changes and vaginal dryness. Although it’s not strictly related to hormonal changes, many women gain weight in the years leading up to menopause, particularly around their midsection.

New research also backs up my mom’s favorite peri-menopausal grumble: forgetfulness. “The data suggests that there may be some very mild problems in verbal memory and processing speed in later stages of [peri-menopause]. So the great number of women who report [them] should know there really is something to their complaints, and they are not alone or going crazy,” says clinical neuropsychiatrist Miriam Weber, an assistant professor at the University of Rochester, who has studied this issue. “The good news is that it looks like [they] are a temporary hit and that things rebound at some point in the first year following the final menstrual period.”