Instead of "blaming it on the rain," Milli Vanilli could have just as easily written a one-hit wonder called Blame It On Your Hormones. Because, let's face it, hormones are one of those things: You can't live with them and you can't live without them. It pretty much feels like they can be blamed not only for a ton of good (which they don't always get much cred for) but also a lot of bad.

Whether it's that you feel your PMS is going to be the death of you this month, you can't seem to lose those last five, your mood is on an emotional rollercoaster, or your libido is non-existent,the common denominator could be, and often is, estrogen. "Estrogen is a hormone that is produced primarily in the ovaries - and there are actually three kinds - but estradiol is the one that's predominantly associated with your ovaries and reproduction," says Jacqueline M. Thielen, MD, a specialist at the Women's Health Clinic at the Mayo Clinic in Rochester, Minnesota.

And estrogen is a pretty big deal when it comes to your health, how you feel and how you look. Here's why: there are essentially estrogen receptors everywhere - from your brain, to your hair, skin, heart, uterus, lungs, bones, breast tissue and, well, just about every organ in the body. "Think of estrogen as a chemical that promotes the transportation of information that helps all these different kinds of cells to function in a particular way," explains Dr. Thielen. Of course, the first big shot of estrogen is during puberty, but this hormone remains a major player in your body for the rest of your life - especially the baby-making years - right on through menopause. Here's a little walk down estrogen lane. We'll help you understand the role this curious hormone plays in each phase of your life.

In Your 20s And 30s
Puberty is long over (phew!) and your cycle is probably on the somewhat reg. Whether you want kids right now or not, your body is in its baby making prime, which means estrogen is in full effect. "Estrogen is key during these years because it is the hormone responsible for getting the uterus ready for implantation and the growth of the fetus," explains Dr. Thielen. But depending on where you are in your cycle, estrogen fluctuates - peaking about a week and a half after your period (particularly estradiol).

If estrogen is flowing in the right amounts and at the right time naturally (as in, you are not on The Pill), your skin is probably clear and your libido is up (a telltale sign is an increase in cervical mucous, your body's natural lubrication, usually around mid-cycle - right before ovulation). If you're on hormonal birth control (a.k.a. The Pill), it basically takes over your body's estrogen production - and could impact some of the above, especially lowering your sex drive. "Your body still makes estrogen, but The Pill tricks the body into thinking it doesn't need to make quite as much and that it doesn't need to trigger your ovaries to produce as much estrogen and to ovulate each month," explains Rebecca Brightman, clinical instructor of Obstetrics, Gynecology and Reproductive Services at Mt. Sinai School of Medicine.

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If estrogen levels have gone haywire, it's probably why your PMS (and you) feel out of control pre-period. "For women who get severe PMS symptoms, that typically means they're more hypersensitive to hormonal fluctuations. For example, if your estrogen levels drops before your period dramatically, it is the fluctuation, not necessarily the exact level of estrogen itself, that bothers women and can lead to common PMS issues such as bloating and irritable mood," says Thielen.

Take comfort in knowing that docs say there is no "normal" range of estrogen, per say. "It really depends on your age and, if you are in your reproductive years, where you are in your menstrual cycle," says Dr. Brightman. "Estrogen peaks mid-cycle, and since menstrual cycles vary in length, estrogen levels fluctuate throughout each cycle." But dramatic drops in estrogen could affect your fertility - because, again, it is essential to ovulation. Estrogen typically starts to drop around 30 and then takes another nosedive at 35. This means that by your early or mid-30s you could notice signs that your estrogen levels aren't what they used to be (somewhat lower libido and some weight gain) though perhaps you wouldn't notice until, say, you have trouble getting pregnant and find out there is reason for concern from your physician.

In And After Your 40s
While menopause typically doesn't occur until 50-something (the experts interviewed say that 51 is actually the average age), perimenopause can occur up to eight years before a woman's last menstrual period. Even if that's not the case, estrogen levels do start to fluctuate dramatically when you hit your 40s, which means you aren't ovulating like you used to, and you could also see other side effects that weren't an issue in your estrogen-rich 20s and 30s - a drop in sex drive as well as more vaginal dryness and just-won't-budge weight, especially around your belly and bottom.

And then there's your mood: "Once you get to an age when your ovaries are not functioning as readily, you can start to notice a monthly pattern of more of a roller coaster of hormonal changes," says Dr. Thielen. At 50-something if menopause hasn't started yet, it inevitably will soon, and while estrogen levels aren't nonexistent, they are quite low. Along with the aforementioned side effects of subpar estrogen levels, Dr. Brightman says symptoms of menopause such as hot flashes also arise, as well as decreased production of collagen and, therefore, lax skin and wrinkles get fast-tracked.

And, while we often think of deficient levels of calcium and vitamin D as the main culprits of bone issues, including decreased bone density, estrogen is key, too. "Women achieve maximum bone mass in their 30s and start to lose bone mass at the onset of menopause," says Dr. Brightman. "Estrogen is responsible for facilitating the absorption of calcium into bone - so a loss of estrogen can cause a depletion of calcium needed to maintain a healthy bone mass." And just like how the crazy ups and downs of estrogen can throw you in a tizzy, the super-low levels during these years have also been linked to changes in mood, anxiety, and depression, explains Brightman.

What all of the above could mean for you: Estrogen levels are going to ebb and flow, depending not only on your age but also other aspects of your life - from your health to stress and diet. This also means that the scale may go up or down, you might want to tackle the world or hide under the covers, and you might want to get it on every night or not at all. The good news is that you don't have to add stressing about what your hormones are (or aren't) doing every second of your life: "Despite that fact that estrogen levels change with age, women are often resilient and know their bodies and know when something isn't right before a doctor can - and if that's the case, a woman should be persistent in seeking answers. They may need a blood test to see why things are changing, and it may not only be their estrogen levels," says Thielen. "But know, if perimenopause is occurring, that doesn't always mean something needs to be fixed, so it's important to know the symptoms and how your body is reacting and then discuss that with your doctor."

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