When I was in my 20s and 30s, I was blissfully naïve about breast cancer. I thought it was something that I didn’t have to think about—or at least, not yet. I figured that when I hit my 40s, I’d start doing regular self-exams, go get mammograms, pay due diligence to something of which I was aware affected a lot of women around my mother’s age. I have no family history of breast cancer and was unaware that I actually had some of the risk factors for developing it: Ashkenazi Jewish heritage, never having had a baby, “dense” breasts…the list goes on. But I had no idea.

When I had my very first mammogram at age 40, the radiologist spotted some “shadows” on my breast which were hard to identify, so she sent me for an ultrasound and then a biopsy. And then, boom—I was diagnosed with breast cancer.

Thankfully, my cancer was found at an early stage and was treatable. I am now totally cancer free—it was removed and then the area was treated with radiation. Since then, I began a quest to learn as much as I could, to make up for all the not knowing in my 20s and 30s. Here are all the things I wish I had known:

1. Ashkenazi Jews and breast cancer—it’s a thing

Ashkenazi Jewish women are more likely to carry three mutations in the genes BRCA1 and BRCA2, which significantly increase the risk of breast and ovarian cancer. About one in every 10 Ashkenazi Jewish women with breast cancer and one in three with ovarian cancer have one of the BRCA1 or BRCA2 gene mutations. If you have a family history of breast or ovarian cancer, on either your mom or your dad’s side, you might want to get tested to see if you are a carrier. After my diagnosis, I was tested, and, fortunately, I don’t carry a gene mutation for developing breast cancer. My cancer was one of the majority of new diagnoses which don’t have a genetic cause (only about 5-10 % of breast cancers are genetic).

2. Even younger women should do breast self-checks

While age is the most significant risk factor developing breast cancer (the likelihood of getting breast cancer increases as you get older), still, about 12,000 women under the age of 40 will be diagnosed with breast cancer in America every year. Eight out of 10 of these women spotted something not right by checking their breasts. Regular self -exams are really important for women under 40 because mammograms aren’t as effective at showing up cancer in younger breasts, which tend to have more fibrous tissue, making cancer more difficult to spot. Before I was diagnosed, I wasn’t particularly diligent about self-exams. Now, I won’t ever miss a month.

Here’s what I’ve learned about how to do a self-check:

• Examine your breasts once a month; right after your period ends.

• Get to know your entire breast. Start at your collarbone and go down to the lower edge of your rib cage. Feel up into your armpit, too. The idea is to know how your breasts look and feel, so you can notice if there are any changes.

• Inside your breast there are nodules (pimple like structures). These feel round and smooth, and if you push them they might feel tender. If you feel something hard, especially if it’s not tender or/and if it is growing in size, call your doctor to schedule an ultrasound or a 3d mammogram (2d mammograms are not as effective in identifying cancer in women under 40).

3. There’s a connection between breast density and breast cancer

Until my diagnosis, I didn’t know that my breasts were “dense,” a fact which may have put me at higher risk for developing breast cancer in the first place. “Dense” simply means that breasts contain more fibrous than fatty tissue. Forty percent of all American women have dense breasts. Breast density is often inherited, so it’s worth asking your mother whether she knows if her breasts are dense.

Unfortunately, early stage cancers tend to spread faster within dense breasts, and women with dense breasts have a higher risk of getting breast cancer than other women. Dense breasts make it harder to detect cancer, too—cancerous cells blend in with all the fibrous tissue. The only way to know for sure if you have dense breasts is to have a first, baseline mammogram.

Many states recommend that women without a family history of breast or ovarian cancer have their first mammogram at age 40. But if you suspect that your breasts are dense AND you have a family history of breast or ovarian cancer as well, you may want to have a screening in your 30s. In some parts of the U.S and around the world, your doctor is legally obligated to tell you if your breasts are dense, so that you can make informed decisions about screenings: For dense breasts, 3d mammograms are better than 2d, and ultrasounds and MRIs work even more effectively than mammograms at identifying cancer.

4. How old you were when you got your first period is important

If your first period arrived sometime before you were 12, you have a higher risk of getting breast cancer later on. This is because the earlier puberty hits, the earlier your breasts form, and the sooner they’re ready to interact with hormones inside and outside your body, as well as with chemicals in products that are hormone disruptors. This longer interaction with hormones and hormone disruptors can increase breast cancer risk. If this is you, you can reduce your risk by eating well—natural, organic foods without hormones (limit red meat intake to once a week if you can). Exercising helps, too: Young women who lead an active lifestyle lower their risk of getting breast cancer, even if they did start their period at a young age.

5. Pregnancy generally lowers breast cancer risk

If your first pregnancy is before the age of 30, your overall risk of getting breast cancer later in life goes down. And if you have more than one child, your long-term risk of getting breast cancer decreases even more. This is because of hormones released during pregnancy that cause your breast cells to mature and become more resistant to hormone receptive breast cancers.

I’ve never carried a baby to term, and I’m over 40, which puts me at higher risk for breast cancer. The fact is that the older you are when you have your first baby (or if you’ve never been pregnant at all), your risk for breast cancer rises: You will have been exposed to more estrogen in your lifetime, and estrogen is a hormone that promotes breast cancer growth. Relatedly, breastfeeding significantly lowers breast cancer risk, even in women who carry a gene mutation for breast cancer (such as BRCA1 or BRCA2).

6. Taking breaks from the pill sometimes might be a good idea

If you’re on the birth control pill or have been recently, you may have a slightly higher risk of developing breast cancer than women who’ve never used the pill. A 1996 study found that the risk was highest for women who began using the pill in their teens. The good news is that 10 or more years after you stop taking the pill, your risk of developing breast cancer returns to what it was before.

7. Alcohol influences breast cancer risk

Limit alcohol intake. I’ve heard this advice for years, as we all have, but I’d never made a connection between my twice a week glass of red wine and my risk for developing breast cancer. It turns out that alcohol can increase levels of estrogen and other hormones associated with hormone-receptor-positive breast cancer (it may also may increase breast cancer risk by damaging DNA in cells): Compared to women who don’t drink at all, women who have three alcoholic drinks per week have a 15% higher risk of breast cancer.

And finally…

I’m so grateful my cancer was spotted while it was still at an early stage, so it was entirely treatable and my prognosis is excellent. I wish I’d known, 20 years ago, what I now know about breast cancer and the risk factors. But, better late than never, and now that I’m aware, I’m doing everything I can to lead a healthy lifestyle—something which, despite my risk factors, may just help prevent a recurrence.

Amy Schreibman Walter

Amy Schreibman Walter is an American writer living in London.