Pregnancy-associated breast cancer is breast cancer that is diagnosed during pregnancy or in the first year after pregnancy. It even has its own acronym, PABC. PABC is rare: it affects about one in 3,000 pregnancies. Even so, it is the second most common cancer during pregnancy. Here is the important point upfront: PABC can be treated and you can continue your pregnancy. 
PABC is most common in women between ages 32 and 38. But as more women are getting pregnant after age 40, PABC is expected to increase, because women over 40 are more likely to get breast cancer. 
During pregnancy and during breast-feeding, hormones increase the size of your breasts. Breast lumps are more common. More than 80 percent of these lumps are not a problem. They are benign. A lump that is firm and painless is a possible red flag. You may discover the lump or your health care provider may find it. 
During pregnancy, you want to avoid radiation from a mammogram, so the first diagnostic test would be an ultrasound imaging study. This test is very accurate for finding PABC. The next step may be a needle biopsy. If the biopsy shows breast cancer, treatment follows soon after. In almost all cases, pregnancy continues. [1,2]
The exact treatment will depend on how advanced the cancer is and where you are in your pregnancy. Treatment may include surgery, radiation (after delivery), and/or chemotherapy.  PABC in the first year after pregnancy is treated like any other breast cancer. The only difference is that if you are breast feeding, you may have to stop if you need certain types of chemotherapy. [1,2]
Here are the basics for PABC during pregnancy: [1,2]
- Surgery is usually the first and safest treatment. The usual surgery is modified radical mastectomy. General anesthesia is usually safe for your baby.
- If you were not pregnant, the next step for most women would be radiation therapy. But radiation is not safe for your baby. If you need radiation therapy, it will be done after delivery.
- If you need chemotherapy, some chemotherapy agents can be given after the first trimester. There is no risk of birth defects for these drugs after this time. There is some risk of preterm labor and growth restriction (small baby). Your pregnancy may be monitored more frequently. Chemotherapy will be stopped after 35 weeks. After that time, it may cause your baby to have a low white blood cell count.
- Some chemotherapy treatments are not safe any time during pregnancy. These include hormone therapy (tamoxifen) and targeted therapy (Herceptin).
Studies show that ending a pregnancy does not improve breast cancer survival for most women. Most studies show that women with PABC do as well as other women with breast cancer. An exception might be a very aggressive or advanced type of cancer. You will need to discuss your options with your obstetrician and your oncologist. [1,2]
Other Things to Know About Breast Cancer and Pregnancy
The links between breast cancer and pregnancy are complicated. Some breast cancers are stimulated by high levels of estrogen and some are not. During pregnancy, your breast cells grow, this can trigger genetic changes that lead to cancer. But these changes are unlikely in younger women. So here is what we know: 
- If you are under age 35, pregnancy reduces your risk for breast cancer. The more pregnancies you have, the more you reduce your risk.
- Breast-feeding reduces your risk for breast cancer.
- After age 35, pregnancy may slightly increase your risk for breast cancer. Genetic changes are more likely to occur.
What if you have breast cancer diagnosed at an age when you still want to have a pregnancy? You should talk to your doctors about what treatment options are best for you. Ask if any of the treatments may affect your ability to become pregnant in the future. For example, tamoxifen, a commonly used targeted therapy, may cause early menopause. Some chemotherapy drugs may cause infertility. If you need these treatments, you can talk to a fertility specialist about storing fertilized embryos before treatment. [2,3]
What if you have already been treated for breast cancer and you want to get pregnant? In the past, some doctors thought that higher levels of estrogen during pregnancy might increase the risk of breast cancer recurrence, especially estrogen sensitive breast cancer. Recent studies suggest this is not the case. 
A 2017 study reported at the American Society of Clinical Oncology reported that there was no difference is cancer-free survival for women who become pregnant after cancer treatment compared to women who did not become pregnant. The study followed over 1,000 women for 10 years.  In fact, there have been several studies that show getting pregnant after breast cancer may improve breast cancer survival. 
If you find a breast lump during pregnancy, let your doctor know. It is unlikely to be cancer. But even if it is, it is unlikely to mean you need to end your pregnancy.
- Obstetrics & Gynecology, Pregnancy-Associated Breast Cancer.
- American Cancer Society, Treating Breast Cancer During Pregnancy.
- Susan G. Komen, Does Pregnancy Affect Breast Cancer Risk and Survival?
- ASCO, Pregnancy after breast cancer does not increase chance of recurrence.