According to the American Cancer Society (ACS), approximately 3 to 7 percent of all breast cancer diagnoses take place in women who are pregnant or breastfeeding. Although it is the most common form of cancer in pregnant women, it is diagnosed in just 1 pregnancy out of every 3,000, according the ACS.

Breast cancer in itself does not appear to harm the fetus, as long as the mother's health remains good. In other words, the cancer can not be transmitted to the fetus. In addition, there is no evidence that a history of breast cancer in a woman has any effect on her baby.

According to the National Cancer Institute (NCI), the average age of a breast cancer patient who is diagnosed during pregnancy or while still breastfeeding, is between 32 and 38 years of age. Thus, with more and more women choosing to delay childbearing, the number of pregnant or postpartum women diagnosed with breast cancer is expected to rise.

During pregnancy, total estrogen levels are 1,000 times that of a menstruating woman. The body’s additional exposure to all the estrogen, in turn, can cause cancer to grow and multiply even faster. For this reason, there are additional factors to be taken into consideration for treatment of a pregnant woman with breast cancer.

Breast cancer diagnosed during pregnancy is often detected at a later stage due to the natural enlargement of the breasts, which makes it more difficult to find or notice changes. These risks to both mother and child make it imperative to make clinical breast exams part of the prenatal and postnatal care. 

According to the ACS, it is relatively safe for women to have mammograms during pregnancy. The amount of radiation is low and is focused on the breast tissue. A lead shield that covers the abdomen is used for extra protection. Because researchers still do not know for certain the effects of low dose radiation on the fetus, a woman's physician can best determine if the mammogram must be performed immediately or if it can wait until after the baby is born. In some cases, the physician may choose another imaging test, such as breast ultrasound, that does not use radiation.

If an abnormal area or lump is detected, a biopsy is necessary to definitively diagnosis cancer. The procedure removes a sample of tissue from the suspected area, typically using local anesthesia that numbs the area. It is usually performed on an outpatient basis with little risk to the fetus. If the biopsy needs to be completed with the woman asleep (general anesthesia), it is still considered to be relatively safe for the unborn baby.

For pregnant women with breast cancer, the type of treatment, as well as the schedule, depends on multiple factors. Whenever possible, surgery is typically the first choice of treatment for breast cancer in all women. The primary goal is to control the cancer and keep it from spreading (metastasizing) in the body.

Traditional breast cancer treatments such as lumpectomy, mastectomy and/or removal of the lymph nodes can be considered in pregnant women. These procedures are completed under general anesthesia, which may pose a higher risk to the fetus. The patient's cancer care team and obstetrician will need to determine the safest treatment method and timing for the surgery.

Breast conserving surgeries, such as lumpectomy, typically require radiation therapy after surgery to prevent the cancer from returning. Since radiation can harm the fetus, this treatment would need to be delayed until after the birth of the baby. Studies have not proven how this delay affects the risk of cancer recurrence and the patient's prognosis. For this reason, women may choose mastectomy which typically does not require radiation therapy.

Based on the results of several studies, chemotherapy may be considered during the second and third trimesters of the pregnancy (fourth through nine months of pregnancy), but not during the first trimester (the first three months), according to both the ACS and the NCI. Chemotherapy treatments during the last two trimesters has not been found to increase the risk of birth defects or stillbirths. However, scientists do not yet know the long term effects of chemotherapy on these children.

Women who undergo breast cancer surgery in their third trimester may have their chemotherapy treatment delayed until after birth. In these cases, the baby may be delivered a few weeks early to begin earlier treatment.

Due to the potential side effects from chemotherapy, treatments should not be given three to four weeks before delivery, according to the ACS. The drugs can result in bleeding and increase the risk of infection. During this time period, chemotherapy is withheld to allow the mother's blood count to increase and return to normal.

The effects of hormone therapy (e.g., tamoxifen) have not been widely studied in pregnant women. Some research has indicated a specific defect may occur in the fetuses of women who become pregnant while taking tamoxifen. Animal studies also have found that tamoxifen may cause fetal changes. Hormone therapy currently is not recommended for treatment of breast cancer in pregnant women and should not be used until after the birth of the baby.

For women diagnosed with breast cancer who are still breastfeeding, it is advisable for them to stop nursing once treatments begin. Chemotherapy drugs can be passed to the baby through the mother’s breast milk. After treatments have stopped, a woman should discuss with her physician when it might be safe to resume breastfeeding.

While some physicians may recommend terminating an early pregnancy to begin treatment, studies have shown that terminating the pregnancy does not improve a woman’s survival rates. Breast cancer treatments during pregnancy depend on how far along in the pregnancy it occurs and the size, location and stage of the breast cancer. A diagnosis of breast cancer during pregnancy may correlate to lower overall survival rates for the mother due to a number of factors. Additional research is being conducted on treatment methods and timing to improve the prognosis for pregnant women with breast cancer.