If a pregnant women is diagnosed with cancer, is it likely the baby will also develop cancer?"

Although it is possible, it is extremely rare for a mother to pass cancer on to her baby during pregnancy. To date, there have only been around 17 suspected incidences reported, most commonly in patients with leukaemia or melanoma

A case in Japan in 2009 was the first to be hailed as proof that it can happen. In that case, a mother was diagnosed with leukaemia soon after she gave birth, and her baby daughter was diagnosed with lymphoma when she was 11 months old. Although two different types of cancer, the cancer cells of the mother and baby carried the identical mutated cancer gene. The baby hadn’t inherited the gene, meaning the cells must have come from the mother. The baby’s cancer cells had an additional mutation making them invisible to her immune system, allowing them to cross the placental barrier and survive without being attacked. 

But in the vast majority of cases where cancer is diagnosed during pregnancy, which are uncommon to begin with, cancer cells can’t pass from mother to baby. Nor can cancer cells pass from a mother to baby through breast milk. Women who have been diagnosed with cancer are advised not to become pregnant, however, because chemotherapy and radiotherapy can harm the unborn baby.