Having to start chemotherapy during pregnancy remains a rare event. The decision to proceed this treatment relies on the drugs used, the time of exposure for the foetus and the gestational age at the time of exposure. As a matter of fact, the mutagenicity potential of the chosen medicine has to be known, and consequently, the risk for the child, without challenging the mother's life, nor her health. This question mainly occurs for breast cancers, leukemias and lymphomas. First trimester of pregnancy is the most critical period for the foetus, that corresponds to organogenesis. Antimetabolites are the main drugs bound to generate malformations. Besides, chemotherapy may induce direct toxicity towards the in utero exposed child. Myelosuppression is the most common toxicity, that can induce infections and/or fetal hemorrhages. Regarding the long term risks of cancer on fertility or child's intellectual development, they are still not well known and must be included into prospective studies, as well as being registered in a specific data file.