Inclusion criteria :  1) female, 2) ≥ 18 years old and <45 years old, 3) histologically confirmed BC, whatever the histological subtype (hormone receptor positive (RH+), negative (RH-), with (HER2+) overexpression or without (HER2-) HER2 overexpression, or triple negative), 4) planned NAC (up to the day of the first neoadjuvant chemotherapy treatment), 5) Eastern Cooperative Oncology Group Performance status ≤2, 6) willing to participate for the entire duration of the study, 7) ability to use a compatible smartphone or Tablet PC to download the application Withings Health Mate (from iOS 10 and Android 5.0 or above), 8) Internet access, 9) valid health insurance, 10) able to read, write and understand French  Exclusion criteria :  1) presence of metastases 2) Presence of bilateral breast cancer, 3) Presence of multifocal breast cancer, 4) a history of cancer other than basal cell skin lesions and cervical dysplasia, 5) pregnancy, likelihood of pregnancy, or breastfeeding, 6) impossibility of follow-up throughout the study for medical, social, family, geographic or psychological reasons, 7) deprived of liberty through a judicial or administrative decision.