The best available evidence suggests that pregnancy after breast cancer does not increase a woman's risk of developing a recurrence from her breast cancer. In particular, the most recent data suggest that this is the case also in women with a hormone receptor-positive breast cancer. There is also no indication of increased risk for delivery complications or for the newborn. The aim of the study is to investigate if temporary interruption of endocrine therapy, with the goal to permit pregnancy, is associated with a higher risk of breast cancer recurrence.The study aims also to evaluate different specific indicators related to fertility, pregnancy and breast cancer biology in young women. A psycho-oncological companion study on fertility concerns, psychological well-being and decisional conflicts will be conducted in interested Centers.

Recent decades have witnessed a delay in childbearing for a variety of reasons including cultural, educational, and professional. As a consequence, breast cancer in young women often occurs before the completion of reproductive plans. Infertility has a significant impact on quality of life, resulting in substantial distress in younger women with breast cancer and influencing treatment decisions in a consistent proportion of patients.The best available evidence suggests that pregnancy after breast cancer does not increase a woman's risk of developing a recurrence.For women desiring pregnancy after a breast cancer, 5-10 years of endocrine therapy may substantially reduce the chance of conception; however, a shorter duration of endocrine therapy in this population has not been studied in a prospective manner.

Birth outcome after breast cancer has not been shown to be different from that of the normal population, but increased risks of delivery complications, cesarean section, preterm birth and low birth weight have been reported.

Endocrine agents are potentially teratogenic: taking into account their median half-life, waiting 3 months after their interruption before attempting conception is considered safe.

The limited evidence available on breastfeeding after breast cancer reports successful lactation from the treated breast in approximately 30% of women without detrimental effect on survival. No prospective definitive data are available.

Inclusion Criteria:

• Age ≥ 18 and ≤ 42 years at enrollment.
• Has received adjuvant endocrine therapy (SERM alone, GnRH analogue plus SERM or AI) for ≥18 months but ≤30 months for early breast cancer.

Note: Patients who have received neo/adjuvant endocrine treatment within a clinical trial and patients who have received pharmaco-prevention are eligible.

• The adjuvant endocrine therapy must have stopped within 1 month prior to enrollment.
• Patient wishes to become pregnant. Note: Patients who have undergone oocyte/embryo/ovarian tissue cryopreservation at breast cancer diagnosis and/or have a previous history of assisted reproductive technology (ART) are eligible.
• Breast cancer for which patient is receiving endocrine therapy must have been histologically-proven stage I-III, endocrine-responsive (i.e., estrogen and/or progesterone receptor positive, according to local definition of positive, determined using immunohistochemistry (IHC)), and treated with curative intent.

Note:

• Patients with synchronous bilateral invasive breast cancer (diagnosed histologically within 2 months) are eligible.
• Patient with invasive breast cancer or synchronous bilateral invasive breast cancer (diagnosed histologically within 2 months) during pregnancy are eligible.
• Patients with BRCA1/2 mutations are eligible.
• Patients could have received neo/adjuvant chemotherapy, or other systemic therapy (e.g., neo/adjuvant HER2-targeted therapy) according to institutional policy and patient's desire.
• Patient must be premenopausal at breast cancer diagnosis, as determined locally and documented in patient record.
• Patient must be without clinical evidence of loco-regional and distant disease, as evaluated according to institutional assessment standards and documented in the patient record.
• Written informed consent (IC) for trial participation must be signed and dated by the patient and the investigator prior to enrollment.
• Written consent to biological material submission, indicating the patient has been informed of and agrees to tissue and blood material use, transfer and handling, must be signed and dated by the patient and the investigator prior to any procedures specific for this trial.
• The patient has been informed of and agrees to data transfer and handling, in accordance with national data protection guidelines.
• Patient must be accessible for follow-up.

Exclusion Criteria:

• Post-menopausal patients at BC diagnosis, as determined locally.
• History of hysterectomy, bilateral oophorectomy or ovarian irradiation.
• Patients with current local, loco-regional relapse and/or distant metastatic breast cancer.
• Patients with a history of prior (ipsi- and/or contralateral) invasive BC.
• Patients with previous or concomitant non-breast invasive malignancy.
• Exceptions are limited exclusively to patients with the following previous malignancies, if adequately treated: basal or squamous cell carcinoma of the skin, in situ non-breast carcinoma, contra- or ipsilateral in situ breast carcinoma, stage Ia carcinoma of the cervix.
• Concurrent disease or condition that would make the patient inappropriate for study participation or any serious medical disorder that would interfere with the patient's safety.
• Patients with a history of noncompliance to medical treatments and/or considered potentially unreliable.
• Patients with psychiatric, addictive, or any disorder that would prevent compliance with protocol requirements.