Breast cancer is one of the most common malignant tumors in women, and the total number of breast cancer cases has been increasing in recent years, and the proportion of young breast cancer has also increased significantly. According to the survey of American Cancer Society, the incidence of breast cancer at the age of ≤40 was 27.5/100,000 in 1980, 32.8/100,000 in 1988, and 34.4/100,000 in 2004, which shows that the incidence of young breast cancer is increasing year by year. Since many patients did not have children before the onset of the disease, the issue of fertility after the disease naturally becomes a great concern for patients and their relatives. Breast cancer is a systemic disease and its treatments such as surgery, chemotherapy, radiotherapy and endocrine therapy may damage ovarian function, leading to menstrual disorders, sexual dysfunction, amenorrhea, and even infertility, the severity of which depends on the patient’s age and treatment method. The degree of ovarian damage caused by chemotherapy depends on the individual follicular reserve. Young patients have a relatively low risk of premature ovarian failure, early menopause, and amenorrhea because of their higher follicular reserve. It has been shown that the incidence of amenorrhea in breast cancer patients aged <40 years is about 31%-38% after 6-9 cycles of three-drug combination chemotherapy; and the older the patient is, the earlier amenorrhea occurs after chemotherapy: <40 years generally occurs 4-8 months after chemotherapy, while >40 years generally occurs 2-4 months after chemotherapy. Chemotherapy-associated amenorrhea usually recovers spontaneously, but the older the age, the less likely it is that ovarian function will return to normal. It was found that 61% of patients aged 35-40 years had normal menstruation 6 months after the end of chemotherapy and 45% after 5 years, while 85% of patients aged <35 years had normal menstruation 6 months after the end of chemotherapy and 100% after 5 years. Furthermore, chemotherapy-associated amenorrhea is also regimen specific, with the lowest amenorrhea rate with adriamycin-containing regimens. Tamoxifen is a selective estrogen receptor antagonist and is commonly used in premenopausal patients with hormone-responsive breast cancer. Studies have shown that after 5 years of tamoxifen use, 50% of women experience perimenopausal-like adverse effects such as hot flashes, night sweats, vulvar itching, and vaginal bleeding, and the drug also inhibits ovulation, causing menstrual disorders and even increasing the incidence of endometrial cancer by 2-4 times. However, compared with some chemotherapeutic drugs, its ovarian toxicity is less, and after discontinuation of the drug, most patients' menstrual and ovulatory functions can still be restored to normal, only that the restoration is less likely near the age of menopause. In view of this, the American Society of Clinical Oncology recommends that physicians should inform their patients of reproductive age of the possibility of infertility before developing treatment plans for them, and for those who wish to have children, they should be prepared to preserve their fertility. Patients who wish to have children should also proactively consult their physicians to obtain the cooperation of both doctors and patients. In view of the toxic side effects of chemotherapy and radiotherapy, when is the best time for breast cancer patients to have children? In a retrospective study conducted by Mulvihill et al. in 66 breast cancer patients, the number of low birth weight fetuses and preterm babies was significantly higher in patients who became pregnant one year after the end of chemotherapy, and in a retrospective study conducted by Angela Ives in women aged 15-44 years who became pregnant after breast cancer diagnosis in Western Australia between 1982 and 2000, 62 of them became pregnant within 2 years of breast cancer diagnosis. Of these, 29 (47%) were aborted, 27 (44%) were delivered, and 6 (9%) were spontaneously aborted. This shows that there is a greater risk of pregnancy within 2 years after the end of chemotherapy for breast cancer. Women diagnosed with breast cancer are mostly advised to plan pregnancy at least 2 years after completion of final adjuvant therapy (chemotherapy, radiotherapy, endocrine therapy, any of the final treatments): 2 years after the end of chemotherapy and/or radiotherapy for those who do not need endocrine therapy, and 2 years after the end of endocrine therapy for those who need it; these recommendations, although recommended as a guideline, have not been followed in a large scale. There is no large-scale evidence-based medical evidence. Alternatively, it could be explained that the first peak of recurrence of breast cancer occurs 2 years after surgery and that this delay in pregnancy was originally intended to prevent possible early recurrence and to ensure the completion of adjuvant therapy. These recommendations are relevant for patients with breast cancer who require adjuvant therapy; however, patients with early-stage breast cancer who do not require adjuvant therapy do not need to adhere to them. Patients with early-stage breast cancer that does not require chemotherapy, radiotherapy, or endocrine therapy may have a pregnancy in the near future after surgery, and there is no uniform opinion on the exact timing. Ms. H underwent modified radical mastectomy for breast cancer in May 2007, postoperative chemotherapy and radiotherapy, and started endocrine therapy in January 2008. Since endocrine therapy needs to last for 5 years and wait for another 2 years after treatment, her ideal time for pregnancy should be after 2015. III. Maternal safety There are fewer reports on whether the pregnancy process has any effect on the patient. In Sweden, Dalberg K et al. reported that from 1973-2002, 2,870,932 singleton babies were recorded in the Swedish medical birth registration system, of which 331 fetuses were born to mothers who had undergone surgical treatment for breast cancer, and the average time from surgical treatment to pregnancy was 37 months. The vast majority of all mothers who had been treated for breast cancer had uneventful deliveries. However, women treated for breast cancer had a significantly higher risk of complications during delivery, cesarean delivery, and preterm delivery (<32 weeks gestational age). It was concluded that women treated for breast cancer have an overall safe delivery, but these women may need to be treated as high-risk pregnancies requiring continuous monitoring and treatment. IV. Fetal safety Lucia et al. reported that patients diagnosed with breast cancer before the age of 45 had only a 3% chance of delivering a live baby, while patients diagnosed with breast cancer before the age of 35 had an 8% rate of full-term pregnancies. This shows that the older the patient, the lower the fetal safety. The study also showed that the incidence of spontaneous abortion was higher in breast cancer patients than in normal controls, and occurred mainly in the first 20 weeks of pregnancy. The mechanism may be due to the disruption of hormone levels in the body by adjuvant therapy, such that the pregnancy cannot be maintained resulting in miscarriage. Dalberg et al. reported that women treated for breast cancer had a significantly increased risk of low birth weight and an increased risk of birth defects in their fetuses. similar results were found in the Danish study. However, long-term follow-up of these fetuses has not been reported. V. Effect of pregnancy on patient prognosis Traditionally, it was believed that pregnancy in breast cancer patients would cause poor prognosis, especially in those with hormone-dependent breast cancer. However, many current studies disprove this view and even suggest that pregnancy has improved prognosis. In France, Kojouharova et al. followed up patients with invasive breast cancer treated at the Strasbourg Obstetrics and Gynecology Hospital between 1993 and 2007. 6 of the 598 patients became pregnant within 2 years of diagnosis (4 miscarriages and 2 births); 17 became pregnant 2 years after diagnosis (3 spontaneous abortions, 3 induced abortions, 1 ectopic pregnancy, and 10 births). Two gave birth and died of distant metastases; one gave birth 3 years after the diagnosis of breast cancer and survived despite the discovery of lung and brain metastases; one had a local tumor recurrence; and 18 patients lived well at 105 months of follow-up. It was concluded that pregnancy is more appropriate for those patients with a long expected survival time and that the prognosis of breast cancer is related to the stage of the disease and not to pregnancy. Mueller, of the Fred Hutchinson Cancer Institute in Seattle, noted that many epidemiological studies have not found a negative effect of childbirth on breast cancer survival. The researchers selected 438 breast cancer patients who gave birth after diagnosis for the study, while 2,775 patients who did not give birth after diagnosis were selected as a control population. The results found that patients who gave birth 10 months after diagnosis had a 46% lower mortality rate compared to those who did not have children. However, overall, the mortality rate of women who gave birth after diagnosis was basically the same as that of those who did not give birth. The age of onset of breast cancer in China is generally earlier than in Western countries, and there is a trend toward younger and younger age, which means that more breast cancer patients will face the challenge of childbirth. "The ideal outcome is to have both, pursuing the longest possible survival and achieving the goal of healthy fertility. Advances in breast cancer treatment have increased the survival rate of breast cancer patients year by year, while the development of reproductive technology has made difficult pregnancies easier.