This is a non-blind self-control trial. We mainly compare the changes of endometrial thickness，changes in menstrual flow, and improvement in pregnancy before and after stem cell treatment in patients with thin endometrium.

According to the medical history and the results of the examination, the selected cases were screened and the informed consent was signed. After hysteroscopy, the collagen/umbilical cord mesenchymal stem cell was spread on a foley catheter, and placed in the uterine cavity, and 5 ml of saline was filled into the balloon. B-ultrasound confirmed that the stent was attached to the uterine wall. The balloon was removed in situ after 1 week. We mainly compare the endometrial thickness before and 6 months after surgery.

Inclusion Criteria:

1. According to the GCP regulations, obtain the informed consent of the subject, volunteer for the test, and have signed the informed consent form.
2. Women aged 20-40 years with fertility requirements
3. Infertility patients who are treated in this hospital
4. After more than 2 times of hysteroscopic adhesion separation surgery, the uterine cavity morphology has basically returned to normal. The patient has a normal menstrual cycle or an estrogenic dose of 6 mg per day, and the highest thickness of the endometrium is 5.5 mm in 6 cycles by ultrasound.
5. or receive adjuvant reproductive treatment, the endometrial thickness is less than 5.5mm for unknown reasons, the treatment of estrogen, colony stimulating factor, aspirin, sildenafil and other drugs up to 8mg / day is invalid.
6. HBVAg negative, HCV negative, HIV negative, syphilis negative
7. Normal bone marrow morphology, normal blood routine
8. Previously failed to receive relevant stem cell therapy

Exclusion Criteria:

1. Those who cannot accept the treatment observation process required by the test
2. There are contraindications for hysteroscopic surgery;
3. congenital uterine malformation; severe adenomyosis; uterine fibroids that may affect embryo implantation; the investigators judged that there are other uterine factors that are not suitable for embryo implantation
4. Patients with chromosomal abnormalities
5. contraindications to estrogen therapy; systemic diseases such as thrombosis, cardiopulmonary disease, hematopoietic diseases, malignant tumors, etc.
6. Patients without fertility requirements