The goal of this clinical trial is to evaluate the efficacy and safety of transvaginal niche resection in improving pregnancy outcomes of a large uterine caesarean scar defect (niche) with secondary infertility. The main questions it aims to answer are:

1. Does transvaginal niche resection improve pregnancy outcomes of a large uterine caesarean scar defect (niche) with secondary infertility?
2. What medical problems do participants have when taking transvaginal niche resection?

Researchers will compare transvaginal niche resection to expectant management (without any additional surgical intervention) to see if transvaginal niche resection works to improve pregnancy outcomes of a large uterine caesarean scar defect (niche) with secondary infertility.

Participants will:

1. Undergo a procedure (transvaginal niche resection) within 2 week after randomization in the intervention group. Women were advised to use contraception in the first 6 months following niche resection to allow uterine healing prior to subsequent pregnancy.
2. Receive usual care at least 9 months which means no additional surgical intervention during this period in the control group. Patients are allowed to become pregnant and to receive fertility therapies if indicated. Any member of the control group who has not become pregnant after 9 months will be given the opportunity to undergo a transvaginal niche resection.
3. Receive a standardized magnetic resonance imaging (MRI) evaluation of the niches. Niches will be evaluated at baseline in all groups and at 3 months after surgery.
4. Be contacted by telephone at 6, 9 and 15 months to assess the primary and secondary outcomes.

Inclusion Criteria:

1. It was diagnosed by MRI as a large niche (defined as niche with a depth of >50% of the myometrial thickness and a residual myometrial thickness (RMT) ≤3mm);
2. Infertility secondary to niche (the inability to conceive within 12 months of unprotected intercourse or repeated failed IVF);
3. Fertility requirements;
4. Signed informed consent.

Exclusion Criteria:

1. Age below 18 years or over 49 years;
2. Contraindications for intraspinal or general anaesthesia;
3. A (suspected) malignancy or combined benign lesions requiring hysterectomy;
4. Atypical endometrial cells or cervical dysplasia;
5. Uterine or cervical polyps;
6. Submucosal fibroids;
7. Hydrosalpinx.