Rationale: Cesarean delivery (CD) rates are increasing rapidly worldwide. The consequences of cesarean section not only affect the next pregnancies such as uterine rupture or malplacentation but also cause many gynecological complications or infertility due to the formation of cesarean scar defect (CSD) or Niche. One of the major factors for Niche formation is the techniques of hysterotomy closure at the time of cesarean delivery. Most common hysterotomy closures include single and double layers closure. However, there is limited literature to prove which technique decreases niche creation and associated gynecological complications.

Objective: To compare the effect of uterine closure techniques during CD on the formation of Niche evaluated on transvaginal ultrasound and gynecological complications.

Study design: a double-blinded, single-center, randomized clinical trial. Setting: Hanoi Obstetrics & Gynecology Hospital (HOGH), Viet Nam. Study population: All women at gestational age ≥ 37 0/7 weeks who undergo a primary CD (planned or unplanned).

Intervention: After informed consent, women will be randomized to either unlocked single layer closure or unlocked double-layer closures of cesarean uterine scar in a 1:1 ratio Primary outcome: The proportion of Niche over time (at 6, 12, 18, 24 months after CD) in the two groups.

Power calculation: A sample size of 389 women is needed for each group. Nature and extent of the burden and risks associated with participation, benefit, and group relatedness: Both uterine closure methods followed by the Vietnam Ministry of Health (VN-MOH), transvaginal ultrasound is also not harmful. It is believed that there are no potential increased risks to patients taking part in this trial because of the standard care and management they receive.

Pregnancies at term, indicated for cesarean delivery for the first time, will be fully consulted information related to this study before surgery. Written consent will be obtained from each eligible pregnant woman before randomization into the study.

After signing the informed consent form, eligible women will be randomly assigned in a 1:1 ratio to receive either single-layer closure or double-layer closure, using block randomization with a variable block size of 4, 6. The computer-generated random list will be prepared by an independent statistician who has no other involvement in the study. To ensure allocation concealment, opaque and sealed envelopes will be prepared and controlled by two administrative staff in the Clinical Trial Unit who have no involvement in clinical work. Whenever there is an eligible participant, these two staff will hand over the envelope in sequence to the clinician. As the result, surgeons will not be blinded, but participants and sonographers will be unaware of closure techniques. Apart from randomization, patients will be followed up and treated according to local protocol.

The cesarean section with two uterine scar closure techniques will be performed by well-trained obstetricians. Participants and sonographers will be blinded for the closure technique.

Intervention (double-layer closure) In both study groups, women will undergo a CD following a standard way with respect to the mode of hysterotomy, non-closure of the peritoneum. In the intervention group, double-layer closure of the uterus will be performed using unlocked multifilament continuous running sutures for both layers and the endometrial layer will be included in the first layer. The second layer is a continuous running suture that imbricates the first layer.

Control group (single-layer closure) The control group will receive a single-layer closure using unlocked continuous running multifilament sutures and the endometrial layer will be included.

Complication during surgery

The cases that have complications during surgery will be excluded from the study. Complications encompass:

• Maternal mortality
• Maternal admitted ICU
• Hysterectomy
• Damage to internal organs (bowel, bladder or ureters).
• Complications during postoperative period requiring further surgery. Follow-up Participants will be contacted via telephone to notify about follow-up visits within 10 days of the first day of every menstrual cycle at 6 months, 12 months, 18 months, 24 months (give or take 2 weeks).

Every regular follow-up visit, the participants will be examined and transvaginal ultrasound followed by saline infusion sonohysterography to evaluate Niche. The ultrasound evaluation is standardized as proposed by the latest standard of ISUOG(2019).

Reproductive outcomes at two years follow-up: Percentage of women that conceived at any time during the follow-up duration, time of conception, and results of said pregnancies (ongoing pregnancy, terminated pregnancy due to medical/ social indication).

Niche evaluation Every regular follow-up visit, the participants will be examined and transvaginal ultrasound followed by sonohysterography to evaluate Niche. The Niche evaluation is standardized as proposed by the latest standard of ISUOG(2019).

Inclusion Criteria:

• Pregnant women who undergo a primary CD (planned or unplanned)
• Age ≥ 18 years.
• Gestational age ≥ 37 0/7 weeks

Exclusion Criteria:

• Previous major uterine surgery (e.g. laparoscopic or laparotomic fibroid resection, septum resection).
• Women with abnormal menstrual bleeding (e.g. cervical dysplasia, communicating hydrosalpinx, uterine anomaly or endocrine disorders disturbing ovulation, drugs, polyps, fibroids, etc.).
• Women with dysmenorrhea, dyspareunia, suprapubic pain.
• Abnormal placenta: Placenta privia, Placenta percreta… in the current pregnancy.