Thin endometrium is a condition in which the lining of the uterus is too thin during preparation for embryo transfer. This may reduce the chance of pregnancy and live birth in patients undergoing frozen-thawed embryo transfer.

This study will evaluate whether intrauterine infusion of granulocyte colony-stimulating factor, also called G-CSF, can improve pregnancy outcomes in patients with thin endometrium. Participants will be randomly assigned to one of two groups. The experimental group will receive mild endometrial stimulation followed by intrauterine infusion of G-CSF 300 μg. The control group will receive the same mild endometrial stimulation followed by intrauterine infusion of normal saline. Both groups will then continue standard endometrial preparation for frozen-thawed embryo transfer.

The main outcome of the study is the live birth rate. The study will also assess endometrial thickness, endometrial blood flow, clinical pregnancy rate, miscarriage rate, and safety outcomes, including adverse events, pregnancy complications, fetal malformations, and neonatal birth defects. Participants will be followed from enrollment through embryo transfer, pregnancy assessment, and delivery.

This is a single-center, prospective, randomized controlled trial conducted at Nanjing Drum Tower Hospital, The Affiliated Hospital of Nanjing University Medical School. The planned sample size is 620 participants.

Thin endometrium remains a clinical challenge in assisted reproductive technology because it may reduce endometrial receptivity and the chance of successful embryo implantation and live birth. Several treatment strategies have been used for thin endometrium, including hormonal treatment, medications to improve blood flow, endometrial stimulation, and intrauterine infusion of granulocyte colony-stimulating factor (G-CSF). However, the clinical benefit of intrauterine G-CSF infusion remains uncertain, partly because previous studies have differed in patient populations, timing of administration, dose, and study design.

This study is designed to evaluate whether intrauterine infusion of G-CSF during the endometrial preparation phase can improve reproductive outcomes in patients with thin endometrium undergoing frozen-thawed embryo transfer. The study uses a prospective, single-center, randomized, parallel-group, blinded, superiority design. Eligible participants will be randomly assigned in a 1:1 ratio to receive either mild endometrial stimulation with intrauterine G-CSF infusion or mild endometrial stimulation with intrauterine normal saline infusion. Randomization will be performed using an interactive web response system.

All participants will receive routine endometrial preparation for frozen-thawed embryo transfer according to local clinical practice. In the intervention group, participants will undergo mild endometrial stimulation followed by intrauterine infusion of G-CSF 300 μg. In the control group, participants will undergo the same mild endometrial stimulation procedure followed by intrauterine infusion of an equal volume of normal saline. Participants, outcome assessors, and statistical analysts will be blinded to group assignment, while the physician performing the intrauterine procedure will not be blinded.

After the intrauterine procedure, endometrial development will be monitored by ultrasound. Embryo transfer will be performed after endometrial transformation according to the frozen-thawed embryo transfer protocol. Participants will be followed for pregnancy outcomes, delivery outcomes, and safety events. The study aims to determine whether the addition of intrauterine G-CSF to mild endometrial stimulation provides a clinically meaningful benefit compared with mild endometrial stimulation and saline control.

Inclusion Criteria:

• Female patients aged 18 to <40 years.
• Patients with normal ovarian function.
• History of endometrial thickness ≤7 mm in at least one previous endometrial preparation cycle.
• At least two good-quality embryos remaining.
• Fewer than 3 previous embryo transfer cycles.
• Willing and able to provide written informed consent.

Exclusion Criteria:

• Uterine diseases or conditions that may affect the uterine cavity, including intramural uterine fibroids affecting the uterine cavity, severe adenomyosis, endometriosis, uterine malformation, endometrial tuberculosis, or severe untreated intrauterine adhesions.
• Endometrial lesions, history of pelvic tumors, or previous radiotherapy or chemotherapy.
• Known allergy to G-CSF.
• Current participation in another clinical study.

Individual participant data will not be shared because the study involves sensitive reproductive, pregnancy, delivery, and neonatal clinical information. The current study protocol and informed consent documents do not include a plan for public sharing of individual participant data. De-identified aggregate data may be reported in scientific publications.

Participants, outcome assessors, and statistical analysts will be blinded to group assignment. The intrauterine procedure will be performed in the same manner in both groups, and participants will not be informed whether they receive G-CSF or normal saline. Outcome assessors will not participate in randomization or the intrauterine procedure, and ultrasound and clinical outcome data will be assessed using anonymized identifiers. Statistical analysts will receive coded group information as Group A and Group B. The physician performing the intrauterine procedure will not be blinded because they must prepare and administer the assigned infusion.