Assessment of the Feasibility of Frozen Embryo Transfers in a Natural Cycle Among Obese Compared to Non-Obese Patients (NATEC)

This retrospective study included 1,831 single blastocyst frozen embryo transfer (FET) cycles performed between November 1, 2022 and August 31, 2025. Three endometrial preparation protocols were used according to ovulatory status, cycle duration, and characteristics of previous FET cycles: modified natural cycle with ovulation trigger (mNC-FET) (n = 770), stimulated cycle FET (SC-FET) (n = 468), and hormone replacement therapy FET (HRT-FET) (n = 593). In natural cycles, if the predefined criteria for ovulation trigger were not met, the cycle was converted to a stimulated cycle. The aim of this study was to determine whether body mass index affects embryo transfer feasibility, reproductive outcomes, and cycle characteristics across different FET protocols.

Study Overview

• Status: Completed
• Conditions: Infertility, Female
• Intervention / Treatment: Procedure: Modified natural cycle; Procedure: Substitued cycle; Procedure: Hormonal replacement therapy

• Study Type: Observational
• Enrollment (Actual): 1831

Contacts and Locations

Study Locations

• France
  • Rouen, France, 76000
    • Clinique Mathilde

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult
• Accepts Healthy Volunteers: No

• Sampling Method: Non-Probability Sample

Study Population

Women undergoing assisted reproductive technology (ART) with frozen embryo transfer (FET) at a tertiary fertility center were retrospectively identified. The study population included adult patients who underwent endometrial preparation using one of the following protocols: modified natural cycle with ovulation trigger, stimulated cycle, or hormone replacement therapy (HRT). Participants were categorized according to body mass index (BMI) into obese and non-obese groups for comparative analysis of embryo transfer feasibility and reproductive outcomes.

Description

Women aged ≥18 years at the time of frozen embryo transfer.

Patients undergoing assisted reproductive technology (ART) with frozen embryo transfer (FET) using a natural, modified natural, stimulated, or hormone replacement therapy (HRT) protocol.

Availability of body mass index (BMI) measurement prior to initiation of the treatment cycle.

Study Plan

How is the study designed?

Number of groups / cohorts

3

Cohorts and Interventions

Group / Cohort	Intervention / Treatment
Modified natural cycle	Procedure: Modified natural cycle; In the modified natural cycle group, patients underwent ultrasound and hormonal monitoring to track spontaneous follicular development. When the leading follicle reached an appropriate size and endometrial thickness was adequate, ovulation was triggered using human chorionic gonadotropin (hCG) to schedule frozen embryo transfer.
Stimulated cycle	Procedure: Substitued cycle; In the stimulated cycle group, mild ovarian stimulation was performed using oral agents and/or low-dose gonadotropins to promote follicular development. Follicular growth was monitored by ultrasound, and ovulation was either triggered with hCG or occurred spontaneously, allowing scheduling of frozen embryo transfer.
Hormonal replacement therapy	Procedure: Hormonal replacement therapy; In the hormone replacement therapy group, endometrial preparation was achieved through exogenous estrogen administration. Once adequate endometrial thickness was confirmed, progesterone supplementation was initiated to mimic the luteal phase and schedule frozen embryo transfer.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
feasibility of embryo transfer	Proportion of treatment cycles in which the planned frozen embryo transfer (FET) was performed as scheduled, regardless of the number of embryos transferred.	scheduled day of embryo transfer (Day 0 of planned FET cycle)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Clinical pregnancy	Presence of an intrauterine gestational sac with fetal cardiac activity confirmed by transvaginal ultrasound.	6 weeks after embryo transfer
Miscarriage rate	Pregnancy loss occurring after confirmation of clinical pregnancy and before 12 completed weeks of gestation.	12 weeks of gestation
Cycle cancellation rate	Proportion of initiated FET cycles in which embryo transfer was not performed due to inadequate endometrial development, ovulation disorder, premature luteinization, or other clinical reasons.	scheduled day of embryo transfer
Conversion from natural to stimulated cycles	Proportion of cycles initially planned as natural or modified natural cycles that required initiation of ovarian stimulation due to insufficient follicular development or ovulatory dysfunction.	time of treatment protocol modification during the stimulation phase (prior to ovulation trigger)

Collaborators and Investigators

• Sponsor: Clinique Mathilde

Study record dates

Study Major Dates

• Study Start (Actual): November 1, 2022
• Primary Completion (Actual): August 31, 2025
• Study Completion (Actual): January 31, 2026

Study Registration Dates

• First Submitted: February 19, 2026
• First Submitted That Met QC Criteria: March 2, 2026
• First Posted (Actual): March 4, 2026

Study Record Updates

• Last Update Posted (Actual): March 4, 2026
• Last Update Submitted That Met QC Criteria: March 2, 2026
• Last Verified: March 1, 2026

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Urogenital Diseases; Genital Diseases; Female Urogenital Diseases; Female Urogenital Diseases and Pregnancy Complications; Genital Diseases, Female; Infertility; Infertility, Female
• Other Study ID Numbers: 2025-16-CMR

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No