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Completed

NCT Number: NCT07450300

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

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.

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Key information

Conditions

Infertility, Female Female Urogenital Diseases Female Urogenital Diseases and Pregnancy Complications Genital Diseases Genital Diseases, Female Infertility Urogenital Diseases

Age range

18 year–45 year

Sex eligibility

Female

Study type

Observational

Primary location

Clinique Mathilde

Rouen, 76000, France

Who can participate

Healthy volunteers accepted: No

Only the study team can determine whether someone qualifies for participation.

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.

Treatment and study plan

Modified natural cycle

Procedure

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.

Substitued cycle

Procedure

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

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.

Primary outcomes

  1. feasibility of embryo transfer

    Time frame: scheduled day of embryo transfer (Day 0 of planned FET cycle)

    Proportion of treatment cycles in which the planned frozen embryo transfer (FET) was performed as scheduled, regardless of the number of embryos transferred.

Secondary outcomes

  1. Clinical pregnancy

    Time frame: 6 weeks after embryo transfer

    Presence of an intrauterine gestational sac with fetal cardiac activity confirmed by transvaginal ultrasound.

  2. Miscarriage rate

    Time frame: 12 weeks of gestation

    Pregnancy loss occurring after confirmation of clinical pregnancy and before 12 completed weeks of gestation.

  3. Cycle cancellation rate

    Time frame: scheduled day of embryo transfer

    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.

  4. Conversion from natural to stimulated cycles

    Time frame: time of treatment protocol modification during the stimulation phase (prior to ovulation trigger)

    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.

Sponsors and collaborators

Lead sponsor

Clinique Mathilde

Other

Registry information

Acronym: NATEC

Important dates

Study start
2022
Primary completion
2025
Study completion
2026
First posted
Mar 4, 2026
Registry last updated
Mar 4, 2026

OpenTrials presents study information sourced from ClinicalTrials.gov. The official registry record should be consulted for the latest information.

View the official ClinicalTrials.gov record (opens in a new tab)

This listing is for discovery and informational purposes only. It is not medical advice, does not guarantee that a study is recruiting, and does not determine eligibility. Contact the study team and a qualified healthcare professional when considering participation.

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