Modified Natural Cycle FET Protocol Guidelines

Protocol Components

The American College of Obstetricians and Gynecologists recommends an endometrial thickness target of ≥7 mm before proceeding to trigger, to ensure optimal embryo transfer conditions 1
The American Society for Reproductive Medicine strongly recommends single embryo transfer (SET) regardless of embryo quality in vitrified-warmed blastocyst cycles, to minimize multiple pregnancy risks 2

The Endocrine Society suggests continuing progesterone until 10 weeks of pregnancy if conception occurs, and estrogen continuation (when used) should be maintained for 3-4 weeks after positive pregnancy confirmation, then gradually tapered over 2 weeks 1
The European Society of Human Reproduction and Embryology recommends continuing luteal support for 1-3 weeks after ultrasound confirmation of viable intrauterine pregnancy, to support early pregnancy development 3, 4

The decision to proceed with embryo transfer should not be based solely on endometrial characteristics, though adequate thickness remains a practical clinical target, according to recent guidelines from the European Society of Human Reproduction and Embryology 5, 6, 7

Initiate progesterone supplementation once adequate endometrial thickness is confirmed, and continue estrogen supplementation throughout the progesterone phase until pregnancy testing, as recommended by the American Society for Reproductive Medicine 8

Endometrial thickness alone should not dictate the timing of embryo transfer; clinicians must prioritize hormonal parameters (e.g., LH surge, progesterone rise) and accurate identification of ovulation when preparing the endometrium for transfer. [9][10]

Single‑embryo transfer (SET) is strongly recommended for all vitrified‑warmed blastocyst cycles to minimise the risk of multiple pregnancy. [11][12]
A single embryo should be transferred regardless of cycle type (spontaneous or hCG‑triggered), embryo quality, patient age, or the number of previous failed attempts. [13][9][11][12]