What Day of Cycle Is Frozen Embryo Transfer?
A frozen embryo transfer (FET) can be timed on a natural cycle, a modified natural cycle, or a hormone-replacement (artificial) cycle. The day depends on the protocol and when the uterine lining is ready.
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Natural Cycle FET Timing
In a natural cycle, the transfer is timed around ovulation. The embryo is typically transferred 5 days after the luteinizing hormone (LH) surge, which corresponds to the implantation window.
Modified Natural Cycle FET Timing
In a modified natural cycle, a trigger shot of human chorionic gonadotropin (hCG) is used to pinpoint ovulation. The FET is then scheduled 5 days after the trigger injection.
Hormone Replacement (Artificial) Cycle Timing
In a hormone replacement cycle, estrogen and progesterone medications prepare the uterine lining without relying on natural ovulation. The transfer day is determined by the number of days of progesterone exposure, usually 5 days after starting progesterone supplementation.
Comparison of FET Cycle Protocols
| Protocol | Key Timing Marker | Typical Transfer Day |
|---|---|---|
| Natural Cycle | LH surge or ovulation | Day 5 post-ovulation |
| Modified Natural Cycle | hCG trigger shot | Day 5 post-trigger |
| Hormone Replacement Cycle | Start of progesterone | Day 5 of progesterone |
What Determines the Exact Day
The exact day of a frozen embryo transfer depends on embryo quality, the development stage at freezing (cleavage vs. blastocyst), and the readiness of the endometrium. Clinics use ultrasound and blood work to confirm the lining is receptive before scheduling the procedure.