Dados do Trabalho

Title

The plasticity of natural cycle for frozen embryo transfer: more friendly than expected

Objective

There is a growing interest in frozen embryo transfer using the natural cycle for endometrial preparation. However, there is little information available to support clinician in the daily practice of this protocol. Eventually, a cycle is suspended or leaves the clinician unsure if it differs from “ideal” physiology or had laboratory monitoring and ultrasound scans below the desirable. Herein, the study objective is to describe natural cycles that resulted in deliveries or ongoing pregnancies to assess the main variables with implications in clinical practice.

Methods

Natural cycles that resulted in live births or ongoing pregnancy (from 20 weeks) were retrospectively evaluated from January 2018 to December 2022. All transfers were performed with good quality embryos according to Gardner et al. (2000) classification. Variables analyzed on LH peak day were: the cycle day, LH dosage, E2 dosage, follicle size and endometrial thickness. Regarding transfer, the variables evaluated were: number of days after LH peak, cycle day and weekday. The total number of ultrasound scans after baseline screening was also tabulated. The LH surge day was determined based on hormone dosage (above 15 or twice as the previous day) or on day before the CL identification based on ultrasound scan. The additional luteal supports were made using 400mg intravaginal micronized progesterone from 4 to 5 days before embryo transfer.

Results

Among the 143 natural cycles that resulted in live births or ongoing pregnancy, 8 had no records and were removed from the study. From the remaining 135 cycles, 128 (94,8%) were performed without inducers and/or triggers (true natural cycles) and had the data analyzed. In 65,6% of cycles the LH surge was determined based on LH dosage and in 34,4% on CL identification. The LH surge day vary from 8 to 23 of cycle. In 71,3% of cases the LH surge occur between days 11-16 but in 18%, from day 17 to 20. Follicle size on the day of ovulation was different from 18 to 21mm in 32,4% of cycles. The LH and E2 most comum dosage on the surge day had variance from 41 to 60 µUI/ml and from 200 to 300pg/ml respectively. In 81,7% of cycles the endometrium thickness was higher than 7mm. Regarding the transfer day, 77% of cycles the transfer was between days 17 and 23. In 76,9% of cycles the transfer was after LH+6 days and 20,7% on LH+7 and the remaining 2,5% of cycles the transfer was on days LH+4/LH+5; none of the cycles had a transfer on Sunday. Finally, the number of ultrasound scans varied from 1 to 10 and in the half of the cases performed 4 or 5 times.

Conclusion

The data highlights the heterogeneity of pure natural cycle markers for endometrial preparation in successful cycles of frozen-thawed embryo transfer. Although it has expected findings in standard natural cycle in relation to the day of the LH peak, LH and estradiol values, follicle size and implantation window; cycles that differs from that are acceptable. This result brings important information on biological variations on natural cycle that can result in live births. Showing that even with the difficulties of the number of ultrasounds and desirable hormone dosages, it is possible to manage a natural cycle with very satisfactory results. Importantly, this plasticity of the pure natural cycle demonstrated by transfers on day LH+6 and LH+7 possibilities a more friendly cycle for the team avoiding weekend transfer.

Keywords

Frozen embryo transfer, endometrial preparation, LH surge, natural cycle.