P–629 Is spontaneous ovulation better than induced ovulation for frozen-thawed embryo transfer cycle?
Authors: Sim P K , Nadkarni P 
Published: August 2021

Study question

Between spontaneous ovulation (SPO) and induced ovulation (INO) comparing clinical pregnancy rate and ongoing pregnancy rate for frozen-thawed embryo transfer (FET) cycle, which is better?

Summary answer

Both spontaneous ovulation and induced ovulation protocols showed no significant difference in clinical pregnancy rates and ongoing pregnancy rates.

What is known already

Recent practice worldwide is moving towards elective freezing of all embryos and subsequent frozen-thawed transfer, both for a perceived higher pregnancy rate as well as the significant reduction of ovarian hyperstimulation. The timing of FET can be determined by either detecting the spontaneous Luteinizing Hormone surge (SPO group) or by the administration of hCG (INO group). There is still an ongoing debate to determine which is the best protocol for frozen-thawed embryo transfer in the non-hormone replacement therapy (non-HRT) cycle.

Study design, size, duration

This retrospective study included 500 FET cycles for patients who had regular menses between June 2017 and June 2020. The FET cycles were grouped by type as follows: SPO (n = 281) and INO (n = 219). The primary outcome was the clinical pregnancy rate and the secondary outcome was ongoing pregnancy rate. Ongoing pregnancy is defined as a viable intrauterine pregnancy at 12 weeks of gestation confirmed on an ultrasound scan.

Participants/materials, setting, methods

This study was conducted in a single IVF centre. Vitrification was used as the cryopreservation method. To standardize outcome measures, only patients having single blastocyst transfer and aged under 38 years old were included. The average age of the patient was 32.9. Gamete donation, embryo donation, pre-implantation testing and assisted hatching cycles were also excluded from the analysis. Categorical data were analysed using Chi-square test SPSS version 25.

Main results and the role of chance

Clinical pregnancy rate for SPO group was 54.8% (154/281) versus 52.9% (116/219) in INO group. Even though clinical pregnancy rate was higher in SPO group as compared to INO group, it did not reach significance level (ꭓ2 = 0.17, p = 0.68). As all patients had single blastocyst transferred, the implantation rate was the same as clinical pregnancy rate. Ongoing pregnancy rate was also found higher in SPO group as compared to INO group (135/281, 48.0% and 97/219, 44.3% respectively) but again failed to reach significance level (ꭓ2 = 0.70, p = 0.40).

Limitations, reasons for caution

The retrospective nature of the study and therefore, the analysis was not adjusted for confounding factors such as blastocyst grading, etiology of infertility, and ethnicity of patients.

Wider implications of the findings: In natural cycle, both spontaneous ovulation and induced ovulation protocols had the same pregnancy outcomes for frozen-thawed embryo transfer. However, induced ovulation can facilitate in scheduling FET timing to avoid weekends and public holidays, if necessary.

Trial registration number

Not applicable