Study question

What is the optimal thawing time in day 3 frozen-thawed embryo transfer (FET)during natural and hormone replacement therapy (HRT) cycles?

Summary answer

Immediate embryo transfer after thawing in the morning had improved pregnancy rate compared with thawing in the evening 1 day before embryo transfer in HRT cycles.

What is known already

Embryo-endometrial synchronization seems to be improved by control of thawing day in FET cycles. However, the optimal timing for thawing still remains uncertain.

Study design, size, duration

In a retrospective study, the relationship between thawing time and pregnancy rate has been studied in 678 FET cycles between Jan 2008 and Dec 2011.FET cycles were divided into four groups according to thawing time and endometrium preparation methods.

Participants/materials, setting, methods

Embryos which were thawed evening 1day before ET were cultured overnight and transferred the next day morning (group A:in natural cycle(n=160), group B:in HRT cycle (n=277)), while embryos from thawed in the morning were transferred within 3h (group C:natural cycle (n=138), group D:in HRT cycle (n=103)).

Main results and the role of chance

There were no differences among four groups in patient age (group A: 34.7±4.0, group B: 34.7±4.5, group C: 33.9±3.5and group D: 34.2±4.2), the mean number of transferred embryos (2.2±0.4, 2.1±0.4, 2.5±0.5and 2.1±0.3), the mean number of cryopreserved embryos (9.4±4.8, 10.9±6.2, 10.9±5.9 and 11.3±5.6), endometrial thickness (10.4±1.9, 10.3±2.0, 9.9±1.5 and 10.1±1.6) and embryo quality. The mean embryo survival index before transfer was comparable in the four groups (72.4%, 71.7%, 70.6% and 71.4%p=0.30). The clinical pregnancy rates in natural cycles were no significant difference between group A (28.1%) and group C (37.7%, p=0.19). However, the clinical pregnancy rates in HRT cycles were significantly higher in group D (50.5%) than group B (31.0%, p<0.01).

Limitations, reasons for caution

The number of cycles between each group was different. In particular, the number of HRT cycles was small.

Wider implications of the findings

In our results, the overall pregnancy rate in FET cycles had improved in immediate embryo transfer after thawing in the morning. Especially, pregnancy rate was significantly higher in the HRT cycles. We do not know the exact reason for the high pregnancy rate in the HRT cycles. Probably, it seems that better embryo-endometrial synchronization was achieved in HRT cycles.