Results
Women and cycle characteristics
During the study period, a total of 445 women aged ≥36 years underwent fresh blastocyst transfers at the Infertility Clinics Northwest Women and Children’s Hospital. Of them, 16 women had only one blastocyst available for transfer (compulsory SBT), and they were therefore excluded from the analysis. eSBT was performed in 240 cases and extra blastocysts were frozen and DBT was carried out in 189 women (Figure 1).
The baseline and fresh IVF/ ICSI cycle characteristics of both groups are shown in Table1. Women in the eSBT group were significantly younger (P =.001) and had fewer previous number of IVF attempts (P <.001) than those in the DBT group. eSBT was more routinely practiced during the latter two years (2017-2018). Women in the eSBT group had more blastocysts available (4.9 ± 2.0 in eSBT versus 4.4 ± 2.3 in DBT, P =0.011). Other characteristics (body mass index, education level, type and causes for infertility, duration of infertility at the time of IVF and ovarian reserve test including antral follicular count and FSH levels) did not differ significantly between the groups.
Infertility treatment outcomes
We assessed 429 women that yielded 297 livebirths. The CLBR was increased in those who underwent single (74.2%,178/240) compared to double (63.0%,119/189) blastocyst-stage embryo transfer. After adjustment for female age and blastocysts quality, number of blastocyst transfer did not have a significant effect on the chance of live birth (aOR: 1.09 (0.68, 1.75)) (Table 2). The total number of children born was 194 after eSBT (162 from singletons and 16 from twin pregnancies) versus 154 (84 from singletons and 35 from twin pregnancies) after DBT. The cumulative multiple livebirth rate was 9.0% after eSBT (16/178) versus 29.4% after DBT (35/119) (OR: 0.24 (0.12-0.45)). No significant difference in the Kaplan-Meier survival curves for time to probability of livebirth between the two study groups (Hazard ratio: 0.85 (0.68,1.08), P =0.152) (Figure 2).
LBRs in initial fresh cycles for single- versus double- blastocyst groups were also calculated and found to be nearly identical at 52.1% (125/240) and 52.4% (99/189), respectively (Table S1). A statistically significant difference, however, was noted in the rate of implantation (62.9% in SBT vs 45.2% in DBT, P <0.001). Patients undergoing one fresh eSBT had a twinning rate of 4% (5 monozygotic twins /125) compared to a twinning rate of 29.3% (29/99) in those after one fresh DBT (OR= 0.10, 95%CI 0.04 to 0.27). Until Oct 2020, 143 women without livebirth in the fresh cycles went on the subsequent frozen-thawed embryo transfer (FET) cycles (n=185). 53 women of the eSBT group and 20 of the DBT group resulted in live births. More cryopreserved blastocysts were left in eSBT group (3.0 ± 2.2 vs. 2.0 ± 2.2, P <0.001).
Subgroup analyses
Female age
For women aged with 36-37 and 38-39 years old, those who underwent eSBT had slightly higher odds of live birth than those who had DBT, but the differences did not reach statistical significance in each subgroups (Table 2). For women ≥40 years old, it seemed that the use of eSBT over DBT slightly reduced the odds of live birth after treatment weighting (aOR: 0.88 (0.29-2.65)), but the difference was also not statistically.
Blastocyst quality
The findings in the pregnancy outcomes persisted with and without accounting for quality of the blastocyst transferred (Table 2). Numbers of blastocyst transfer did not have a significant effect on the chance of live birth in women across the two categories of quality of blastocyst transfer (aOR: 0.47 (0.19-1.15) in good-quality group and aOR: 1.56 (0.86-2.82) in fair-quality group).
Calendar period
More patients underwent eSBT during the latter calendar period which reflects the change in IVF practice in our clinic. Therefore, we also assessed the impact of the eSBT strategy on older women undergoing IVF/ICSI from 2015 to 2018. Despite a successive rise in eSBT cycles from 39% in 2015 to 45% in 2016 and exceeding 60% in 2017-2018, the cumulative LBRs were maintained at around 50% (varied from 48.9 to 53.5%), while multiple birth rate dropped from 25.7% in 2015 to 12.0% 2018, though the differences between each year categories were not statistically significant (P =.233) (Figure S1).
Perinatal outcomes
Perinatal outcomes after fresh and frozen transfers in women with live birth were shown in Table 3. The odds of preterm delivery (0.37 (0.21-0.64)) and low birth rate (0.31 (0.16, 0.60)) after eSBT were significantly lower than that of those with DBT. There was no significant difference in the risk of cesarean section between eSBT and DBT (OR:0.73 (0.41-1.31).