Clinical Outcomes Following Cryopreservation of Blastocysts
Clinical Outcomes Following Cryopreservation of Blastocysts
Between 2009 and 2011, there were 46 890 fresh blastocyst transfer cycles and 33 739 initiated thaw blastocyst cycles (12 852 slow freezing and 20 887 vitrification cycles). Table I presents the maternal demographic and obstetric characteristics in the fresh, slow frozen and vitrified blastocyst groups. The mean maternal age was 35.2 ± 4.7 years for the fresh group, 33.6 ± 4.4 years for the slow freezing group and 34.2 ± 4.4 years for the vitrification group (Table I). The difference of mean maternal age between fresh group and vitrification group was 1.0 years (95% CI for the difference 0.96–1.04). Similarly the difference of mean maternal age between vitrification group and slow freezing group was 0.6 years (95% CI for the difference 0.55–0.65). Due to the large sample size, all differences between the fresh, slow freezing and vitrification group were highly significant (P < 0.001) (Table I).
95.6% of the initiated vitrified blastocyst warming cycles resulted in embryo transfer, which was significantly higher than the 90.6% in the slow freezing group (ARR: 1.05, 95% CI: 1.03–1.08, P < 0.001) (Table II and Table IV). On average, 1.3 ± 0.6 vitrified blastocysts were warmed per cycle, while 1.5 ± 0.8 slow frozen blastocysts were thawed per cycle. A higher proportion of single embryo transfer was reported in the vitrification group (88.6%), compared with 83.8% in the slow freezing group and 75% in the fresh group (Table II).
The 46 890 fresh blastocyst transfers, 11 644 slow frozen blastocyst transfers and 19 978 vitrified blastocyst transfers resulted in 16 845, 2766 and 6537 clinical pregnancies, which led to 13 049, 2065 and 4955 live deliveries, respectively (Table III). The clinical pregnancy rate and live delivery rate per embryo transfer cycle were highest in the fresh blastocyst transfer group (35.9 and 27.8%, respectively) (Table III). The vitrified blastocyst group had a significantly higher clinical pregnancy rate per thaw cycle compared with the slow frozen blastocyst group (31.3 and 21.5%, respectively; ARR: 1.43, 95% CI: 1.37–1.50) and clinical pregnancy rate per embryo transfer cycle (32.7 and 23.8%, respectively; ARR: 1.38, 95% CI: 1.32–1.45) (Table III and Table IV). The likelihood of a live delivery per thaw cycle was 47% higher for vitrified blastocyst transfer cycles compared with slow frozen blastocyst transfer cycles (ARR: 1.47, 95% CI: 1.39–1.55). Similarly, of vitrified blastocyst transfer cycles, 24.8% resulted in a live delivery. This was significantly higher (ARR: 1.41, 95% CI: 1.34–1.49) than the proportion in the slow frozen blastocyst group (17.7%) (Table III and Table IV).
The perinatal outcomes of singletons following transfer of fresh, slow frozen and vitrified blastocysts are presented in Table V. The mean birthweight of singletons following vitrified blastocyst transfer (3421.6 ± 590.5 g) was significantly heavier than that of singletons following fresh blastocyst transfers (3276.2 ± 611.6 g) (P < 0.001) (Table V). A significantly lower proportion of low birthweight singletons was reported in the vitrified blastocyst group compared with the fresh blastocyst group (ARR: 0.67, 95% CI: 0.58–0.78). Compared with singletons born after transfer of fresh blastocysts, the likelihood of SGA births was 40% lower for singletons born after transfer of vitrified blastocysts (ARR: 0.60, 95% CI: 0.53–0.68), and 36% lower for singletons born after slow frozen blastocysts (ARR: 0.64, 95% CI: 0.53–0.78). Both slow frozen and vitrified blastocyst transfer group resulted in higher risk of LGA births compared with the fresh blastocyst transfer group (ARR: 1.77, 95% CI: 1.57–2.00, ARR: 1.54, 95% CI: 1.40–1.69, respectively) (Table VI). The mean gestational age was comparable in singletons after transfer of fresh, slow frozen and vitrified blastocysts (38.4, 38.5 and 38.5 weeks, respectively) (Table V). The risk of preterm birth (<37 completed weeks of gestation) decreased among singletons in the vitrified blastocyst group compared with the fresh blastocyst group (ARR: 0.86, 95% CI: 0.77–0.96) (Table VI).
Results
Between 2009 and 2011, there were 46 890 fresh blastocyst transfer cycles and 33 739 initiated thaw blastocyst cycles (12 852 slow freezing and 20 887 vitrification cycles). Table I presents the maternal demographic and obstetric characteristics in the fresh, slow frozen and vitrified blastocyst groups. The mean maternal age was 35.2 ± 4.7 years for the fresh group, 33.6 ± 4.4 years for the slow freezing group and 34.2 ± 4.4 years for the vitrification group (Table I). The difference of mean maternal age between fresh group and vitrification group was 1.0 years (95% CI for the difference 0.96–1.04). Similarly the difference of mean maternal age between vitrification group and slow freezing group was 0.6 years (95% CI for the difference 0.55–0.65). Due to the large sample size, all differences between the fresh, slow freezing and vitrification group were highly significant (P < 0.001) (Table I).
95.6% of the initiated vitrified blastocyst warming cycles resulted in embryo transfer, which was significantly higher than the 90.6% in the slow freezing group (ARR: 1.05, 95% CI: 1.03–1.08, P < 0.001) (Table II and Table IV). On average, 1.3 ± 0.6 vitrified blastocysts were warmed per cycle, while 1.5 ± 0.8 slow frozen blastocysts were thawed per cycle. A higher proportion of single embryo transfer was reported in the vitrification group (88.6%), compared with 83.8% in the slow freezing group and 75% in the fresh group (Table II).
The 46 890 fresh blastocyst transfers, 11 644 slow frozen blastocyst transfers and 19 978 vitrified blastocyst transfers resulted in 16 845, 2766 and 6537 clinical pregnancies, which led to 13 049, 2065 and 4955 live deliveries, respectively (Table III). The clinical pregnancy rate and live delivery rate per embryo transfer cycle were highest in the fresh blastocyst transfer group (35.9 and 27.8%, respectively) (Table III). The vitrified blastocyst group had a significantly higher clinical pregnancy rate per thaw cycle compared with the slow frozen blastocyst group (31.3 and 21.5%, respectively; ARR: 1.43, 95% CI: 1.37–1.50) and clinical pregnancy rate per embryo transfer cycle (32.7 and 23.8%, respectively; ARR: 1.38, 95% CI: 1.32–1.45) (Table III and Table IV). The likelihood of a live delivery per thaw cycle was 47% higher for vitrified blastocyst transfer cycles compared with slow frozen blastocyst transfer cycles (ARR: 1.47, 95% CI: 1.39–1.55). Similarly, of vitrified blastocyst transfer cycles, 24.8% resulted in a live delivery. This was significantly higher (ARR: 1.41, 95% CI: 1.34–1.49) than the proportion in the slow frozen blastocyst group (17.7%) (Table III and Table IV).
The perinatal outcomes of singletons following transfer of fresh, slow frozen and vitrified blastocysts are presented in Table V. The mean birthweight of singletons following vitrified blastocyst transfer (3421.6 ± 590.5 g) was significantly heavier than that of singletons following fresh blastocyst transfers (3276.2 ± 611.6 g) (P < 0.001) (Table V). A significantly lower proportion of low birthweight singletons was reported in the vitrified blastocyst group compared with the fresh blastocyst group (ARR: 0.67, 95% CI: 0.58–0.78). Compared with singletons born after transfer of fresh blastocysts, the likelihood of SGA births was 40% lower for singletons born after transfer of vitrified blastocysts (ARR: 0.60, 95% CI: 0.53–0.68), and 36% lower for singletons born after slow frozen blastocysts (ARR: 0.64, 95% CI: 0.53–0.78). Both slow frozen and vitrified blastocyst transfer group resulted in higher risk of LGA births compared with the fresh blastocyst transfer group (ARR: 1.77, 95% CI: 1.57–2.00, ARR: 1.54, 95% CI: 1.40–1.69, respectively) (Table VI). The mean gestational age was comparable in singletons after transfer of fresh, slow frozen and vitrified blastocysts (38.4, 38.5 and 38.5 weeks, respectively) (Table V). The risk of preterm birth (<37 completed weeks of gestation) decreased among singletons in the vitrified blastocyst group compared with the fresh blastocyst group (ARR: 0.86, 95% CI: 0.77–0.96) (Table VI).