Maternal and neonatal outcomes for the different transfer strategies
Maternal and neonatal outcomes for all FET cycles and in each group are summarized in Table 1. Maternal and neonatal outcomes for the four groups were statistically different for biochemical pregnancy, clinical pregnancy, monozygotic twins, miscarriages, maternal complications, live births, twin births, preterm births, low birth weight and SGA (P<0.05). Results of pairwise comparisons for maternal and neonatal outcomes between the groups are shown in Table S1. Percentages for each maternal or neonatal outcomes for each group are shown in Figure 2.
Among the four groups, the biochemical pregnancy rate, the clinical pregnancy rate, and the live birth rate were the lowest in the C-1 group (24.1%, 21.5% and 11.8%, respectively) and were the highest in the B-2 group (59.1%, 53.0% and 33.6%, respectively). However, this was accompanied with higher risks of miscarriages (21.2%), maternal complications (11.9%), twin births (28.9%), preterm births (21.2%) and low birth weight (17.7%) (P<0.008, Bonferroni-corrected p-value for multiple pairwise comparisons). The biochemical pregnancy rates, the clinical pregnancy rates and the live birth rates in the C-2 group and B-1 group were statistically different (46.7% vs 50.0%, 42.1% vs 44.3%, 29.0% vs 23.0%, respectively), with different risks for monozygotic twins, miscarriages, twin births, preterm births, low birth weights and SGA (P<0.008). These results demonstrated that the double cleavage-stage embryo transfer strategy and the single blastocyst-stage embryo transfer strategy were preferential among the four groups. Hence additional comparisons between the C-2 and B-1 groups were performed.
In this study, 20 cases (0.3%) of congenital malformations were reported among live births and included one case (1.1%) in the C-1 group, 12 cases (0.3%) in the C-2 group, 6 cases (0.3%) in the B-1 group, and one case (0.1%) in the B-2 group. However, due to the limited sample size, comparisons of congenital malformations between the groups were not performed.
In addition, comparisons were performed between the double-cleavage transfer (C-2) and the single-blastocyst transfer (B-1) group. After adjusting for maternal age, BMI, infertility type, cause of infertility, duration of infertility, type of fertilization in the current cycle and previous ART cycles, the biochemical pregnancy rates and the clinical pregnancy rates between two groups were not statistically different (aOR for the biochemical pregnancy rate, 1.07; 95% CI, 1.00-1.15; and aOR for the clinical pregnancy rate, 1.04; 95% CI, 0.97-1.12); and the live birth rate in the B-1 group was lower compared to the C-2 group (aOR, 0.78; 95% CI, 0.72-0.85). With regards to negative outcomes, compared to the C-2 group, the B-1 group had a higher risk for monozygotic twins (aOR, 3.02; 95% CI, 1.79-5.10) and miscarriages (aOR, 1.29; 95% CI, 1.11-1.51) but had a lower risk for twin births (aOR, 0.06; 95% CI, 0.04-0.09), preterm births (aOR, 0.51; 95% CI, 0.41-0.65), low birth weights (aOR, 0.30; 95% CI, 0.22-0.40) and SGA (aOR, 0.51; 95% CI, 0.33-0.77)(.
Maternal age is often considered as one of the most important determinants for maternal and neonatal outcomes for any type of ART cycle. Therefore, maternal or neonatal outcomes between the C-2 group and B-1 group and probabilities for each maternal age group (20-29 years old, 30-34 years old, 35-37 years old, 38-39 years old, 40-42 years old and >42 years old) were calculated. Results are shown in Figure 3 (Detailed results are shown in Table S2 and Table S3).
With regards to the C-2 group, the probability of biochemical pregnancy and clinical pregnancy declined from 0.55 and 0.50 at 20-29 years old to 0.27 and 0.22 at >40 years old. Of the cycles with clinical pregnancy, the probability of monozygotic twins decreased from 0.008 to 0.005, the probability of miscarriages increased from 0.11 to 0.31 and the probability of maternal complications increased from 0.09 to 0.20 at the same maternal age intervals. The probability of live birth declined from 0.30 at 20-29 years old to 0.11 at >40 years old. Among those cycles with live births, the probability of twin births declined from 0.28 to 0.11, and the probabilities of preterm births, low birth weights, and SGA slightly fluctuated from 0.14 to 0.18, 0.12 to 0.15 and 0.05 to 0.06, respectively. (Detailed results are shown in Table S4).
With regards to the B-1 group, the probability of biochemical pregnancy and clinical pregnancy declined from 0.56 and 0.51 at 20-29 years old to 0.28 and 0.22 at >40 years old. Among those cycles with clinical pregnancy, the probability of monozygotic twins decreased from 0.023 to 0.011, the probability of miscarriages increased from 0.13 to 0.34 and the probability of maternal complications increased from 0.08 to 0.16 at the same maternal age intervals. The probability of live birth declined from 0.25 at 20-29 years old to 0.08 at >40 years old. Among the cycles with live births, the probability of twin births, preterm births, low birth weight, and SGA slightly fluctuated from 0.01 to 0.02, 0.07 to 0.10, 0.04 to 0.05 and 0.02 to 0.03, respectively.