Results
A total of 3920 cases met the inclusion and exclusion criteria, including 1822 cases in the fresh embryo transfer group and 2098 cases in the FET group. In the FET group, there were 1353 cases of NC-FET and 745 cases of HRT-FET. S1 shows that the differences in prepregnancy BMI, the number of pregnancies and deliveries, the endometrial thickness on the day of hCG injection/transformation, and whether women had polycystic ovarian syndrome (PCOS) or a history of previous uterine operations were statistically significant among the three groups (P<0.001). The prepregnancy BMI and proportion of women with PCOS in the FET group were significantly higher than those in the fresh embryo transfer group and the HRT-NC group (P < 0.001). The proportions of blastocyst transfer and ICSI in the FET group were significantly higher than those in the fresh embryo transfer group (P<0.001 and P=0.001, respectively). The proportion of first-pregnancy and primipara was higher in the fresh embryo transfer group than in the FET group, and more people in the FET group had a history of uterine manipulation than those in the fresh embryo transfer group (P < 0.001). The comparison of endometrial thickness on the day of hCG injection/transformation showed that the endometrium of the fresh embryo transfer group was the thickest, followed by that of the NC-FET group, and the endometrium of the HRT-FET group was the thinnest.
There was a significant difference in the incidence of preeclampsia among the three groups, and further comparison indicated that the incidences of severe preeclampsia and early-onset preeclampsia were also statistically significantly different. The risks of preeclampsia, severe preeclampsia and early-onset preeclampsia were significantly higher in the HRT-FET group than in the fresh embryo and NC-FET groups (P>0.05) .The rates of postpartum haemorrhage and caesarean section among the three groups were statistically significantly different (P < 0.05), with the rate of the HRT-FET group being significantly higher than those of the fresh embryo and NC-FET groups (P < 0.001). Further analysis of the causes of postpartum haemorrhage in the three groups showed that the most common cause was uterine inertia, followed by placental factors. In the fresh embryo transfer group, 28% developed postpartum haemorrhage due to placental factors, with placenta previa being the most common placental factor. In the NC-FET group and HRT-FET group, the incidence of postpartum haemorrhage due to placental factors was 18.1% and 27.2%, respectively, of which placental implantation was the most common cause. When comparing the proportions of women with cervical insufficiency, the data revealed that the proportion of women with cervical insufficiency in the HRT-FET group was significantly higher than that in the fresh embryo transfer group (P<0.05). The preterm birth rate was higher in the HRT-FET group than in the fresh embryo transfer group (P=0.022) and the NC-FET group (P=0.026) (Table 1).
Table 2 shows that the rate of neonatal transfer to the NICU in the HRT-FET group was higher in the fresh embryo transfer group and NC-FET groups (P < 0.001), whereas there was no difference between the fresh embryo and NC-FET groups (P > 0.05). The most common reasons for neonatal transfer to the NICU in the fresh embryo, NC-FET and HRT-FET groups were suspected neonatal infection, neonatal jaundice and neonatal respiratory distress, respectively. The three groups were statistically significantly different (p<0.05) in the delivery rates of large for gestational age and small for gestational age neonates and in neonatal birth weight, with the neonatal birth weight being significantly lower in the fresh embryo transfer group than in the NC-FET group (p=0.004) and HRT-FET group (P=0.001). The proportion of large for gestational age neonates was higher in the HRT-FET group than in the fresh embryo transfer group (Table 2).
Table 3 demonstrates that there were no statistically significant differences in the incidences of placental structural abnormalities (velamentous placenta, accessory placenta, battledore placenta, circumvallate placenta, etc.), premature rupture of membranes or placental abruption among the three groups (P > 0.05). In terms of placental adhesion and placental implantation, the incidence was significantly different among the three groups (P < 0.001), in which the rate was higher in the HRT-FET group than in the fresh embryo transfer group. In women with no previous uterine operation, the incidence of placental adhesions was highest in the HRT-FET group (4.3%), followed by the fresh embryo transfer group (2.2%) and the NC-FET group (0.8%), and the incidence of placental implantation was significantly lower in the NC-FET group than in the HRT-FET group (p=0.001). In addition, the difference in the proportion of women who developed placenta previa among the three groups was statistically significant (P=0.020), and the incidence of placenta previa was higher in the fresh embryo transfer group than in the HRT-FET group (P=0.007).
Of the live birth cases enrolled, 521 placentas were examined for placental pathology after delivery. An analysis of the pathological reports revealed that there was no statistically significant difference in the incidence of infarction, calcification, interstitial haemorrhage, syncytiotrophoblastic nodule hyperplasia, fibrin deposition or chorionic villus infarction among the three groups (all P > 0.05).The incidence of chorioamnionitis was significantly different among the three groups (P=0.020), with a lower incidence of chorioamnionitis in the HRT-FET group than in the NC-FET group (P<0.05) (Table 4). After adjusting for potential complications, including premature rupture of membranes, gestational diabetes, cervical insufficiency, premature birth, urogenital system infection, and a history of intrauterine operation, it was evident that there was no significant association between the different embryo transfer methods and chorioamnionitis (odds ratio (OR)=1.117, 95% confidence interval (CI): 0.899-1.387, p=0.320).