Main Findings
During the IVF-ET process, in order to attain a higher clinical pregnancy rate, the strategy of twin embryo transfers are implemented. However, using this strategy also increases the probability of multiple pregnancies1, 5, 6. Multiple pregnancy is a serious complication encountered during assisted reproductivity. It can increase the incidence of abortions, fetal deaths, fetal malformations, fetal intrauterine growth restrictions, and the incidence of pregnancy complications such as maternal anemia and pregnancy-induced hypertension compared to single pregnancy5-9.
In order to reduce the occurrence of multiple pregnancies, the number of embryos transferred should be reduced. However, this could reduce the pregnancy rate. How to reduce the number of transferred embryos without affecting the pregnancy rate is important for the success of assisted reproductive technology. Although single embryo transfer logically reduces multiple pregnancies, the clinical pregnancy rate and live birth rate of single embryo transfer are significantly reduced in older women. Hence, single embryo transfers are rarely used in clinical practice. Several studies have shown that the implantation rate and clinical pregnancy rate of blastocyst transfer are higher compared to cleavage embryo transfer, while the rate of multiple pregnancies and ovarian hyperstimulation are lower10-12. Previous studies have compared the clinical outcomes of twin embryo transfer and single blastocyst transfer. However, these were small cohort studies that were not segregated into the four different transfer groups or segregated based on age13, 14. Hence, we analyzed a very large sample cohort derived from our two centers and divided them into the following four groups: single embryo transfer, double embryo transfer, single blastocyst transfer and double blastocyst transfer groups. We compared their clinical pregnancy rate and perinatal outcome and stratified them based on the age of the patient.
Our results demonstrated that the biochemical pregnancy rate, clinical pregnancy rate and live birth rate in the single embryo transfer group were significantly lower compared to the other three groups, while the abortion rate was significantly higher. The biochemical pregnancy rate, clinical pregnancy rate and live birth rate of the double blastocyst transplantation group were significantly higher compared to the other three groups and were statistically different. However, the twin pregnancy rate and the incidence of maternal and fetal complications were the highest. Hence, we do not recommend these two transplantation strategies. This is consistent with the current clinical choice.
We then compared the twin embryo transfer group with the single blastocyst transfer group. We found no significant differences in biochemical pregnancy rate and clinical pregnancy rate between the two groups, however, the live birth rate in the twin embryo transfer group was higher compared to the single blastocyst transfer group, while the twin pregnancy rate, premature delivery rate, low birth weight rate and SGA in the twin embryo transfer group were significantly higher compared to the single blastocyst transfer group. This does not support our intention to deliver healthy babies. In order to reduce the incidence of maternal-fetal complications and multiple pregnancies, single blastocyst transplantation strategy seems to be the best choice. This may be because blastocyst transfer is more in line with the physiological environment compared to cleavage embryo transfer. This is because under natural physiological conditions, the cleavage embryo develops in the fallopian tube and does not enter the uterine cavity until the blastocyst stage. In addition, endometrium and embryo development are more synchronous and is more conducive to embryo implantation. There is no screening during embryo to blastocyst stage culture, with only good quality embryos developing to the blastocyst stage.
Maternal age is an important factor for the outcome of assisted reproductive technology15-17, hence we grouped patients based on age. We found that in the different age groups, the biochemical pregnancy rate and clinical pregnancy rate of single blastocyst transplantation were slightly higher compared to double embryo transplantation, while the live pregnancy rate was slightly lower compared to double embryo transplantation. With the increase in age, the biochemical pregnancy rate, clinical pregnancy rate and live birth rate for the two groups had a downward trend. In the > 42 age group, the live birth rate for single blastocyst embryo transfer was only 8%, while it was only 11% for the double embryo transfer group. The miscarriage rate increased significantly with the increase in age. For the age group over 42 years old, the miscarriage rate in the double embryo transfer group was 31%, while it was 34% in the single blastocyst transfer group. The rate of maternal complications increased significantly with the increase in age. For the age group over 42 years old, the rate of maternal complications was 20% in the double embryo transfer group, while wit was 16% in the single blastocyst transfer group. What’s more, it seemed that the single blastocyst transfer group was more likely to result in monozygotic twins than the double embryo transfer group, and the rate of monozygotic twins in both of two groups decreased with the increase of age. In addition, there were no significant changes in the birth rates of preterm birth, low birth weight and SGA with an increase in age, however, the birth rate of preterm birth, low birth weight and SGA in the twin embryo transfer group was significantly higher compared to the single blastocyst transfer group for all ages.