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.