Interpretation
Female age and quality of transferred embryos were usually accepted as
the two most important causes of bias in assay comparisons. In the
study, IR, CPR, MPR and LBR were decreased significantly and AR
increased significantly with age. The differences of IR, CPR, AR, LBR
and EPR between groups S and L were significant in < 35 age
group, while the differences of these rates were almost similar in the
groups 35-39 and > 39. These results indicated female age
was a critical impact factor for clinical outcomes, and different female
age distribution of patients in different studies may be connected with
different study results. Previous studies demonstrated that D3 embryos
with ≤ 6 blastomeres were associated with low IR, CPR, and LBR and high
AR compared with those with > 6
blastomeres.10, 12, 16 So, the quality of transferred
embryos may be another factor contributing to the discrepancy in
different studies. One limitation of this study was that patients’
characteristic of
the
number of oocytes from the OPU was not mentioned. Because, the limited
number of oocytes available after ovarian stimulation was usually
associated with advanced maternal age and/or premature ovarian
insufficiency, and dramatically decrease of the number and implantation
potential of oocytes were together with a dramatic pick up of aneuploidy
rates over 70%.17 In addition, women with advanced
maternal age had higher risk for miscarriage (over 40%) as well as
drastically lower rate of clinical pregnancy.17Therefore, the limited number of oocytes available after ovarian
stimulation may be an important risk factor affecting the LBR.
With the development of in vitro culture technology, elective single
blastocyst transfer has become a tendency, and this practice has led to
a remarkable increase in LBR and decrease in MPR.18,
19 However, the failure to form blastocysts in patients with few day 3
cleavage stage embryos would lead to an increase in cycle cancelation.
Therefore, a cleavage transfer policy is still adopted in most centers,
as this practice can be used for a wider range of infertility patients.
Two or three cleavage stage embryos are transferred in traditional
cleavage-stage transfer policy which increases the CPR with the
occurrence of high MPR at the same time. Previous study found that
morula embryos, developed from post-thawed cleavage embryos, resulted in
a higher IR, CPR and LBR than non-morula embryos in FET
cycles,20 and extended culture of D3 embryos with ≤ 6
blastomeres to blastocysts, particularly 6-cell embryos, resulted in a
similar clinical pregnancy rate as that of blastocysts derived from D3
embryos with ≥ 7 blastomeres.12 The CPR, IR, and LBR
were similar in the elective single morula embryo transfer group and
elective single blastocyst embryo transfer group.21,
22 A trend toward higher clinical and ongoing pregnancy rates was noted
after day 4 embryo transfers when compared with day 3 embryos in ET
cycles.23 The development of the fair embryos
transferred after 20-22 h in the culture had a highly positive impact on
the pregnancy rate.24 Our study also found that
transferred embryos with more blastomere growth had a higher IR, CPR,
MPR and LBR. The higher MPR in the > 2 group (47.5%)
caught our attention and suggested that the embryos with more blastomere
growth number were associated with better embryo viability. Even if two
embryos with less blastomere growth number (≤ 2 group) were transferred,
the MPR remained high (29.0%). Therefore, transferring a well-developed
embryo after long post-thawed culture period may be a viable embryo
transfer strategy to decrease MPR and cycle cancellation rate while
maintaining CPR and LBR. The reasons of higher IR in the L group may be
that embryos with hardened zona pellucida have a relatively long time to
hatch out and implant in the synchronous endometrium. Moreover, the
development of embryos in vitro lags behind that in vivo, so earlier
thawing is beneficial to synchronize the development of embryos and
endometrium.
Neonatal characteristics of gestational age, preterm birth rates,
cesarean section rate, birthweight, low birthweight rate, sex ratio and
congenital malformation rate were similar in the long culture period
group compared with these in the short culture period group. These
results indicated that long post-thawed culture period might be harmless
to the clinical outcomes of FET compared with short post-thawed culture
period. Multiple pregnancies are considered the most significant adverse
event associated with assisted reproductive technologies and is linked
to an increased risk of maternal and neonatal morbidity. The study also
showed the twin group had a lower average gestational age and
birthweight as well as a higher cesarean section rate, preterm labor
rate and low birthweight rate than the singleton group. Therefore,
elective single well-developed embryo transfer after long post-thawed
culture period or elective single blastocyst transfer after blastocyst
culture are recommended to decrease the risk of maternal and neonatal
morbidity caused by multiple pregnancy.