This is the first study to compare pregnancy outcomes between fresh ET and FET cycles for specific patients transferred with single poor cleavage-stage embryos. The results of this study showed that the rates of clinical pregnancy and live birth of patients in FET cycles were significantly reduced, but the birth weight and proportion of macrosomia were significantly increased compared with fresh ET. Therefore, for patients with a high probability of forming poor cleavage embryos, including women with previous low response to COS or reduced ovarian reserve, fresh ET is recommended as a priority for better pregnancy and neonatal outcomes unless there are special situations.
Studies have shown that FET cycles result in a higher live birth rate compared with fresh ET cycles [11, 18], but other studies have not reached this conclusion [6, 7]. For patients undergoing ICSI [19] or women with a thin endometrium [20], the live birth rate after FET was significantly higher than that in fresh ET cycles. However, two systematic reviews and meta-analyses showed no statistical difference in the live birth rate between the FET and fresh ET groups in normal responders [6, 7]. Currently, the cumulative live birth rate (CLBR) is regarded as a significant indicator for assessing the success of assisted reproductive technology. A recent RCT study showed that the cumulative ongoing pregnancy rate in patients with the freeze-all strategy was inferior to that of the conventional strategy, consisting of fresh and frozen-thawed blastocyst transfer [21]. Moreover, Boynukalin et al. showed that the freeze-all strategy resulted in a higher CLBR compared with a fresh-transfer strategy among patients with > 10 oocytes retrieved, and comparable outcomes were observed in patients with ≤ 10 oocytes retrieved [12]. Therefore, when interpreting these results, some factors need to be taken into consideration, including the specific study population, ovarian response, transferred embryo stage, COS protocol, and fertilization method, which may explain the inconsistent results of many studies.
The above-mentioned studies mostly focused on the cycles transferred with blastocysts. Similarly, the evidence of studies that included cycles treated with cleavage stage embryos is still not unequivocal. Among ovulatory women without polycystic ovary syndrome (PCOS) undergoing IVF, the live birth rate did not differ significantly between fresh ET and FET cycles [10, 22]. However, Chen et al. showed that FET was associated with a higher rate of live birth than fresh ET among infertile women with PCOS [9]. In addition, Liu et al. revealed that FET is better than fresh ET in the GnRH antagonist cycle in women with 3–10 oocytes retrieved [23]. However, FET was associated with lower implantation rate and live birth rate in infertile patients who underwent the GnRHa long protocol compared to fresh ET [24]. The patients recruited in the above-mentioned studies were transferred with high-quality cleavage embryos. Currently, there is no study to explore whether the process of freezing and thawing will significantly affect the pregnancy outcomes of FET compared with fresh ET for the specific population who transferred with poor-quality cleavage embryos. The results of this study showed that for patients transferred with poor-quality cleavage stage embryos, the live birth rate of FET cycles was significantly lower than that of fresh ET cycles. This may be because the vitrification procedure may have more potentially negative effects on pregnancy outcomes for cycles transferred with poor-quality cleavage stage embryos than high-quality embryos. Future randomized controlled trials are needed to confirm this conclusion and focus on exploring the potential physiological mechanisms behind these observed clinical outcomes.
So far, the findings of studies exploring the effect of FET versus fresh ET cycles on obstetric and perinatal outcomes are largely consistent. Studies have shown that FET is associated with an increased risk of preeclampsia, macrosomia, large for gestational age (LGA), and higher birth weight than fresh ET [9, 12–14, 16]. A recent study showed that the freeze–thaw process seems to be associated with excess fetal growth, leading to higher birth weight and greater risk of LGA, at least in the last trimester of pregnancy [25]. One possible reason is that in the first trimester, the placental developmental parameters, including placental volume (PV) and utero-placental vascular volume (uPVV), were comparable between pregnancies resulting from FET and fresh ET cycles. However, uterine artery indices in the second and third trimester after FET were lower than those in pregnancies after fresh ET, and this is associated with higher placental perfusion [26], which may result in higher birth weight after FET. A recent study showed that imprinted genes are differentially expressed in fresh ET placentae and frozen ET placentae, and altered imprinted gene expression may affect glucose transport and cell proliferation, which play an important role in placental development [27]. Another study by Sacha et al. found more anatomic and vascular placental pathology in pregnancies from FET than in pregnancies from fresh ET cycles [28]. Whether these differences are the potential mechanisms for the significant increase in birth weight from FET compared to fresh ET, further research is needed to explore how they affect obstetric and perinatal outcomes. The results of our study are consistent with the conclusions of previous studies and found that the proportion of macrosomia and birth weight from the FET is significantly higher than that of fresh ET cycles.
Recently, two studies have explored the effect of freezing on neonatal growth compared with fresh ET cycles up to 5 years of age. The study by Ainsworth et al. confirmed an association of increased birth weight after FET versus fresh ET, but the association did not persist when controlling for maternal prenatal and antenatal factors associated with increased infant birth weight. They found no effect of FET versus fresh ET on neonatal weight and childhood weight gain trajectory [29]. An observational cohort study by Hannu et al. suggested that childhood growth did not differ between term singletons after FET versus fresh ET when correcting for exact age at measurement and adjusting for maternal BMI and paternal height [30]. These results provide evidence that FET does not cause a clinically significant effect on long-term growth of the offspring and reassurance of the safety and feasibility of the steadily increasing use of FET in assisted reproduction.
There are several limitations to this study that need to be considered. First, in order to implement statistical analysis of these clinical data and interpret the results more conveniently, this study only recruited women who were transferred with single poor cleavage-stage embryos. However, considering the sample size included in the present study, we believe that the results of this study are still valuable for guiding clinical practice. Additionally, the retrospective nature of this study is another major limitation. Therefore, the conclusions need to be confirmed through further randomized controlled trials.