An increasing number of couples are being treated with assisted reproductive techniques (ART) for biological and social reasons. Strong and positive relationship between embryo quality and clinical pregnancy rate (CPR) has been reported. Even though transferring good-quality embryos (GQEs) is preferable, for a large number of patients without GQEs, the transfer of poor-quality embryos (PQEs) is unavoidable [1, 2].

A study comparing outcomes in patients undergoing single embryo transfer and double embryo transfer have conflicting results including both an improvement or no benefit on live birth rate [3]. A systematic review and meta-analysis of 85 studies concluded that, in women under 40 years old or in the presence of GQE, single embryo transfer (SET) should be implemented. In the absence of GQEs, double embryo transfer (DET) may be preferable [4]. A retrospective study reported that live birth rate (LBR) were significantly higher with SET than with DET [5]. Although SET has become a suggested approach over time, multiple embryo transfers are still common in clinical practice [6]. Some in vitro fertilization (IVF) clinics are supporting multiple embryo transfers, as it has been previously reported that DET transfer increases pregnancy and LBR [7, 8]. In a systematic review and meta-analysis of 5 randomized controlled trials, single blastocyst stage transfer was associated with a higher ongoing pregnancy rate and birth rate than single cleavage stage transfer [9].

Advanced age, poor ovarian reserve, and a history of previous failed ART cycles are predictors of poor prognosis. Multiple embryo transfer is often considered in this patient group [10]. However, this patient group has a limited number of embryos, with good quality embryos either absent or few. Therefore, the clinician may decide to transfer PQE in addition to GQE to improve IVF success.

There are limited studies evaluating the negative impact of PQE transfer with GQE on pregnancy rates in fresh in vitro fertilization–intracytoplasmic sperm injection (IVF-ICSI) cycles. Thus, the aim of the current study was to evaluate the effect of transferring 1 GQE plus 1 PQE on clinical pregnancy and LBR in fresh ART cycles. This information may provide guidance for the management of residual PQEs that are not suitable for cryopreservation.

The study included 1631 cases aged 20 to 40 years. Age data were examined categorically in 2 groups as 35 years and $\ge$35 years. Table 1 presents demographic characteristics, infertility etiology, menstrual cycle day 2 estradiol (E₂) values, the number of oocytes retrieved, the number of embryos obtained, and the day of the transferred embryo results according to the groups. The groups were found to be statistically similar in terms of infertility etiology, menstrual cycle day 2 E₂ values, and the day of the treatment when the transfer was made (p $>$ 0.05) (Table 1).

The ages differed significantly based on the quality of the transferred embryos (p 0.01). The number of oocytes retrieved in the 1 GQE group and 2 GQEs group were statistically similar and the number of oocytes retrieved was statistically higher than in groups 3, 4 and 5. Group 1 and Group 2 were statistically similar in terms of MII oocyte numbers. Groups with statistically different MII oocyte numbers were identified by post-hoc analysis. The number of embryos obtained from Group 1 cases was statistically significantly lower than the number of embryos obtained from Groups 2 and 3. The number of embryos obtained from Group 4 cases was found to be significantly lower than all groups.

CPR and LBT for the 5 groups are presented in Table 2. CPR for cases where 2 GQEs were transferred (Group 2) were found to be significantly higher than those for cases where 1 GQE and 1 PQE were transferred (Group 3) (48.6% vs. 26.2%, respectively, p 0.05). CPR were statistically similar between single embryo transfer (Group 1) and 1 GQE + 1 PQE transfer (Group 3) groups (33.7% vs. 26.2%, respectively, p $>$ 0.05). LBR were statistically similar among the groups in which at least 1 PQE was transferred (Group 3, 4 and 5). LBR for Group 3 in which 1 PQE with 1 GQE was transferred were found to be significantly lower than Group 1 and Group 2 (16.7% vs. 26.3%–36.7%, p 0.05). There was no statistical evidence that transferring a PQE over a GQE affected pregnancy rates. Transfer of a PQE together with a GQE resulted in a statistically significant lower live birth rate compared to transfer of a single GQE or 2 GQEs (p 0.001).

The cases included in the study were between 20–40 years of age. The study group was divided into 2 categories as 35 years of age and $\ge$35 years of age. The regression analysis results to determine independent predictors of CPRs and LBRs for each age category are presented in Tables 3,4. Day 5 ET improved the CPRs in both age groups (relative risk (RR), 95% confidence interval (CI): 1.875, 1.267–2.775, p = 0.002 and RR, 95% CI: 2.973, 1.277–6.918, p = 0.011). Day 5 embryo transfer improved LBRs only in 35 years old cases. Two GQE embryo transfer was found to be effective than other transfer options for CPRs. Two GQE ET was found to be decisive in improving LBRs only for cases under 35 years of age. Regression analysis demonstrated that 2 GQE embryo transfers had a statistically significant effect on LBRs only in cases under 35 years of age (RR, 95% CI: 2.314, 1.164–4.601) (Tables 3,4).

Based on the results of our study, we determined that GQE transfer combined with a PQE transfer did not have a positive effect on CPR. On the contrary, an additional PQE negatively affects LBR.

The benefit of 2 embryo transfers is debatable in cases where a GQE is transferred with a PQE in IVF-ICSI cycles. The importance of the interaction between the embryo and the endometrium in the implantation process has been demonstrated. Poor quality embryo transfer may negatively affect endometrial receptivity and may also prevent implantation of a GQE [12, 13]. The effect of transferring a PQE as a second embryo in fresh IVF-ICSI cycles on pregnancy rates has been studied. Some studies have reported that PQE has no negative effect on GQE [14, 15]. A study reported that the transfer of a PQE accompanied by a GQE compared to a single GQE and double GQE transfer did not show a difference in LBR [14]. Pregnancy rates and LBR have been reported to be similar when the second embryo transferred alongside a GQE is either a GQE or a PQE [15]. In contrast, transfer of 1 GQE and 2 GQEs has been reported to be associated with higher implantation and pregnancy rates than transfer of 1 GQE with 1 PQE [16]. In the group of patients with poor prognosis, it has been found that an additional PQE to GQE does not negatively affect implantation rates, but increases live births and multiple pregnancies [17]. In a meta-analysis, it has been reported that DET including GQEs and PQEs does not lead to an increase or decrease in CPR and LBR compared to a single GQE transfer, but is associated with a higher multiple pregnancy rate [18]. Similarly, Hill et al. [19] reported that a concomitant PQE transfer does not have a negative effect on good quality blastocyte transfer and causes a significant increase in the risk of multiple pregnancies. It has been stated that single GQE embryo transfer is beneficial if GQE is obtained in women under the age of 40 years. It has been added that DET can be planned if GQE is not obtained. The same study emphasized that there is insufficient evidence for women over 40 years of age [5]. In a group of patients who underwent frozen-thawed embryo transfer, no improvement in LBRs was observed with the addition of PQE; but in patients who underwent embryo transfer at the blastocyst stage, an increase in LBR was reported with GQE + PQE double embryo transfer [20].

Our study demonstrated that an additional PQE transfer together with GQE did not have a positive effect on CPR and LBR. On the contrary, it was concluded that an additional PQE negatively affected LBR.

The retrospective nature of our study may be a limitation. Patients were followed up with a telephone survey, which is less reliable than medical records. We did not record any data to evaluate endometrial receptivity. It would be a more valuable study if we could discuss endometrial receptivity and pregnancy success with the data of our cases. There may be bias in the embryo evaluation process. In one review, the overall median distribution of artificial intelligence models was reported as 74% (range 64%–98%, inter quartil range (IQR) = 68.5–83.5) while the distribution of embryologists was 64% (range 53%–76%, IQR = 55.5–69.5). Studies conducted with the results of processes involving artificial intelligence models may change these results over time. However, the strength of our study can be stated as being performed in a single center and by the same competent embryologists [21].

In conclusion, our results demonstrated that an additional PQE transfer in fresh embryo transfer cycles did not have a positive effect on CPRs and LBRs. On the contrary, an additional PQE negatively affected LBRs with GQE transferred together. Evaluating the issue in different infertile patient groups and frozen embryo transfer cycles may provide more accurate and different results.

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

BEB, İŞ and NUT designed the research study. BEB, ÇYA, Gİ and HTK performed the research. BEB, HTK and NUT analyzed the data. All authors contributed to writing and editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by Zeynep Kamil Women’s and Children’s Diseases Training and Research Hospital Institutional Review Board (Date: 17.04.2019; Number: 47). Since the study was retrospective, informed consent was not obtained. Patients’ personal data were shared only with the study team.

This research received no external funding.

The authors declare no conflict of interest.