To ensure optimal outcomes with eSET, patient selection plays an important role. Selective application of eSET in a small group of good-prognosis patients may be effective in reducing the overall multiple rate of an entire IVF population without substantially reducing the likelihood of achieving a live birth [8]. Good-prognosis patients may be considered as women aged ≤35 years, in their first or second IVF attempt, and with at least two good quality embryos available for transfer. Women aged 36–37 years may also be considered good-prognosis patients for eSET if good quality embryos, particularly blastocysts, are available for transfer as blastocyst stage embryo transfer generally increases the chance of implantation and live birth compared with cleavage stage embryo transfer. Kresowik et al. [9] reported a live birth rate of 64.6 % and a multiple birth rate of 3.4 % following a mandatory single embryo transfer (mSET) policy for all women aged <38 years, with at least seven zygotes, no prior failed fresh cycle, and at least one good quality blastocyst [9]. In women aged ≥38 years, eSET may result in a significant reduction in live birth rate compared with DET [8].

Success with an eSET would be compromised if the embryo quality suffered. Morphological methods used to select the most viable embryos for transfer may be far from predictive of the implantation potential of these embryos. A paradigm shift using morphological factors along with metabolic, protein and genetic markers in culture media aims to enhance embryo selection and IVF success rates [10] and is particularly useful in selecting single embryos for transfer. Several advanced techniques for embryo selection such as rapid polymerase chain reaction (PCR)-based comprehensive chromosome screening (CCS) and trophectoderm biopsy prior to SET have been reported to enhance embryo selection, with a resultant increase in the ongoing pregnancy rate (55.0 % vs. 41.8 %, respectively; p < 0.01) and a decreased miscarriage rate compared with traditional blastocyst SET (24.8 % vs. 10.5 %; p < 0.01). These novel screening techniques may provide a practical way to eliminate multi-zygotic multiple gestation without compromising clinical outcomes per cycle [11]. Image acquisition and time-lapse analysis of the embryos optimise accurate embryo selection of viable embryos by identifying the morphokinetic parameters specific to embryos capable of implanting and thus, make it possible to determine the exact timing of embryo cleavages in a clinical setting [12]. New technology, based on embryo developmental and morphological characteristics, using multilevel images combined with a computer-assisted scoring system (CASS) has the potential to overcome the disadvantages with standard embryo evaluation with a superior ability to predict implantation and live birth [13].

Improvements in culture protocols facilitate extended culture to the blastocyst stage and by enabling self-selection of viable embryos and improved uterine and embryonic synchronicity, result in higher implantation rates [14, 15]. Excellent pregnancy rates have been reported with SET blastocyst culture with live birth delivery rates comparable to double cleavage stage transfer (27.2 % vs. 24.8 %) and decreased complications related to multiple births [15]. A significant threefold increase in day 5 single embryo transfers over an 8-year period (4.5 % in 2001 to 14.8 % in 2009; p < 0.0001) has been associated with a significant decrease in the rate of multiple births from 44.8 % to 41.1 % (p < 0.0001) [3].

Elective SET with cryopreservation has been suggested to be more effective in maximising the cumulative live births and significantly less expensive than DET in good-prognosis patients and therefore, from a cost-effectiveness perspective, should be adopted as a treatment of choice [8, 16]. In order to maintain the reduction in the rate of multiples achieved with fresh eSET, eSET should be performed in subsequent frozen-thawed embryo transfer cycles [8]. Patients should be informed of the reductions in both multiple pregnancy rate and overall live birth rate after a single fresh eSET when compared with DET in good-prognosis patients [8].

The significance, growing awareness and positive clinical outcomes obtained following SET mandate the performance of a meticulous, atraumatic ET technique that aims to successfully deliver a single good quality embryo in the uterine cavity without associated difficulty. The significance of the eSET technique stems from the fact that since the possibility of embryo selection in the uterine cavity is eliminated, efforts entailed in the preceding clinical and laboratory protocols and the embryo selection would be rendered useless and the cycle wasted if the ET technique was suboptimal. This is especially true of a fresh first cycle eSET, which can perhaps be salvaged with additional embryos, if available, but will leave fewer embryos for cryopreservation. However, in the case of the unplanned difficult single embryo transfer, the situation may rarely improve and could also compromise the quality of embryo transferred. Though there is no universally acceptable or standard technique for ET, factors documented to have a positive and negative impact on ET must be strictly respected to achieve the desired outcome.

Factors that impact the clinical outcome following ET include (1) routine evaluation of the uterine cavity to detect abnormalities, (2) mock embryo transfer immediately before the actual ET, (3) evaluation of uterine position and dimensions, (4) ultrasound guidance during ET, (5) depositing embryos in the mid-portion of the endometrial cavity, (6) the use of soft catheters, (7) avoidance of uterine contractions, blood, or mucus on the catheter, (8) ensuring an absolutely atraumatic transfer technique and (9) the experience and skill of the clinician performing ET. Evidence detailing the significance of each of these factors is presented below.

A routine uterine cavity evaluation enables a thorough exploration of the uterine cavity to check for abnormalities, such as submucosal leiomyomas, adhesions, polyps and congenital abnormalities, that may interfere with a successful outcome. Endometrial cavity abnormalities have been reported with an incidence of 22.9 % following outpatient hysteroscopy in patients with a previous IVF-ET cycle, the correction of which markedly improves the outcome. Sufficient evidence to support the surgical removal of all abnormalities to improve the IVF-ET outcome and the value of performing this procedure before an initial cycle in patients without previous implantation failure is lacking. However, it would seem logical in an effort to minimise the number of cycles a patient must undergo. Three-dimensional saline sonohysterography may be particularly useful in the evaluation [17].

Before proceeding to ET, it is essential to have adequate knowledge about the uterine position, anteverted (AV) or retroverted (RV) by ultrasonography (USG). An RV uterus at mock embryo transfer will often change position at real embryo transfer to become AV [18], changing the course of the ET catheter. Misdirecting the ET catheter can be avoided by accurate knowledge of the uterine position at the time of embryo transfer. Following a comparative evaluation of 996 consecutive mock and real abdominal ultrasound-guided-(USG)-ET embryo transfer cycles, Henne and Milki [18] demonstrated a highly significant (p < 0.0001) change in the position of RV uteri at mock transfer (26 % of 55 % ETs) to AV at the actual transfer compared to the conversion of only 2 % of the 74 % of patients with an AV uterus at mock embryo becoming RV at the actual transfer. The change in uterine position was also noted in frozen-thawed embryo transfers (12 % of AV uteri at mock embryo transfer to RV and 33 % RV uteri to AV at real transfer; p = 0.01). Accordingly, patients with an RV uterus at mock embryo transfer should still present with a full bladder for embryo transfer, since a significant number will convert to an AV position [18]. Moreover, ultrasound evaluation of the uterocervical angulation and uterine cavity length prior to the actual transfer can optimise the ET technique and may reduce the rate of ectopic pregnancies [19, 20].

A mock/trial transfer is essential before actual ET as it enables a thorough knowledge of the uterine position (AV/RV), uterocervical length and angulation that may be of value in accurately guiding the course of the catheter during the actual ET. Additionally, it is of value in revealing intracavitary abnormalities that may interfere with pregnancy and in directing possible surgical management prior to ET. While the value of a mock transfer a few days before the actual procedure has been challenged owing to the change in the uterine position [18], a trial catheterization on the day of ET could prevent most of the unanticipated procedural difficulties during the transfer [17]. Moreover, a USG-trial transfer (UTT) in the office, in preparation for an IVF cycle, has shown to be beneficial as significant differences have been noted between patients when comparing difference in length (DL) to previous pregnancy status and the total cavity depth (sounding depth + DL) (p < 0.05) [21].

Cervical stenosis may be associated with a technically difficult ET, reducing the chances of pregnancy after assisted reproductive procedures. Hysteroscopic revision of the cervical canal results in easier ET by facilitating the course of the transfer catheter through the cervical canal and thus, improved pregnancy rates in patients with cervical stenosis and histories of difficult ET [22].

The amount of fluid volume for day 3 transfer has been shown to have a significant impact on pregnancy and implantation rates. A high fluid volume (40–50 μL) for loading the transfer catheter resulted in significantly higher pregnancy (40 % vs. 23 %; p = 0.012) and implantation rates (24.4 % vs. 14.7 %; p = 0.011) compared to a low fluid volume (15–20 μL; n = 94) [23].

Significantly higher clinical pregnancy rates (39.2 % vs. 22.6 %, respectively, p < 0.001), implantation (20.5 % vs. 12.2 %, respectively, p < 0.001) and live birth rates have been reported following removal of cervical discharge before ET, compared to patients in whom the cervical canal was not cleansed. This suggests that removal of cervical debris prior to ET may have a significant effect on the rate of implantation, pregnancy and live birth [24].

Microbial examination of samples from the fundus of the vagina, the cervix, the embryo culture medium prior and post-embryo transfer, the tip of the catheter and the external sheet shows that the presence of vaginal-cervical microbial contamination at the time of ET is associated with significantly decreased pregnancy rates (Enterobacteriaceae: 22.2 % vs. 51 %; Staphylococcus spp.: 17.6 % vs. 44 %; p < 0.001) when compared to negative culture groups [25]. While catheter contamination by upper genital tract microbes has been suggested to affect the success of ET and administration of antibiotics like amoxicillin and clavulanic acid before ET can significantly reduce microbial colonisation and catheter contamination rates [26], this intervention did not translate into better clinical pregnancy rates [26, 27]. Hence, the routine use of antibiotics at embryo transfer prior to ET is not recommended [26, 27].

The use of ultrasound guidance to perform ET has been one of the most significant advances in the ET technique over the traditional ‘blind’ clinical touch method. Despite the lack of a standard evidence-based protocol, there is substantial evidence that both transabdominal [6, 7, 28, 29] and transvaginal [30] USG-ETs significantly increase clinical pregnancy, embryo implantation, ongoing pregnancy and live birth rates compared to clinical touch alone [6, 7, 28–30]. Occasional studies have demonstrated no benefit with USG-guided ET over the clinical touch method with reference to clinical pregnancy and implantation rates compared to previous ultrasonographic length measurement [31] and in the hands of an experienced operator [32, 33]. However, success in patients with a prior history of difficult uterine sounding or embryo transfer still relied heavily on USG-ET [32]. Of note, the 25 % chance of pregnancy using the clinical touch method alone increased to 32 % (from 28 % to 46 %) when USG-ET was performed instead [34].

Ultrasound-guided ET brings the following advantages to make this an indispensable technique to achieve an optimal outcome:

Indeed, tactile assessment of ET catheter placement has been considered unreliable as the outer guiding catheter inadvertently abutted the fundal endometrium in 17.4 % of transfers, indented the endometrium in 24.8 % and the transfer catheter embedded in the endometrium in 33.1 % transfers. Unavoidable sub-endometrial transfers occurred in 22.3 % of transfers, while USG-ET avoided accidental tubal transfer in 7.4 % of transfers [39]. Measurement of cavity depth by USG is clinically useful to determine the depth beyond which catheter insertion should not occur. The transfer distance from fundus (TDF = cavity depth minus depth of catheter insertion), measured by USG, is highly predictive of pregnancy, unlike that measured by mock transfer as cavity depth by US has been reported to differ from cavity depth by mock by at least 10 mm in >30 % of cases [34]. Moulding the embryo transfer catheter according to the uterocervical angle, measured by ultrasound, increases clinical pregnancy and implantation rates and diminishes the incidence of difficult and bloody transfers compared with the ‘clinical feel’ method. Patients with large angles (>60°) had significantly lower pregnancy rates compared with those with no angle [40]. Significantly higher pregnancy rates per transfer have been reported when ultrasound visualisation was considered to be excellent/good (when the catheter could be followed from the cervix to the fundus by transabdominal ultrasound with the retention of the embryo-containing fluid droplet), compared to fair/poor transfers (where the sequence of events could not be documented). Performance of embryo transfer with a soft catheter under ultrasound guidance with good visualisation resulted in a significant increase in clinical pregnancy rates [36].

Though transvaginal USG-ET may be associated with increased patient comfort due to the absence of bladder distension, the total duration of transfer is statistically significantly higher compared to transabdominal USG-ET [41].

Two-dimensional (2D) USG-ET is the standard for image-guided transfers to monitor catheter passage through the cervix into the endometrial cavity [42], although three-dimensional (3D) USG offers better precision and optimal positioning of uterine catheter tip placement. This is an enhancement in the ET technique and has been shown to improve overall pregnancy rate compared with 2D sonography [42, 43]. Moreover, the disparity of ≥10 mm in transfer distance from the fundus (TDF) between 2D and 3D images may significantly impact the pregnancy outcome [43]. Irrespective of the USG mode used, the important role of USG-ET in optimising pregnancy outcomes warrants perfection in this technical skill.

The type of catheter used for ET (soft/rigid and echogenic/non-echogenic) may influence the degree of trauma to the endometrial cavity during ET. Significantly higher pregnancy (p < 0.0005) and implantation rates (p < 0.01) have been reported with the ultrasoft catheters compared to the more rigid Frydman catheters [35]. A blinded comparison of endocervical and endometrial damage following the use of soft ET catheters [IVF Sydney Set (Cook, Limerick, Ireland), Elliocath (Ellios, Paris, France), Frydman classic 4.5 (CCD, Paris, France)] and rigid ET catheters [Memory Frydman 4.5 (CCD, Paris, France)] demonstrated significantly more frequent endocervical lesions with the soft (63 %) and rigid (85 %) Frydman catheter groups compared to other groups (Elliocath: 29 %, IVF Sydney Set: 26 %; p < 0.0001). Severe endometrial lesions were significantly less frequently observed when soft catheters were used (85 %, 53 %, 32 % and 11 % for Memory Frydman, Frydman classic, Elliocath and IVF Sydney Set, respectively; p < 0.0001) [44]. Blood on an ET catheter is a marker for endometrial microtrauma; all ET catheters can lead to endocervical or endometrial damage, but severe endometrial lesions may less frequently be encountered when soft catheters are used [44]. Though no significant difference in implantation, clinical or ongoing pregnancy rates has been observed following ET with the echogenic catheters (Sure View catheter [44], the echogenic Wallace catheter [45] or the Cook Echo-Tip catheter [46]) and standard catheters without echogenic enhancement, echogenic catheters offer the benefit of superior visualisation due to their ultrasonic contrast properties. This minimises the need for catheter movement to identify the tip [44–46] and significantly shortens the duration of the ET procedure (defined as the interval between when the loaded catheter is handed to the physician and embryo discharge), thus simplifying USG-guided ET [45].

In addition to easy visualisation of the catheter tip, El-Shawarby et al. [47] reported a significantly lower rate of retained embryos in the catheter following ET with the Rocket catheter compared to the Wallace catheter (p < 0.05), although there was no difference in clinical pregnancy and implantation rates [47]. The use of a soft pass catheter was the only variable independently and significantly associated with pregnancy success (OR = 2.74) [48].