11.1 Introduction

Despite the various advances and the increasing success rates of assisted conception treatment, implantation continues to be a rate limiting step. For implantation to occur a blastocyst must attach to and invade the endometrium and as such both the embryo and endometrium are considered critical to the process of implantation. However, there are other factors to consider. Many conditions of the uterine cavity may influence the ability of the embryo to implant such as uterine submucosal fibroids and endometrial polyps, which are well recognized to exert an adverse effect. In addition, an embryo’s implantation potential may be affected by sperm and oocyte quality. Iatrogenic factors such as laboratory conditions and embryo transfer technique play an important role in successful implantation, hence this chapter will focus on the embryo transfer procedure. This is the final step in the treatment cycle, the culmination of both clinicians’ and embryologists’ efforts and a day of great hope for the patients. The importance of the embryo transfer procedure is not to be underestimated. This chapter will highlight the importance to clinicians of not adopting a “one-size-fits-all” approach when planning embryo transfer. It is incumbent on reproductive medicine specialists to focus on the embryo transfer procedure, try to preempt any potential issues that may adversely affect success rates and adopt an individualized plan for embryo transfer when necessary.

11.2 To Transfer in Fresh or Frozen Cycles?

In recent years, the number of frozen-thawed embryo transfer (FET) cycles performed has increased significantly facilitated by improved laboratory techniques. Embryo cryopreservation has undoubtedly resulted in increased IVF/ICSI cycle cumulative pregnancy rates.

A factor contributing to the increasing incidence of elective embryo cryopreservation has been the accumulating evidence to suggest that IVF outcomes may be improved by performing FET cycles as compared to fresh embryo transfers (1, 2). Studies have suggested that controlled ovarian hyperstimulation adversely affects endometrial receptivity (3–5). Kolibianakis et al. demonstrated that endometrial advancement on the day of oocyte retrieval is present in all cycles stimulated with GnRH antagonists, recombinant FSH and hCG (5). The chance of pregnancy is significantly decreased in the presence of extreme endometrial advancement at the time of oocyte retrieval (5). This adverse affect of ovarian stimulation on endometrial receptivity is attributed to supraphysiological levels of both estrogen and progesterone inducing morphological and biochemical endometrial alterations resulting in a more advanced endometrium as compared to natural cycles (6–8). This endometrial advancement leads to asynchrony between embryo stage and endometrium which may be avoided by preferentially cryopreserving embryos and transferring them later into a more physiologically normal environment (6–8). Studies also suggest that superovulation may be detrimental to implantation by altering genes crucial for the endometrium–embryo interaction (9, 10). Many studies have suggested a freeze-all policy in which all embryos are electively cryopreserved for transfer in a subsequent frozen-thaw cycle in the hopes of providing a more physiological environment for embryo transfer and improving implantation and pregnancy rates (11–13).

For patients who experience repeated implantation failure despite the transfer of good-quality embryos, preimplantation genetic screening and elective “freeze-all” should be considered and discussed with the couple. It is interesting to note that a recent prospective cohort study identified a statistically significant improvement in ongoing pregnancy and implantation rates following a freeze-all policy for patients with recurrent implantation failure (14).

For patients planning preimplantation genetic screening, cryopreservation of embryos facilitates trophectoderm biopsy. The current testing method of blastocyst-stage trophectoderm biopsy with the use of next-generation sequencing (NGS) and 24 chromosome screening has resulted in significantly improved implantation rates as compared to the initial day 3 single blastomere biopsy and the initial genetic testing method FISH (fluorescence in situ hybridization).

Undoubtedly, ovarian stimulation with exogenous gonadotropins is a fundamental step in the IVF process resulting in the production of multiple follicles, with each secreting estrogen into the circulation, resulting in supraphysiological estrogen levels. There is now a growing body of evidence to support the association of elevated peak E2 levels with not only decreased success of IVF but also with increased risk of adverse perinatal outcomes including small for gestational age babies and preeclampsia (15–17). Preliminary data suggest that cryopreservation with subsequent embryo transfer in unstimulated cycles will decrease the risk of these perinatal complications (18). As reproductive medicine specialists it is mandatory that at the first consultation with the patient, a detailed full medical history including obstetric details such as, previous pregnancy complications and mode of delivery is taken. This growing body of evidence regarding supraphysiological estrogen levels in ART and subsequent pregnancy complications may in time become a factor when deciding on fresh or frozen embryo transfers for patients. Reproductive medicine specialists must aim for healthy, full term pregnancies with safe outcomes for both mother and baby. This responsibility does not end with a positive pregnancy test.

Certainly elective embryo cryopreservation and subsequent frozen embryo transfer appears to be a reasonable option for patients with high estrogen levels or early elevated progesterone levels during stimulation and particularly for those patients at high risk of ovarian hyperstimulation syndrome (OHSS) (8). It is well established that an elective “freeze-all” approach is the preferable and safe treatment option for patients at risk of OHSS.

Patients demonstrating early elevations of progesterone during superovulation may also benefit from an elective “freeze-all” approach due to the fact that progesterone is crucial to endometrial receptivity and early elevation of progesterone levels during stimulation may adversely affect embryo implantation due to changes in endometrial receptivity and gene expression (8). Studies have confirmed that late follicular phase progesterone elevation prior to hCG administration is associated with decreased pregnancy rates in IVF due to a shift in the window of implantation (19, 20). In this clinical scenario an elective “freeze-all” approach should be discussed and recommended to the patient.

During stimulation clinicians should be vigilant for risk factors of ovarian hyperstimulation syndrome and monitor serum progesterone levels closely and recommend elective “freeze-all” where appropriate. Recurrent implantation failure despite the transfer of good-quality embryos is very distressing for patients and very frustrating for clinicians. In view of the available evidence concerning endometrial receptivity and IVF/ICSI it is a sensible approach to offer these patients elective cryopreservation of their embryos with later transfer into a more physiologically normal uterine environment.

The most common treatment protocols for frozen embryo transfers include natural cycles with or without HCG trigger or endometrial preparation with hormonal treatment (artificial cycles), with or without gonadotrophin – releasing hormone agonist suppression. Recent studies comparing artificial and natural cycles concluded that the optimal means of endometrial preparation for frozen-thawed cycle remains unclear and both options may be offered to women with regular ovulatory cycles (21–24). This topic is dealt with in-depth in a further chapter.

11.3 Molecular Assessment of Endometrial Receptivity

For patients who have experienced implantation failure despite the transfer of good-quality embryos clinicians can now offer patients the endometrial receptivity assay (ERA) in the hopes of providing the patient with a “personalized” embryo transfer plan. The ERA is a technique using a customized array to identify markers of endometrial receptivity (25). It is based on the analysis of the expression of 238 genes thought to be involved in endometrial implantation (25). This test is performed in the hopes of determining a personalized “window of implantation” and is performed by obtaining an endometrial biopsy sample on day LH + 7 in a natural cycle or on the sixth day of progesterone administration during a HRT cycle. Results are expressed as prereceptive, receptive, or postreceptive. According to the result, the timing of the embryo transfer should be adjusted facilitating a “personalized” embryo transfer. The ERA test requires large randomized studies to validate its use prior to introduction into routine clinical practice.

11.4 Endometrial Cavity Fluid Identified during IVF Treatment

Endometrial cavity fluid is reported to occur during controlled ovarian stimulation in approximately 5 percent of IVF cycles (26, 27). It may be a transient finding during controlled ovarian stimulation or may be found in association with hydrosalpinx and notably, can be identified for the first time at embryo transfer. It is well established that implantation and pregnancy rates are lower if endometrial cavity fluid is associated with hydrosalpinx and if identified on the day of embryo transfer (27–29). There are many causes of fluid in the endometrial cavity including reflux from a hydrosalpinx, cervical obstruction, isthmocele/niche resulting from previous cesarean section(s), subclinical endometrial infection. However it is also important to bear in mind that it may be a physiological and transient finding. The mechanisms by which the presence of fluid within the endometrial cavity adversely affect implantation rates include embryotoxicity from endotoxins or micro-organisms, reduced endometrial receptivity and not surprisingly the presence of fluid in the endometrial cavity could prove a mechanical hindrance to the apposition of the embryo to the endometrium (30). Best practice would include assessment of patients prior to commencing IVF treatment to determine if there are any risk factors which may lead to the presence of fluid in the endometrial cavity. Risk factors for tubal disease include a history of pelvic inflammatory disease, ectopic pregnancy, and endometriosis and these factors should alert clinicians to the possibility of hydrosalpinges, a well-recognized risk factor for the finding of fluid in the endometrial cavity. If the presence of fluid in the endometrial cavity proves transient with no evidence of hydrosalpinx and no fluid identified on the day of embryo transfer, it is appropriate to proceed with embryo transfer. However, if fluid is present in the endometrial cavity on the day of the planned procedure, it is best to postpone, cryopreserve the embryo and address the cause.

11.5 Embryo Transfer (ET) Procedure

The embryo transfer procedure itself is regarded as a simple procedure, but difficult transfers may occur and are associated with reduced pregnancy rates (31–33). The description of a transfer as “difficult” is subjective but is a term often used to describe transfers taking longer than usual, causing pain, requiring change of catheter, cervical dilatation, or use of a tenaculum. Many women presenting for IVF have had previous attempts at cervical instrumentation. Clinicians should ask specifically about any history of difficulty instrumenting the uterus as it may forewarn of a difficult embryo transfer. The woman may have been informed about this by previous clinicians or may remember difficult, painful, or prolonged attempts. In women with recurrent implantation failure where despite the repeated transfer of good-quality embryos, implantation does not occur, the details of previous embryo transfers should be elicited, paying particular attention to any technical difficulties encountered. In the absence of any particular difficulty encountered in previous attempts, there is no evidence that a change of embryo transfer technique will improve the implantation rate.

If the patient has a history of a difficult embryo transfer, many clinics will perform a “trial” embryo transfer procedure. This may be described as the passage of an empty embryo transfer catheter through the cervix, thus mimicking the real embryo transfer procedure (34, 35). Very useful information can be gleaned from the “trial” procedure including the type of speculum and embryo catheter best suited to the patient, need for a tenaculum, and very importantly, the direction and curve of the catheter required to negotiate the internal cervical os. This information is recorded and will assist the clinician in ensuring a smooth embryo transfer on the assigned transfer day. A randomized controlled trail (RCT) involving 335 patients demonstrated that the patients who had a trial embryo transfer procedure had a significantly lower incidence of “difficult” embryo transfer and significantly higher implantation and pregnancy rates (35). The trial procedure can be performed before commencing ovarian stimulation or immediately before the actual transfer.

In an effort to preempt potential problems on the day of embryo transfer clinicians should take into account risk factors for a difficult procedure such as cervical stenosis following cervical surgery, acute anteversion/retroversion or to acute anteflexion/retroflexion of the uterus. Clinicians should be aware of several techniques which may be employed when difficulty is encountered.

Performing embryo transfers under transabdominal ultrasound guidance has many advantages and has been shown to increase ongoing clinical pregnancy rates (36). Many ET catheters have echogenic tips facilitating direct visualization with ultrasound at the cervico-uterine angle which can facilitate insertion of the catheter. Ultrasound guidance will assist in the identification of cases where the catheter tip curls backward, when difficulty is encountered passing through the cervical os and will prevent the inadvertent injection of the embryo into the cervical canal.

Reproductive medicine specialists commonly see patients with secondary infertility with a history of having had one or more previous lower uterine segment cesarean sections and with ultrasound confirmation of an isthmocele/niche. The presence of an isthmocele/niche may render the embryo transfer procedure technically more difficult. Clinicians first need to be aware of its presence and perform the transfer under ultrasound guidance to ensure the catheter bypasses the isthmocele and enters the uterine cavity.

Transabdominal ultrasound guidance will require a second operator and the need to bladder fill. Bladder filling is a simple measure which in itself may be advantageous for those women with acute anteversion or anteflexion of the uterus but will not be helpful in cases of acute retroversion or retroflexion where an empty bladder is preferable (34).

The application of a tenaculum to the anterior lip of the cervix and applying traction gently downward may help to straighten an acutely flexed uterus but may compromise pregnancy rates by inducing uterine contractions (37).

If difficulty is encountered negotiating the internal cervical os a rigid embryo transfer catheter as opposed to a soft catheter may be more beneficial and may assist in avoiding the need to apply a tenaculum to the cervix (38). If the transfer procedure still proves difficult despite employing the above described measures it is best to abandon the procedure, (re)freeze the embryo and formulate a management plan. Alternative methods to transcervical embryo transfer include transmyometrial and tubal transfer but should be reserved for cases which are extremely difficult or impossible (39–41).