Trauma to the cervical canal during ET can cause bleeding and can stain the embryos, possibly decreasing their contact and interaction with the endometrium. Stimulation of the internal ostium by the pressure applied by the catheter is thought to trigger uterine contractions, which can lead to expulsion of the embryos from the endometrial cavity. Obviously, endometrial trauma can cause the same problems and decrease the chances of successful implantation. Therefore, the ideal ET catheter should be atraumatic to the endocervix and endometrium.

“Soft” ET catheters, such as various models of Wallace^® (Smiths Medical, St. Paul, MN, USA), Frydman (Laboratoire CCD, Paris, France), and Cook (Cook Ob/Gyn Inc., Bloomington, IN, USA), are usually preferred over “stiff” or “hard” catheters such as the Tight Difficult Transfer (TDT) catheter (Laboratoire CCD), Rocket (Rocket Medical, Watford, England) and Emtrac-A (Gynetics). Despite the use of similar material, the latter models have firmer inner catheters than those of the former models, and therefore, are considered as “firm” ET catheters. In the past, the Erlangen^® ET catheter with a metallic outer sheath and the TomCat^® (Sherwood Medical, St. Louis, MO, USA) catheter, which was originally designed to drain the bladder of male cats, were also used for human ET. Today, to the best of our knowledge, there are only two brands, which carry metallic outer sheaths: IVF-ET Flex by Dr. Peter Steiner and the Gyneflex by Gynetics. Both are rarely used.

While some physicians prefer to use a firm catheter routinely, most would reserve them for technically difficult transfers, e.g., in the presence of a convoluted or stenotic cervical canal. There are numerous retrospective studies and prospective trials comparing various soft and firm ET catheters. According to two systematic reviews and meta-analyses, the use of soft ET catheters is associated with significantly higher clinical pregnancy rates [4, 5]. Abou-Setta et al. [4] pooled randomized controlled trials (RCTs) comparing soft versus firm ET catheters [4]. The “soft” catheters used in the original studies were Wallace, Frydman, Cook (Sydney IVF and K-soft models), and Gynetics Delphin. The comparators included TDT, Gynetics Emtrac A, Tom Cat, Rocket, or Erlangen as the “firm catheters” [6–15]. The odds of achieving a clinical pregnancy was 1.39-fold higher (95 % confidence interval (CI) = 1.08–1.79, p = 0.01) with the soft ET catheters. One can question whether these results are still valid because nowadays, the Erlangen and TomCat catheters are used rarely, if at all. When the three trials comparing Erlangen or TomCat with Cook and Wallace catheters were excluded from the analysis, the difference was still significantly in favor of soft catheters (Fig. 9.1).

Although it is unclear how soft catheters yield higher pregnancy rates than firm catheters, a logical assumption is that the former inflict less trauma to the endometrium. This was indeed shown in a small study where women underwent a diagnostic hysteroscopy immediately following a mock embryo transfer during the postovulatory period [16]. When the outer sheath was not pushed beyond the internal cervical os, the soft Wallace catheter did not cause any trauma to the endometrial lining, while tunnel-like, groove-like, and ulcer-like lesions were frequently observed following mock ET with Frydman or TomCat catheters [16]. However, one needs to ensure that the outer sheath of a soft catheter is not introduced into the endometrial cavity.

Despite providing a higher overall chance of clinical pregnancy, the risk of failure to negotiate the cervical canal is higher with soft catheters, although the absolute difference was short of statistical significance in the meta-analysis by Abou-Setta et al. [4] (OR = 7.51, 95 % CI = 0.94–60.11, p = 0.06) [4]. Difficulty in negotiating the cervical canal with a soft catheter led to the use of a tenaculum or stylet or sounding the uterus more often than firm catheters (OR = 5.4, 95 % CI = 1.28–222.8, P < 0.0001) [4]. According to two randomized controlled trials and a retrospective study, changing the catheter or the use of a stylet was necessary in more than 25 % of ETs when the transfer was first attempted with a soft Wallace catheter, which is considered to be the benchmark of soft ET catheters [17–19]. The use of a stylet, either by the effect of the stylet per se or the difficulty of ET, which led to the requirement for a stylet, seems to be associated with a decreased chance of implantation (OR = 0.67, 95 % CI = 0.50–0.90, p = 0.01) and clinical pregnancy (OR = 0.63, 95 % CI = 0.42–0.92, p = 0.02) [18].

An alternative method of negotiating the cervical canal and internal ostium is to first proceed with the plastic outer sheath slightly beyond the internal ostium, followed by pushing the inner catheter loaded with embryos into the endometrial cavity. Whether such a strategy converts a “soft” catheter to a “firm” catheter and eliminates the advantage of the former was investigated in a retrospective study [20]. In that study, 18 (45 %) of the 40 women who underwent ET with this alternative technique achieved a clinical pregnancy as compared to 51 (50 %) of 102 women who underwent an “inner catheter first” ET with the same catheters, despite a similar number of embryos transferred. The difference was short of statistical significance (OR = 0.82, 95 % CI = 0.39–1.7, p = 0.59). Although one retrospective study is inadequate to provide solid evidence, soft catheters may be able to maintain their pregnancy rates when used in an “outer sheath first” fashion for difficult cases.

Indeed, catheters, which are specifically designed to be used in the “outer sheath first” fashion, are marketed more recently. These include the Labotect (M) (Goettingen, Germany) ET catheter, all models of Kitazato (Shizuoka, Japan), and the Cook Sydney IVF and Guardia Access ET models. These ET catheters have a slightly curved outer sheath to fit the natural curve of the cervical canal, and there is a small bulb at the tip to facilitate negotiating cervical crypts and the internal ostium. Based on three randomized controlled trials comparing the Sydney IVF or Labotect catheters with the Wallace catheter, similar clinical pregnancy rates are achieved with the new “outer sheath first” catheters and the soft Wallace catheter (Fig. 9.2) [17, 21, 22].

Two meta-analyses reported that clinical pregnancy and embryo implantation rates were significantly improved with ET under ultrasound guidance compared to ET with the “clinical touch” method [23, 24]. This has led to the production of ET catheters with increased echogenicity. Two different techniques are used to increase echogenicity of ET catheters. The more commonly used technique involves integration of a metal ring close to the tip of the inner catheter. Examples include Kitazato catheters, Cook Echotip, and Rocket EchoCat series. The metal ring is <2 mm and only provides increased echogenicity toward the tip. On the other hand, the SureView^® series by Smiths Medical is unique in providing increased echogenicity through the entire length of the inner catheter. This is achieved by mixing air bubbles into the material.

Some physicians are hesitant to use ET catheters with echogenic rings due to concerns about endometrial trauma, which can be inflicted by the metallic ring. This concern does not seem to be justified, because Karande et al. [19] compared the Cook Echo-Tip with the regular Wallace catheter in an RCT. Some 251 women were randomized, and implantation (30 % vs. 35 %), clinical pregnancy (57 % vs. 55 %), and ongoing pregnancy rates (49 % vs. 47 %) were similar between the two catheters. However, it is also possible to interpret these results as a failure of the echo tip catheter to improve clinical outcome [19].

There are two RCTs comparing the SureView catheter with the regular Wallace catheter [25, 26]. In addition to implantation and clinical pregnancy rates, visualization of the catheter and ease of transfer were assessed in both trials. Descriptions used to categorize the ease of ET procedures and catheter visualization were the same in the two trials. ET was categorized as very easy when the catheter passed smoothly through the cervix. If the outer Teflon sheath was used to negotiate the cervical canal, ET was defined as easy, and if a tenaculum was required, ET was defined as difficult. A single experienced physician conducted all ETs in both the trials. They both aimed to dislodge the embryos at 15 mm from the fundal endometrium.

Although both the trials reported improved visualization of the catheter, the incidence of easy transfers was not significantly different between SureView and regular Wallace, despite a trend in favour of the former. However, the difference was short of statistical significance even when data from the two trials were pooled for a meta-analysis (pooled odds ratio (OR): 1.11, 95 % Confidence interval (CI): 0.91–1.34). Neither trial reported significantly increased clinical pregnancy rates with SureView. When the results from the two trials were combined, SureView and regular soft Wallace catheter again yielded similar clinical pregnancy rates (Fig. 9.3). It should be noted that embryo implantation rates were higher in the SureView arms of both trials; however, the difference was significant in only one [25].

Overall, according to the best available evidence, the echogenic SureView catheter does not seem to increase clinical pregnancy rates. These results should be interpreted with caution for several reasons. Both, the number of RCTs and the sample sizes of each RCT are limited. A single experienced physician conducted all ETs in both trials. Any additional advantage of echogenic catheter can be more important for less experienced physicians. Selected participants of the two RCTs could have obscured an additional advantage of echogenic catheters. While Allahbadia et al. [26] did not report the participants’ body mass index (BMI), the vast majority (>95 %) of the participants in the trial by Coroleu et al. [25] had a BMI < 30 kg/m². Echogenic catheters can be more advantageous in a selected group of patients, e.g., obese women or women with a retroverted uterus.