The biological and biochemical mechanisms by which growth hormone (GH) can lead to better clinical outcomes in assisted reproductive technology (ART) remain unclear, but are believed to be related to oocyte development and/or subsequent embryo quality, along with potential effects in the endometrium. Several clinical studies over the past decade have focused on the use of GH for women classified as poor-prognosis in ART programs [1-9] as a desperate means to establish a pregnancy in severely infertile patients. However, the data around any beneficial effect is perplexing, with many studies claiming positive effects and an almost equivalent number of other investigations reporting no significant effect. In 2019, the most powerful prospective, randomized, double-blind, placebo-controlled trial was published, but failed to confirm any benefit from adjuvant GH on clinical outcomes such as the clinical pregnancy (CP) and live birth (LB) rate [8]. However, this prospective randomized controlled trial (RCT)—known as the Live birth, In vitro fertilisation and Growth Hormone Treatment (LIGHT) study—failed to reach its enrolment target, as patients categorized as poor-prognosis resisted inclusion and only 34.9% of the required sample size was recruited [8,10]. Nonetheless, the report demonstrated that GH patients reached oocyte retrieval faster than non-GH patients, as also shown in other studies [9], which may indicate that GH has an effect on folliculogenesis; however, this possibility was not supported by differences in embryo quality [8]. Nonetheless, the links between GH and folliculogenesis, oocyte quality and responsiveness to gonadotrophins is still unclear [2,9,11]. The authors also indicated that while there was no difference in the number of oocytes retrieved or transferred, the low recruitment numbers may mask any perceived improvements [8].

It is increasingly difficult to recruit patients into prospective, well-designed ART studies and to convince them to devote a portion of their narrow reproductive lifespan to research. The LIGHT study performed admirably given that it was conducted in a setting where patients could avail themselves of GH outside of the trial via the clinician and then not consent to study inclusion [8]. Consequently, the unique commercial availability of ART services means that patient demand overpowers the ability to recruit, leaving the field with vastly underpowered studies. This may mean that alternative, weaker studies, such as retrospective cohort studies, may be required to advance the field provided they are stringently designed, conducted and analysed. Two recent retrospective studies by our group showed that CP and LB rates were significantly greater with GH (3.42-fold and 6.16-fold higher, respectively) following the transfer of fresh autologous embryos in stimulated cycles [12,13]. Age at embryo transfer (ET) and morphological quality of the transferred embryo were the only other significant independent predictors of the likelihood of CP and LB, but no significant differences were observed in the proportion of high-quality embryos generated in the GH arm, echoing the most recent prospective study [8]. While these examinations [12,13] were undertaken as retrospective, observational analyses and have significant limitations, they represent one of the largest current GH data-sets published. However, these reports focused on only fresh ET cycles and not subsequent frozen cycles, which are also lacking in the LIGHT study.

Some studies have already investigated the role of GH co-treatment directly with hormone replacement therapy (HRT) in frozen ET (FET) cycles, and indicated that GH may affect endometrium thickness, CP and LB rates [14-17]. Due to their study design, those investigations suggested that GH may have additonal positive effects on outcomes that are related to the endometrium, rather than embryo quality, as the embyros were generated previously without GH exposure. However, only one previous study has examined the cumulative effect of GH on outcomes in fresh and subsequent FET cycles, and demonstrated higher productivity (cumulative) in terms of CP and LB rates in GH-treated patients [1]. We have chosen to build upon this latter study and determine whether GH administration in the stimulation cycle can lead to a higher likelihood of CP and LB in subsequent FET HRT cycles in a new cohort of patients from our clinic. The current retrospective study may provide insight into any “carry-over” effect from GH administration during stimulation cycles into subsequent FET cycles, using a strict single ET and HRT regimen.

The cohort of any ART clinic can be divided into two distinct groups: those with a poor prognosis and those with a good prognosis. This distinction is based on the likelihood of achieving CP and/or LB, and incorporates key determinants of fertility. Female age is the single factor most definitively associated with the CP and LB rate [26], and is also indicative of a woman’s ovarian reserve of primordial follicles remaining in the ovary. The AFC and the AMH level are measures that predict ovarian reserve and underpin the prescription dosage of exogenous gonadotrophins that are replaced after hypothalamic gonadotrophin-releasing hormone suppression in IVF stimulation [27- 29]. The categorization of women as poor-prognosis is generally accomplished by applying the Bologna criteria, which state that poor-prognosis women meet two of the following three parameters: advanced maternal age (≥ 40 years), low ovarian reserve (AMH ≤ 3.6– 7.9 pmol/L or AFC < 5–7 follicles), or poor ovarian response with conventional stimulation (≤ 3 oocytes retrieved) [30]. However, the applicability of these criteria is debated and researchers are developing more nuanced approaches, such as Patient-Oriented Strategies Encompassing IndividualizeD Oocyte Number (POSEIDON), to define “low”-prognosis patients [31]. Nonetheless, these classifications have not been able to define subsets of poor-prognosis patients who can benefit from various ART interventions including adjuvant co-treatment [31]. In our previous studies, we introduced the concept of reduced embryo quality as an additional diagnostic parameter for poor-prognosis patients, whereby those with > 50% of embryos with marked fragmentation are predicted to have poorer outcomes [1,12,13]. In the current study, we used the same criteria to define a poor prognosis, and it is these patients who are offered adjuvant therapies such as GH in order to enhance their primary ART outcomes.

We reported that after adjustment for critical variables such as female cycle age, embryo age, AFC and transferred embryo quality, the CP and LB rates in the GH-treated group ([+]GH) were lower but not statistically significantly different from the younger, good-prognosis patients who did not receive adjuvant therapy ([–]Adj [Good]). Furthermore, the likelihood of CP and LB was lowest in the group of poor-prognosis women who did not receive GH ([–]GH), which was maintained following adjustment for the same covariates. Interestingly, in the cycle age-matched analysis, the (+)GH group showed similar outcomes to those of the good-prognosis group, (–)Adj (Good), and significantly better outcomes than those of the (–)GH group. In this comparison, the (+)GH group showed a up to 2-fold increase in the likelihood of LB in the multivariate analysis, following adjustment for age, AFC and transferred embryo quality. The increased likelihood of LB in embryos previously generated under the action of GH was supported by a corresponding reduction in the miscarriage rate; this reduction was not significant, but may provide support for the premise that GH cotreatment improves oocyte maturation during folliculogenesis in the preceding stimulated cycle [32,33]. This trend for GH was also evident in our previous studies of fresh ET generated under the action of GH [12,13].

In accordance with this previous work on fresh transfers [12,13], the current findings indicated that the use of GH as an adjuvant cotreatment during IVF stimulation had a positive outcome on fresh ET and in addition, in subsequent FET cycles. As GH was not administered during the FET cycles, this suggested that the potential beneficial effects of GH may extend to embryo quality as presented previously [5,9,16], rather than involving an effect on endometrium receptivity [15,17]. However, in our previous studies on embryos generated under GH action, we failed to demonstrate that the adjuvant altered embryo quality to a measurable extent, as the proportion of high-quality embryos on morphological assessment was the same with or without GH [12,13]. This echoes the observations of the LIGHT study, where no significant increase in measurable embryo quality was detected [8]. However, they showed that patients receiving GH reached oocyte retrieval more rapidly than those who did not receive adjuvant therapy, which may indicate a potential effect on folliculogenesis [8]. Furthermore, in our previous reports, patients receiving GH had higher oocyte and embryo utilization rates, which indicated that more “usable” embryos were either transferred or cryopreserved. While this is not a direct measurement of embryo quality, it does imply that those with GH may have more opportunities for ET (and ensuing pregnancies) in later cycles. Nonetheless, several other reports have demonstrated that GH enhanced embryo quality as assessed by morphological grading [5,16], although in our study this enhancement was not detectable. It is possible that such embryos (and the oocytes from which they were derived) have higher competency for generating pregnancies, which is not measured by current morphological grading systems.

Other investigations have found that GH cotreatment increased CP and LB rates for poor-prognosis patients by 10%–20% [1,2,12]. A retrospective analysis of younger poor responders and patients with previous unsuccessful IVF attempts estimated that GH increased CP rates by 25%–30% [34], but had little effect on those with advanced maternal age. Our own recent retrospective analysis also showed little benefit in women over 41 years of age [12], which has implications for GH use in poor-prognosis patients with at least one Bologna criterion, advanced age [30]. However, in prospective studies with 80–145 participants, other researchers have shown that GH had no influence on CP or LB rates in ART [3,4,6]. Moreover, systematic reviews of the literature are also divided on the therapeutic effects of GH, possibly because of the heterogeneity among clinical studies. Two recent meta-analyses indicated that GH had no significant influence on the LB rate [35,36], but yielded a slight increase in the pregnancy rate [35]. Several other systematic reviews and meta-analyses have indicated that GH co-treatment significantly increased the OR for CP from 1.6 to 3.3, and the OR for LB from 1.7 to 5.4 [37-40]. Previously, we demonstrated that GH independently increased the likelihood of CP in fresh transfers with an approximate OR of 2.5 (95% CI, 1.42–4.37) and increased the likelihood of LB, with an OR of 4.8 (95% CI, 2.30–9.79) [13]. These ORs are in line with the studies outlined above and are also in accordance with the observations of the current FET study, in which GH independently increased the chances of LB by about 2.7-fold.

Importantly, very few studies have investigated the role of GH cotreatment either directly in FET cycle set-ups or in subsequent FET cycles derived from GH used in stimulated cycles. Two recent reports suggested that direct GH co-treatment during the HRT phase of FET cycles led to increased endometrium thickness, with subsequent increases in the implantation and CP rates [14,15], and increased LB rates in a prospective study with 230 participants [14]. Furthermore, in an RCT with freshly transferred donor embryos, Altmae et al. [17] showed that donor recipients who received GH as part of their HRT regimen had greater endometrium thickness, along with higher pregnancy and LB rates. Cui et al. [15] also performed in vitro experiments and showed that GH upregulated insulin-like growth factor 1, along with the proangiogenic factor vascular endothelial growth factor in an endometrial carcinoma cell line, which may modify endometrial receptivity [41] and aid decidualization. The potential effect of GH on endometrial receptivity is intriguing, but requires further research, as the more traditional hypothesis has centered around improved embryo quality [9].

Given the administrative and financial resources required, the LIGHT investigators were unable to examine the subsequent FET outcomes from the stimulated cycles with GH therapy in their RCT [8]. This could have yielded insights on how these embryos performed later and provided a reflection of embryo quality beyond standard morphology. However, our group examined this issue in a retrospective fashion in 2010 [1], and now in 2019, in two separate cohorts from both studies we have demonstrated positive effects in subsequent FET cycles that were conducted without the influence of exogenous GH. These data may indicate a potential “carry-over” effect from GH used during stimulation, which is likely to be mediated by undetectable changes in embryo quality, suggesting that the potential beneficial mechanism of GH for clinical outcomes is more likely to be related to embryo quality than endometrial receptivity.

Unfortunately, like all retrospective analyses, these studies have significant limitations, which should be cautiously considered when interpreting the findings. The potential influential effects of GH are strictly associative rather than causative, as this study was not designed as an interventional RCT. Furthermore, there was significant heterogeneity in terms of the factors linked to a poor prognosis factors and the combination thereof in the adjuvant treatment groups. These groups tended to have a higher proportion of patients with multiple factors linked to a poor prognosis, and this heterogeneity is a significant limitation. In addition, the definition of a poor prognosis is not precisely in line with international criteria, weakening the study design, but we propose that the generation of embryos of reduced quality should be directly consider as a criterion for poor prognosis. Prospective studies have the advantage of focusing on homogenous populations with well-defined criteria of a poor prognosis. However, as discussed here, the current criteria are a matter of debate; future studies could incorporate factors outside of the traditional Bologna criteria and possibly take a more nuanced approach like that of the POSEIDON study [31]. In addition, while our study design attempted to minimize patient selection bias, its retrospective nature means that this could not be completely averted. The selection of the (–)GH control group meant that this group was composed of patients who were offered adjuvant therapy due to one or more of the five reasons outlined in the Methods section, but their first chronological cycle within the study period did not include an adjuvant. This reflected our attempt to select a poor-prognosis cohort, but to randomly choose cycles without adjuvant intervention. The secondary aim of this process was to prevent any confounding that could result from including multiple treatment cycles in individual women. Other potential factors that were not examined included patient’s socioeconomic status and parity. Affordability could be a critical confounder, as patients who used adjuvants in the stimulated cycle were required to pay for the rather expensive GH.

Conversely, there are significant strengths associated with the current study including the use of a large data-set (n = 1,119 cycles) with a specific and well-established HRT regimen, along with a focus on the transfer of single autologous embryos that had been vitrified using the same technique. In addition, we analysed only the first chronological FET cycle for each patient within the study time frame, as a mechanism to minimize patient selection bias. The study was also comprehensive in examining the independent influence of several potential confounding variables such as age, AFC, AMH level, transferred embryo quality, endometrial thickness, E₂ and P4 levels, which were also used as covariates in multiple regression analyses (data not shown).

Taken together, while not as robustly designed and as powerful as a prospective RCT, the current study design limited any perceived bias, and is one of the largest studies of GH in FET cycles to date. It is increasingly difficult to recruit ART patients into well-designed RCTs, as demonstrated by the LIGHT study [8]. This is an issue for all ART studies, as patient demand in commercial clinics overpowers the ability to recruit, leaving researchers in the field with vastly underpowered studies. If this cannot be addressed, progress may only be achieved by useful retrospective studies that are stringently designed, conducted and analysed. Nonetheless, the current investigation provided further evidence to suggest that GH administration in stimulation cycles may potentially cause changes in developing oocytes that lead to better LB rates when the embryos derived from these oocytes are transferred later in frozen cycles. This potential change may be reflected by enhanced embryo quality that is not detectable by conventional morphological assessments, but provides improved competency for the generation of pregnancies.