In vitro fertilization (IVF) has generally been used for years with idea for retrieving many eggs, created by ovarian stimulation, to enable production of many embryos, such that multiple embryos can be transferred, cryopreserved for future cycles, to find best embryos by allowing natural selection through blastocyst culture. Ovarian stimulation is utilized to increase the number of oocytes which might compensate the drawbacks of IVF procedure, enabling the selection of one or more embryos for transfer.1 In recent past, long protocol (agonist suppression) along with gonadotropin (Gn) stimulation was the most frequently used stimulation protocol.2,3 Such high dose of Gn (up to 450 IU per day) failed to demonstrate improvement in the outcome.4,5 Even in recent days, the antagonist protocol where Gn use is less than Long protocol, has proved to be of comparable success as the previous protocol, but with less complications. Long protocol always associates with the risk of complication like ovarian hyperstimulation syndrome (OHSS),6,7 and so also can lead to high dropout, emotional stress, as well as abdominal discomfort.1 Whether such a long stimulation cycle raises the long-term health risk like ovarian cancer, is still uncertain. Some authorities opine that there is increased incidence of low birth rate and birth defect following IVF treatment.8,9 Edwards et al. as far back in 1996 called for the use of milder stimulation protocols which they thought to be safer, more patient friendly, and with minimized risk of treatment, as compared to standard stimulation protocols.10
The problem of use of milder stimulation protocol in practice is availability of diminished number of eggs which may lead to diminished pregnancy rate. However, improved culture condition and instrumentation in many IVF laboratories and recent trend of diminishing the number of embryo transfer (ET) together reduce the need of large quantities of oocytes. The supportive evidence regarding potential negative effect of supra-physiological estradiol (E2) level on endometrial receptivity,11,12 corpus luteal function,13,14 oocyte and embryo qualities,15,16 impress that mild ovarian stimulation might lead to beneficial effect on implantation potential as well as corpus luteal function.
CLINICAL INDICATION OF MILD OVARIAN STIMULATION
- Normogonadotrophic women with—(a) repeated failure of conventional stimulation, (b) repeated implantation failure with standard protocol
- Poor responders and hypergonadotrophic women
- Hyper-responders—polycystic ovarian (PCO) disease and high anti-Mullerian hormone (AMH).
Normogonadotrophic women may face recurrent IVF failure. Embryos those develop from high stimulation may present with high degree of chromosomal anomalies. There may be mitochondrial defects in eggs which may be inherent or age related leading to recurrent failure, after high stimulation (conventional stimulation). This may also lead to defect in endometrial receptivity.
Poor responders may be normogonadotrophic or hypergonadotrophic, with or without diminished ovarian reserve. Normo-Gn poor responders may face downregulation of follicle-stimulating hormone (FSH) receptors due to higher FSH dose received in conventional stimulation, leading to poor number of follicular growth. Women with diminished ovarian reserve (DOR) may have poor number of antral follicles available to be stimulated for ovulation.
Women with PCOD as well as high AMH level in serum are hyper-responders. With conventional stimulation, there is every possibility of development of OHSS. Moreover, such a large number of follicles often contain eggs of lower quality; even they may be empty follicles.
DIFFERENT PROCEDURES OF MILD OVARIAN STIMULATION
- Natural cycle
- Natural cycle with FSH boost/add-back
- Minimal Gn stimulation (low dose), alone or with adjuvants like oral ovulation-inducing (OI) agents.
Development of Milder Stimulation Protocols
The introduction of gonadotropin-releasing hormone (GnRH) antagonist for downregulation of pituitary, brought into clinical practice of controlled ovarian stimulation (COS), has made planning of milder stimulation protocol possible.17 Unlike GnRH agonist, GnRH antagonist does not cause initial flare of endogenous Gn release, instead cause reversible suppression of Gn secretion. The administration of GnRH antagonist in mid and late stimulation/follicular phase prevents any premature luteinizing hormone (LH) surge. This has made planned ovum pick-up (OPU) possible even in a natural menstrual cycle. This approach utilizes the endogenous intercycle FSH rise rather than suppressing it, resulting in reduction in the medication needed. The use of GnRH antagonist had made the stimulation period shorter (number of day of Gn stimulation) and diminished number of Gn ampoules required for stimulation and does not cause cyst formation. Though the initial studies suggested a detrimental effect on pregnancy rate following antagonist use, as compared to agonist,18,19 a recent meta-analysis comprising of 22 randomized controlled trials (RCTs) showed no significant difference in probability of live birth.20
Natural Cycle Stimulation for IVF
The first IVF baby was born out is a natural cycle oocyte recovery.21 Thereafter, ovarian stimulation was used instead of natural cycle to improve success rate per cycle.3,22 Natural cycle IVF requires simple monitoring of spontaneous cycle and retrieval of single oocyte prior to spontaneous LH surge or following attenuated LH surge by antagonist along with human chorionic gonadotropin (hCG) trigger. This excludes the possibilities of OHSS and multiple pregnancies. The cost per cycle becomes almost one-fourth as compared to a stimulated cycle.23,24 Ongoing pregnancy rate was found to be quite low, for which it has not become popular. Repeated collection of oocytes in multiple natural cycles followed by multiple frozen embryo transfer (FET) might increase the success rate, particularly in elderly poor responders. The most drawback of natural cycle, IVF is premature LH surge leading to premature ovulation, and reduced chance of successful oocyte pick-up (OPU).25 It is safe and less stressful method of stimulation and in selected cases, the cumulative pregnancy rate and live birth rate may be 45% and 32% respectively.24 The use of antagonist in a natural cycle can be called to be a modified natural cycle. Modified natural cycle is quite useful in patients having previous poor ovarian response to conventional ovarian stimulation.
Natural Cycle with FSH Add-back/Boost
The ongoing growth of dominant follicle is supported by addition of exogenous Gn (FSH add back) and use of antagonist to prevent premature LH surge. In most of the centers, GnRH antagonist and Gn (75–300 IU per day) initiated a follicular diameter of 12–14 mm. This protocol is also popular in patients with poor response with conventional stimulation or poor responders with poor ovarian reserve test (ORT). This protocol yields best success rate in young couple with severe male infertility, as an only fertility compromising factor. Modified natural cycle IVF in consecutive cycles in selected population may result in improved effectiveness.
EXOGENOUS GONADOTROPIN FOR MILD OVARIAN STIMULATION
Mild ovarian stimulation in which low-dose gonadotropin (FSH/hMG–human menopausal gonadotropin) administration is delayed until the mid-follicular phase is based on the FSH window concept.26 Exogenous FSH administration is limited to the mid to late follicular phase with the aim of preventing a decrease of FSH levels and thus inducing multifollicular development.27 The use of GnRH antagonists for suppression of premature LH rise enabled this concept to be introduced into IVF.28 A pilot study showed that multiple dominant follicles could even be induced when the initiation of FSH was postponed until CD 7.29 However, there was a tendency toward a lower percentage of women presenting with multiple dominant follicle development compared with patients started on CD 3 or 5.30 A fixed daily dose of 150 IU rFSH compared with 100 IU/day was found to be more effective in consistently inducing multiple follicular growths when ovarian stimulation was initiated on CD 5.29
A large randomized study to compare the efficacy of mild strategy (mild ovarian stimulation with antagonist) as compared to conventional stimulation protocol with agonist showed comparable live birth rate at the end of 1 year along with reduction in Gn requirement multiple pregnancy and overall cost involved.31
Gonadotropin with Clomiphene Citrate or Aromatase Inhibitors
The antiestrogen clomiphene citrate (CC) was the first preparation used for ovarian stimulation in IVF.22,32 Important advantages of CC compared with gonadotropins remain including its oral administration, low price and widespread availability. CC acts to increase pituitary FSH secretion by reducing negative estrogen feedback. An ovarian stimulation protocol combining CC with gonadotropins could lead to a reduction in the amount of gonadotropins required due to the combined synergistic effects. Additionally, because gonadotropins may counterbalance the undesired antiestrogenic effects of CC on the endometrium, it may be counterbalanced with Gn addition.33,34 This combination might lead to improved pregnancy rates compared with CC alone. CC has now been largely replaced by more effective hMG/FSH protocols in combination with GnRH analog (GnRH-a) cotreatment.3,35
The available clinical data available for aromatase inhibitors (AI) in IVF treatment is limited. One uncontrolled study with 22 good responders with limited financial means, where Letrozole was used for first 5 days of cycle (CD 3–7), followed by hMG from CD-7 yielded 27% pregnancy rate. In other RCTs, AI did not show any significant benefit when used in IVF program.36,37 A third RCT38 showed more number of available oocytes following AI used.
Late Follicular Phase hCG or LH
Human chorionic gonadotropin or LH administration in low doses may replace FSH administration in late follicular phase, as mild stimulation approach. This is based on acquired LH responsiveness of granulosa cells in dominant follicles in late follicular phase.39 The administration of recombinant LH was found to be sufficient to maintain follicular growth in late follicular phase after initial stimulation with FSH.40 Though this approach has been postulated to reduce OHSS,41 this has not been substantiated till date.
CONVENTIONAL STIMULATION VERSUS MILD OVARIAN STIMULATION
The pros and cons of standard and mild ovarian stimulations are given in Table 1.
Some observations indicated relation between degree of ovarian stimulation and embryo quality, either morphologically or by chromosomal constitution,16,42 by disrupting natural selection of good quality oocytes.
In mouse oocytes, high dose of Gn stimulation during in vitro maturation increased incidence of chromosomal abnormality had been observed.43,44 High E2 level following conventional ovarian stimulation has negative impact on implantation potential,45,46 as well as chromosomal constitution of human embryos.47 One study indicated that high stimulation may disrupt the chromosomal segregation mechanism of embryo.48,49
The retrieval of modest number of oocytes following mild stimulation leads to distinctly higher implantation rate compared to conventional stimulation.50 This may be due to retrieval of more homogenous group of good quality oocytes instead of pathological reduction of ovarian response. The fear of getting low number of oocytes following mild stimulation thus may be compensated in getting increased oocyte number, but lower pregnancy rate with high dose stimulation.51,52
It is true that with mild stimulation, cryopreservable embryos are less, leading to unavailability of subsequent FET. However, lesser number of good quality embryos leads to similar results as compared to conventional stimulation protocol, so far as the number of pregnancy is concerned.16
Luteal Function and Endometrial Receptivity
Supraphysiological steroid levels are widely held responsible53 for poor endometrial receptivity with high dose of Gn in COS. This results in impaired embryo implantation when compared with natural cycle with oocyte donation.54 E2 level of more than 3,000 pg/mL on hCG day has been shown to reduce implantation rate, independent of embryo quality.55 In contrary, improved implantation rate has been observed with mild stimulation approached12 due to more physiological response. Increased pregnancy rate has been observed following FSH step down in high responders due to decrease in E2 in preimplantation period.56
It is obvious that the cost involved in mild stimulation is much less as compared to conventional group. Moreover, mild stimulation carries negligible chance of OHSS which otherwise can impart more cost in treatment. Less chances of multiple pregnancies and preterm birth in mild strategy57 leads to reduced cost involvement.
Mild stimulation which is a more shorter and patient-friendly protocol with little complication might decrease the treatment-related stress. In natural cycle, excess chance of cycle cancellation is not also that stressful, if patient is counseled properly. Furthermore, mild stimulation has been found to reduce significant dropout rate per cycle. A mild IVF treatment strategy was found to be associated with fewer symptoms of depression after overall treatment failure, than a standard IVF treatment.31,58 As there is less psychological burden in mild stimulation, even a failure as accepted by the patient positively with a hope of having success in the following cycles, which compensates the low pregnancy rate per cycle.59
It will be not far when ovarian stimulation will be replaced by in vitro maturation of the oocytes, after retrieval of immature eggs from unstimulated or minimally stimulated cycles, avoiding requirement of Gn stimulation for invivo follicular growth and oocyte maturation.60,61 Till date, this procedure has not produced increased pregnancy rates due to poor implantation. Moreover, safety of this technique for the offsprings has not been identified.62,63
Since the mid-90s, the long agonist stimulation protocol has been widely used worldwide. This was lengthy, expensive protocol, with advantages of more programmed IVF, less cycle cancellation and more number of oocytes of available with better pregnancy rate.64 When compared against complications and disadvantages, long protocol showed more OHSS, more short- and long-term risks, more physical and emotional burden along with increased dropouts. It involved long continued repeated injections, multiple blood sample tests, and repeated ovarian ultrasound scan, requiring more attendance to the clinic.
The mild stimulation on the other hand, was devoid of more disadvantages of the long protocol, requiring shortened stimulation period. Though the retrieved oocytes were less in number, the pregnancy rate per ET seems to be comparable between two approaches (P-679–outlook–Hohmann). The embryo quality of mild regime was much better.16,65
Another aspect is very important as in many countries like Italy, there are IVF legislations which do not allow embryo banking. Hence, availability of excess number of oocytes is of no use.
Mild ovarian stimulation for IVF is gaining ground gradually, as evidenced in recent literature. It is very useful for selected group of patients like good responders of young age, some having polycystic ovarian disease (PCOD), but the drawback being reduction in pregnancy rate per cycle. The best part is less chance of complication, increased patient compliance and economic benefit, which is making it more popular day by day. It is difficult to draw a conclusion about the best agents for mild stimulation till date. A better understanding of physiology of follicular dynamics may lead to individual approaches.66 It should be the aim to develop at least 3 follicles in mild stimulation to produce competitive IVF treatment outcomes. Reduction in dose of medication becomes highly beneficial so far embryo quality and implantation rate is concerned.
- Fauser BC, Devroey P, Macklon NS. Multiple birth resulting from ovarian stimulation for subfertility treatment. Lancet. 2005;365:1807–16.
- FIVNAT 1996 report. French National Register on In Vitro Fertilization. Contracept Fertil Sex. 1997;25:499–502.
- Macklon NS, Stouffer RL, Giudice LC, et al. The science behind 25 years of ovarian stimulation for in vitro fertilization. Endocr Rev. 2006;27:170–207.
- Wikland M, Bergh C, Borg K, et al. A prospective, randomized comparison of two starting doses of recombinant FSH in combination with cetrorelix in women undergoing ovarian stimulation for IVF/ICSI. Hum Reprod. 2001;16:1676–81.
- Delvigne A, Rozenberg S. Epidemiology and prevention of ovarian hyperstimulation syndrome (OHSS): a review. Hum Reprod Update. 2002;8:559–77.
- Aboulghar MA, Mansour RT. Ovarian hyperstimulation syndrome: classifications and critical analysis of preventive measures. Hum Reprod Update. 2003;9:275–89.
- Wang YA, Sullivan EA, Black D, et al. Preterm birth and low birth weight after assisted reproductive technology-related pregnancy in Australia between 1996 and 2000. Fertil Steril. 2005;83:1650–8.
- Kapiteijn K, de Bruijn CS, de Boer E, et al. Does subfertility explain the risk of poor perinatal outcome after IVF and ovarian hyperstimulation? Hum Reprod. 2006;21:3228–34.
- Edwards RG, Lobo R, Bouchard P. Time to revolutionize ovarian stimulation. Hum Reprod. 1996;11:917–9.
- Simon C, Cano F, Valbuena D, et al. Clinical evidence for a detrimental effect on uterine receptivity of high serum oestradiol concentrations in high and normal responder patients. Hum Repro. 1995;10:2432–7.
- Devroey P, Bourgain C, Macklon NS, et al. Reproductive biology and IVF: ovarian stimulation and endometrial receptivity. Trends Endocrinol Metab. 2004;15:84–90.
- Fauser BC, Devroey P. Reproductive biology and IVF: ovarian stimulation and luteal phase consequences. Trends Endocrinol Metab. 2003;14:236–42.
- Beckers NG, Platteau P, Eijkemans MJ, et al. The early luteal phase administration of oestrogen and progesterone does not induce premature luteolysis in normo-ovulatory women. Eur J Endocrinol. 2006;155:355–63.
- Valbuena D, Martin J, de Pablo JL, et al. Increasing levels of estradiol are deleterious to embryonic implantation because they directly affect the embryo. Fertil Steril. 2001;76:962–8.
- Baart EB, Martini E, Eijkemans MJ, et al. Milder ovarian stimulation for in-vitro fertilization reduces aneuploidy in the human preimplantation embryo: a randomized controlled trial. Hum Reprod. 2007;22:980–8.
- Tarlatzis BC, Fauser BC, Kolibianakis EM, et al. GnRH antagonists in ovarian stimulation for IVF. Hum Reprod Update. 2006;12:333–40.
- Ludwig M, Katalinic A, Diedrich K. Use of GnRH antagonists in ovarian stimulation for assisted reproductive technologies compared to the long protocol meta-analysis. Arch Gynecol Obstet. 2001;265:175–82.
- Al-Inany HG, Abou-Setta AM, Aboulghar M. Gonadotrophin-releasing hormone antagonists for assisted conception. Cochrane Database Syst Rev. 2006:19:CD001750.
- Kolibianakis EM, Collins J, Tarlatzis BC, et al. Among patients treated for IVF with gonadotrophins and GnRH analogues, is the probability of live birth dependent on the type of analogue used? A systematic review and meta-analysis. Hum Reprod Update. 2006;12:51–71.
- Cohen J, Trounson A, Dawson K, et al. The early days of IVF outside the UK. Hum Reprod Update. 2005;11:439–9.
- Aboulghar MA, Mansour RT, Serour GA, et al. In vitro fertilization in a spontaneous cycle: a successful simple protocol. J Obstet Gynaecol. 1995;21:337–40.
- Nargund G, Waterstone J, Bland J, et al. Cumulative conception and live birth rates in natural (unstimulated) IVF cycles. Hum Reprod. 2001;16:259–62.
- Pelinck MJ, Hoek A, Simons AH, et al. Efficacy of natural cycle IVF: a review of the literature. Hum Reprod Update. 2002;8:129–39.
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- Schipper I, Hop WC, Fauser BC. The follicle-stimulating hormone (FSH) threshold/window concept examined by different interventions with exogenous FSH during the follicular phase of the normal menstrual cycle: duration, rather than magnitude, of FSH increase affects follicle development. J Clin Endocrinol Metab. 1998;83:1292–8.
- Macklon NS, Fauser BC. Regulation of follicle development and novel approaches to ovarian stimulation for IVF. Hum Reprod Update. 2000;6:307–12.
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- Hohmann FP, Laven JS, de Jong FH, et al. Low-dose exogenous FSH initiated during the early, mid or late follicular phase can induce multiple dominant follicle development. Hum Reprod. 2001;16:846–54.
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