To blastocyst or not to blastocyst? That is the question

Michael M. Alper1,5, Peter Brinsden2, Robert Fischer3 and Matts Wikland4

1 Boston IVF, Waltham, MA, USA, 2 Bourn Hall, Bourn, Cambridgeshire, UK, 3 Fertility Center Hamburg, Hamburg, Germany and 4 Fertility Center Gothenburg, Gothenburg, Sweden

Abstract

Recent advances in culture media preparations have allowed for cleavage embryos to be developed to the blastocyst stage. Blastocysts are regarded as having increased implantation potential, and two blastocysts are typically transferred, which reduces the occurrence of high order multiple gestations. However, with current techniques, most cleavage embryos do not become blastocysts and it is not clear how many of these embryos would have implanted had they been replaced at the cleavage stage. Furthermore, experience with blastocyst cryopreservation is lacking and the overall benefit of blastocyst culture is unknown, unless we consider the combined pregnancy rates of both fresh and frozen blastocysts.

Key words: blastocyst/cryopreservation/IVF/transfer

Introduction

The culture of human embryos to the blastocyst stage of development has been advocated to increase the efficiency of IVF by both reducing the number of embryos replaced and diminishing the incidence of multiple gestations.

Culture of embryos beyond the cleavage stage has been known for some time. However, the recent understanding of the different metabolic needs of the cleavage and blastocyst stage embryos has resulted in improved media preparations. Many embryos can be grown successfully to the blastocyst stage, allowing embryo transfer to occur on day 5 or 6 following oocyte retrieval. The suggested benefits of blastocyst transfer include: (i) selection of embryos with increased implantation potential; (ii) replacement of fewer embryos; and (iii) reduced probability of high order multiple gestation by replacement of a maximum of two embryos. The purpose of this report is to comment on the current data on human blastocyst transfer and to offer our opinions on the practice in order to stimulate discussion.

Is blastocyst transfer advisable?
Designing a study to address the benefits of routine blastocyst versus cleavage stage transfer is definitely possible, with the understanding that for any given patient it is impossible to know whether an embryo that failed to reach the blastocyst stage would have implanted if replaced on day 3. Randomizing patients to both groups prior to cycle start would be essential. Patients would have (all) embryos cultured to either day 3 or 5. Two blastocysts or day 3 embryos would be replaced. Alternatively, the study could compare transferring one blastocyst versus one cleavage stage embryo. Embryos for freezing in both groups would be tallied. The total number of babies per oocyte retrieval over time (derived from both frozen and fresh transfers) would be most important since it is the oocyte retrieval and the antecedent costs of monitoring and medication that represent the most expensive and riskiest part of the treatment cycle. For example, a recent study described the experience of offering blastocyst transfer to `non-selected' patients (Marek et al., 1999Go). This study reported higher pregnancy and implantation rates with blastocyst transfer (as expected). Of interest is that this study failed to demonstrate a difference in outcome between the two groups in women <35 years of age. The difference in pregnancy and implantation rates was confined to the age group 35 years or older, but it is not the older patient that represents the main target group to prevent twin gestation. However, the pregnancy rate from combined fresh and frozen transfers from both the blastocyst group and retrospectively determined controls is unknown.

Reproductive medicine is unique. Many treatments are empirical and untested in randomized clinical trials. Usually, a good idea is tested on a small group of patients in whom the procedure is likely to be successful—a recent report on successful human blastocyst transfer is no exception (Gardner et al., 1998Go). A new discovery such as blastocyst transfer, patient pressure and competition between IVF centres force clinicians to consider the new technology, even in the absence of good data—again, blastocyst transfer is no exception.

Obviously, any investigator who believes in a given technology and who wants it to work would limit it to patients in whom it is likely to be successful. Many reports to date have employed blastocyst transfer in good prognosis patients. A recent report (Schoolcraft et al., 1999Go) illustrates this point. Patients had at least 10 follicles on the day of human chorionic gonadotrophin (HCG), the mean number of oocytes per patient was 22, and the mean number of embryos transferred was 1.3. Although the pregnancy rate (66%) and implantation rate (48%) were very impressive, an excellent success rate with the transfer of cleavage embryos would also be expected in this highly selected group of patients.

One of the few prospective studies to date has failed to show a benefit of day 5 versus day 3 transfer (Huisman et al., 2000Go). Embryo transfer was electively done on day 3, 4, or 5, based upon the day of the week when oocyte retrieval occurred in 1787 cycles. The authors found no difference in pregnancy, implantation, or multiple gestation rates, whether a day 3, 4 or 5 transfer occurred. The study was limited because a single culture media was used in both groups, although 43% of patients had blastocysts transferred on day 5. Furthermore, patients in whom all the embryos had excellent morphological criteria showed a higher implantation rate with day 5 transfer; the pregnancy rate in this subgroup was not stated. A recent randomized study to compare the transfer of two to three cleavage embryos or blastocysts failed to demonstrate a difference in pregnancy rates, or implantation and twinning rates (Coskun et al., 2000Go).

High order multiple gestation and twin pregnancy remain serious problems with IVF. In the USA, data from the national registry reported a 13.7% incidence of triplets or greater in women <35 years undergoing IVF (SART, 1999). The emotional, financial and medical toll associated with these high-risk gestations is enormous. The principle purported advantage of blastocyst culture is the ability to replace fewer embryos to reduce high order multiple gestations; but replacing fewer embryos is not a new concept. Investigators have demonstrated that limiting the number of (cleavage) embryos to two in patients undergoing their first IVF cycle results in comparable pregnancy and implantation rates, but fewer multiples, compared to replacing three embryos (Fujii et al., 1998Go). Incorporating an embryo grading system to reduce the number of transferred embryos has also been proposed. Hu and colleagues (1998) have shown that in women who were aged <35, replacing either four poor grade, two fair grade, or two good embryos resulted in respectable 35% pregnancy rates with no high order multiple gestations (Hu et al., 1998Go). Similarly, others (Kodama et al., 1995Go) reported that, by reducing the number of transferred cleavage embryos to two (or three if all were poor quality), one could achieve similar pregnancy and twin rates, but reduce triplet rates from 9 to 2%.

When more than two blastocysts are replaced, high order multiple gestations are possible and the benefits of the technique are lost. Careful review of the literature supports this. A recent report (Milki et al., 2000Go) compared replacing 2.4 ± 0.6 blastocysts to 4.6 ± 1.3 cleavage stage embryos and found a higher pregnancy rate with blastocyst transfer (68 versus 46%) but also a higher triplet rate (12 versus 4%). Similarly, another group replaced 2.2 ± 0.5 embryos and had 3% triplet deliveries (Kodama et al., 1995Go). Patient pressure and competitive forces may continue to place stresses on IVF centres to replace more than two blastocysts, which may defeat the primary advantage of this approach.

Blastocyst culture is thought to filter out the better embryos. Approximately 40% of embryos in younger women reach blastocyst stage. How many of the other embryos could potentially result in a baby if they were transferred on day three? Morphology is not a good predictor of blastocyst formation—50% of good quality day 3 embryos will become blastocysts but so will 20% of poor quality embryos (Rijnders and Jansen, 1998Go). Also, recent research shows that 34% of blastocyst embryos show aneuploidy, corroborating the notion that morphology is not a strong predictor of outcome (Sandalinas et al., 2000Go). Furthermore, the morphology of both cleavage and blastocyst stage embryos does not correlate with the degree of mosaicism found in these embryos (Evsikov and Verlinsky, 1998Go). Since the uterus may be a better milieu than current blastocyst media preparations, we should be cautious in assuming that failure to reach the blastocyst stage indicates that the embryo never had implantation potential. It is also important to note that between 2–40% of patients will have their embryo transfer cancelled because no embryos developed to day 5. Would some of these patients have had a reasonable chance of success if they underwent a transfer of their embryos at the cleavage stage?

Many IVF pregnancies occur from the replacement of cryopreserved embryos. Since less than half of embryos reach blastocyst stage, there will be fewer embryos available to freeze. Furthermore, experience with blastocyst cryopreservation is lacking. At Boston IVF, the ongoing pregnancy rate for day 3 cryopreserved/transferred embryos was 69/282, or 24.5% compared with 4/39, or 10.3% with cryopreserved/transferred blastocysts (M.M.Alper, unpublished data). The overall benefit of blastocyst culture cannot be determined until we have more data on the success of blastocyst cryopreservation. The combined success of both fresh and frozen embryo transfer must be compared between cleavage and more advanced embryos to determine which approach is superior.

One of the challenges clinicians face is how to manage patients with more advanced age, multiple IVF failures, diminished ovarian reserve and problems of embryo quality. Many patients have less than average prognosis with IVF. At this point in time, blastocyst transfer does not appear to offer an advantage to these patients. In a small prospective study, poor responders undergoing blastocyst transfer had only a 10% pregnancy rate (Karande et al., 1999Go). A small study of women who underwent blastocyst transfer after two failed IVF attempts failed to show a statistically significant increase in pregnancy rates compared to matched controls undergoing cleavage stage transfers (Gutknecht et al., 1999Go). It is probable that blastocyst culture will have limited application in poorer prognosis patients since these couples produce fewer embryos and given the currently poor blastulation rate, very few embryos would ultimately be available for transfer.

The goal of IVF is to transfer a single embryo that results in one healthy baby, and the technology must be performed in a cost conscious and safe manner. Although recent focus has been on preventing the high order multiple gestations, twin pregnancies are associated with potentially adverse pregnancy and delivery outcomes in addition to the psychosocial burden that may arise. In a small, randomized and prospective study in young women who produced two or more high grade embryos, transfer of one embryo resulted in a respectable ongoing pregnancy rate (11/26, 38.5%) with no dizygotic twinning (Gerris et al., 1999Go). However, current methods of choosing the single best embryo to transfer have been based upon embryo morphology (Van Royen et al., 1999Go). Clearly, morphology has its limitation in assessing the viability of both cleavage stage and blastocyst embryos.

Conclusions

We conclude that our current understanding of human blastocyst transfer is limited primarily to patients with good prognosis. Furthermore, there are not many prospective trials comparing transfer of two blastocysts with two cleavage stage embryos. Recent published studies are experimental and do not take into account the large number of embryos with some potential that are discarded, nor do they take into consideration cycles with cryopreserved embryos. We caution clinicians to evaluate the data carefully before offering blastocyst transfer routinely, while we await an improved understanding of embryo development.

Notes

5 To whom all correspondence should be addressed at: Boston IVF, 40 Second Avenue, Waltham, MA 02451, USA.E-mail: michael.alper{at}bostonivf.com Back

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