In vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) have become procedures used throughout the world and are expanding for the treatment of female and male subfertility. It is well recognized that assisted reproductive technology (ART) is associated with higher fetal and maternal risks.
The first IVF pregnancy was achieved after Day 2 embryo transfer. Since then, scientific research continued to improve the outcome of IVF. Improvement of ovarian stimulation using gonadotropin-releasing hormone agonist (GnRHa) or GnRh-antagonist protocols helped to increase pregnancy rates. Modification of embryo transfer techniques and development of softer catheters were steps forward toward better results. The improvement of laboratory standards and modifications of the culture media helped to achieve high-quality embryos.
Sequential media was developed for blastocyst culture to develop blastocyst transfer, and later, single culture media was used to achieve blastocyst culture. During the past decade, there was an increased trend for conducting embryo transfer at the blastocyst stage. Advantages and possible increased risks of blastocyst culture were evaluated by several studies with some controversial data.
Why Blastocyst Culture?
Blastocyst culture allowed embryo transfer at the stage of the blastocyst, which was believed to result in higher pregnancy rates. The development of a blastocyst encouraged clinicians to transfer one embryo, which satisfied the request of the regulatory bodies to avoid multiple pregnancies.
Blastocyst transfer is considered advantageous because it mimics the natural physiology of a blastocyst reaching the uterine cavity on days 5–6. Hence, it may provide better embryo-endometrium synchrony. This may increase implantation rates.
One additional advantage is that blastocyst culture means that the activation of the embryonic genome at the eight-cell stage was successfully achieved, which assures the IVF team that they are transferring an embryo with high probability of implantation.
IVF/ICSI Outcome After Blastocyst Transfer
The first prospective randomized study of infertile women (below 36 years) comparing embryo transfer (ET) at cleaved embryo versus blastocyst transfer showed significantly higher pregnancy and delivery rates after the blastocyst stage. Thirty-two percent versus 21.6% (relative risk [RR] 1.48, 95% confidence interval [CI] 1.04–2.11). Several randomized studies showed no difference in clinical pregnancy rate between Day 3 and Day 5 embryo transfer.
In routine practice, many IVF centers continue culture to the blastocyst stage if there are four or better-quality cleaved embryos. So, patients with poor prognoses are excluded from extended culture and blastocyst transfer. In several cycles, blastocyst culture will not result in a blastocyst, and the embryo transfer is canceled.
But, as in vitro culture conditions are different from those in vivo, cleaved Day-3 embryos may fail to reach the blastocyst stage at in vitro culture media. The same embryo may survive and proceed normally at in vivo conditions.
Type of Media and Blastocyst Culture
The culture of embryos at the preimplantation stage has always been a key element of laboratory embryology and has made a significant contribution to the success of many assisted reproduction procedures. Despite a scientific and commercial challenge stimulating research worldwide to optimize embryo culture conditions, there is no consensus even on principles such as composition and exchange of media, the required physical and biological environment, and even incubation temperature.
Problems of Blastocyst Culture and Blastocyst Transfer
Blastocyst culture provides some theoretical advantages and disadvantages, while it promotes embryo self-selection, it also exposes those embryos to possible harm due to the in vitro environment.
Premature Birth After Blastocyst Transfer
After adjusting for confounding factors, the risk of preterm birth was significantly greater after blastocyst stage than after cleaved-stage transfer. IVF pregnancies after blastocyst transfer were associated with a higher incidence of premature births (RR 1.27, 95% CI 1.22–1.31) and very preterm births (RR 1.22, 95% CI 1.10–1.35) as compared to cleaved-stage embryos. The preterm birth rate was higher with blastocyst transfer versus day 3 transfer (17.2 versus 14.1% = <0.001).
The incidence of monozygotic twinning after single embryo transfer was lower for days 2–3 transfer (1.71%, 95% CI 1.45-1.98, n = 162) than for days 5–6 transfer (2.5%, 95% CI 2.28-2.74; n = 472). A meta-analysis has shown that monozygotic twin is increased after transfer at the blastocyst stage.
Weight of the Baby
There is a controversy concerning the weight of the baby at birth after blastocyst and cleaved embryo transfer. There is a questionable possibility of increased large-for-gestational-age babies. The birth weight after fresh blastocyst transfer was significantly higher than that after transfer at the cleavage stage.
In a large population-based study, perinatal mortality was significantly higher after blastocyst transfer as compared to cleaved embryo transfer (OR 1.61; 95% CI, 1.14-2.29).
In a large population study, there was no increased risk of congenital malformations after blastocyst transfer.
Epigenetic Disturbances at In Vitro Cultured Gametes and Embryos
The phenotypic effects of any observed epigenetic differences between and without ART are largely unclear. The more conceptional period is critical not only for embryonal, placental, and fetal development, but also for the outcome at birth. Suboptimal in vitro culture conditions may also lead to persistent changes in the epi-genome influencing disease susceptibilities later in life.
Therefore, when considering the safety of human ART from an epigenetic point of view, our main task should not be whether or not several rare imprinting disorders increase, but rather we should be aware of the functional relationship between interfering with epigenetic reprogramming in very early development and adult disease.
The risks of placenta previa and placental abruption were higher after blastocyst transfers as compared to cleavage-stage transfer (OR, 2.08; 95% CI, 1.7-2.55) and (OR 1.62; 95% CI 1.15-2.29), respectively.
The live birth rate after fresh single embryo transfer was significantly higher in the blastocyst transfer as compared to cleaved embryos.
The higher-quality randomized studies and the meta-analysis comparing the live birth rate after blastocyst and cleaved embryos showed no significant difference in the cumulative pregnancy rate per one started cycle.
The question is, why do we continue extended culture to blastocyst? The key answer is the higher pregnancy rate in the fresh transfer. However, the cumulative pregnancy rate per one stimulated cycle is often ignored. By extending the culture of the embryos to the blastocyst stage, there is a risk of arrest of the embryos at any stage before reaching the blastocyst stage, and embryo transfer will be canceled. It is possible that if embryos were transferred at the cleavage stage, they may have survived in vivo.
Finally, neonatal complications of the baby and maternal possible complications are often ignored. A possible explanation for the adverse perinatal outcome could be that blastocyst culture may trigger genetic and epigenetic changes in the tropho-ectodermal cells that can lead to abnormal placentation.