The importance of the day of embryo transfer during in vitro fertilisation

Embryo development and transfer policies

IVF, with or without intracytoplasmic sperm injection (ICSI), involves the handling of sperm cells and oocytes (gametes) and embryos outside the human body. In IVF, thousands of sperm cells are used to inseminate oocytes in the laboratory, whereas in ICSI, one sperm cell is selected and injected into each oocyte.23 About 23 hours after fertilisation, the first cell division occurs. On the second day after insemination or injection, the embryo should comprise four cells (blastomeres), and on the third day the embryo ideally presents with eight equally sized blastomeres—although some embryos cleave a little faster, and some slower.

The cleavage stage of embryo development comprises the period from two cells to the morula stage, which occurs around day 4 of culture (fig 1). During the morula stage the embryo has around 16 cells, and then a process called compaction begins, where the cells start to adhere tightly to each other, forming a compacted structure. This process is crucial for the next stage of development, as it increases cell-to-cell communication and prepares the embryo for differentiation. Cell division increases exponentially. In the subsequent blastocyst stage, around day 5 or day 6 of embryo development, a fluid filled cavity called a blastocoele forms within the embryo, which now has more than 100 cells. Some cells differentiate into an inner cell mass, from which the fetus will eventually develop, and other cells form an outer layer of trophectoderm cells, which later in development will contribute to the placenta.3

Fig 1

Stages of embryo development

• Download figure
• Open in new tab
• Download powerpoint

Embryo transfer can occur at different stages of development. Traditionally, embryos were transferred on day 3 after insemination or sperm injection, aligning with the cleavage stage of embryo development. After improvements to in vitro culture conditions and embryo cryopreservation techniques, the standard practice has shifted towards transferring embryos at the blastocyst stage, usually on the fifth or sixth day after oocyte retrieval.456

Rationale behind transfer approaches

The rationale for embryo transfer at the blastocyst stage lies in the potential for an enhanced embryo selection process. In IVF, not all embryos develop into good quality blastocysts. Some embryos show arrested development and some exhibit lower implantation potential, as can be observed in the laboratory by morphological characteristics such as the presence of too few or too many cells during development, and the presence of cell fragments. Extended culture to the blastocyst stage helps in identifying embryos with high implantation potential. Moreover, transferring embryos at the blastocyst stage is thought to align more closely with the natural timing of implantation in the endometrium, potentially increasing the chance of live birth for each transfer and reducing the number of embryo transfers needed to achieve pregnancy.456

Committing to a blastocyst stage transfer carries the risk of poor embryo development, however, as debate is still ongoing about whether IVF culture conditions are truly optimised for human embryos. Embryos that might not survive the challenges of extended in vitro culture might survive in vivo if transferred to the uterus at the cleavage stage. Additionally, the number of supernumerary embryos available for cryopreservation at the cleavage stage is typically higher than those available after reaching the blastocyst stage.4

Cumulative live birth rate

Success rates in IVF are increasingly measured by the cumulative live birth rate, which includes live births from all fresh and frozen-thawed embryo transfers resulting from a single oocyte retrieval. This metric better provides a more comprehensive reflection of the overall success rate of an IVF treatment cycle. Traditionally, IVF success rates were reported based on the live births for each fresh or first embryo transfer, excluding the outcomes of supernumerary frozen-thawed embryo transfers. With advances in cryopreservation methods, the success of embryo freezing and thawing has increased considerably, leading to higher cumulative live birth rates.7 Today, both professionals and women consider it essential to report outcomes from both fresh and frozen-thawed embryo transfers to obtain a better idea of the true success of an IVF treatment.89