What Is a Blastocyst? Why Is This Stage Ideal for Embryo Transfer in ICSI/IVF?

In the process of IVF/ICSI, one of the key steps influencing treatment outcomes is the embryo transfer—returning embryos cultured in the laboratory into the uterus. Today, transferring embryos at the blastocyst stage has become a widely adopted standard, supported by research showing that it may offer higher success rates compared with earlier-stage transfers.

This article explains what a blastocyst is, and why transferring embryos at this stage can be an important factor in increasing the likelihood of pregnancy.

From Egg Collection to the Blastocyst Stage

Embryo development in IVF/ICSI begins on the day eggs are retrieved and fertilized with sperm in the laboratory. On Day 1, fertilization produces a single-cell zygote. If the embryo is of good quality, it continues dividing into multiple cells over the following days. By Day 5 or 6, the embryo may reach the blastocyst stage.

What Is a Blastocyst?

A blastocyst is an embryo that has developed for about 5–6 days after fertilization—the peak of in-lab embryo culture. At this stage, the embryo has two key structures:

• Inner Cell Mass (ICM): the cluster of cells that will form the baby
• Trophoblast (TE): the outer layer of cells that will form the placenta, critical for implantation and nutrient exchange

These structures are essential for continued growth in the mother’s body and play a role in pregnancy outcomes.

Why Transfer Embryos at the Blastocyst Stage?

1. Natural selection
   Embryos that reach this stage generally show stronger growth potential. Those with lower quality usually stop developing earlier, making this stage a natural filter for stronger embryos.
2. Alignment with the body’s natural process
   In natural conception, embryos typically reach the uterus around Day 5–6 for implantation. Transferring blastocysts mirrors this natural timing and may improve the chance of implantation.
3. Chromosome screening (PGT-A/PGTseq-A)
   At the blastocyst stage, embryologists can biopsy a few cells from the trophoblast (the part that forms the placenta) for PGT-A (Preimplantation Genetic Testing for Aneuploidy). This allows selection of embryos with normal chromosomes, helping to reduce miscarriage risk and support healthy pregnancies—without affecting the cells that develop into the baby.
4. Lower risk of multiple pregnancy
   Because blastocysts often have stronger implantation potential, doctors can transfer just one embryo while maintaining a good chance of success. This reduces the risk of twins and the complications associated with multiple pregnancies.

The Role of a High-Quality Laboratory

Reaching the blastocyst stage requires a carefully controlled lab environment. Temperature, humidity, air quality, and pH must be precisely managed—under the supervision of experienced embryologists accredited by international organizations. Every detail impacts embryo growth.

Modern tools and technologies further support this process, such as:

• Time-Lapse Incubators, which allow continuous monitoring of embryo development without exposing embryos to external conditions.
• GM-CSF enriched culture media, which mimics natural conditions in the female body. GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor) is a naturally occurring protein during pregnancy that helps protect embryos. Adding it to culture media may support implantation, particularly in women with recurrent miscarriage, implantation failure, or advanced maternal age

Why SAFE Fertility Clinic Emphasizes Blastocyst Transfer

At SAFE Fertility Clinic, we view blastocyst transfer not simply as waiting for embryos to grow longer, but as a strategic process:

• Culturing embryos in an internationally accredited laboratory environment
• Selecting embryos with stronger potential and better synchrony with the uterus
• Enabling safe and effective genetic screening with PGT-A / PGTseq-A
• Together, these factors support higher chances of successful implantation and pregnancy.

With over 20 years of expertise in fertility care, a team of experienced specialists (Doctors and Embryologist), and state-of-the-art laboratory facilities, SAFE Fertility Group PCL is dedicated to supporting couples seeking embryo transfer in Thailand. As a leading clinic for advanced embryo care in Bangkok, we help you move closer to building the family you dream of.

Article by: Kamolrat Mee-o-partmongkon, M.D. (Dr.Bo)

SAFE Fertility Group PCL, Gaysorn Amarin and Ramintra Branch

Medical License Number 41300

Reference

• PMC PubMed Central : Is Blastocyst Culture Responsible for Higher Pregnancy Rates? A Critical Analysis of the Day of Optimal Embryo Transfer and Embryo Quality
  https://pmc.ncbi.nlm.nih.gov/articles/PMC9355446/
• PubMed : Trophectoderm Biopsy: Present State of the Art
  https://pubmed.ncbi.nlm.nih.gov/40004463/