Category
Dec 4, 2025
Many patients believe the hard work ends with the egg retrieval. However, for our embryologists, the true "miracle of life" is just beginning. Over the next 5 to 6 days, your embryos undergo a critical transformation inside our incubators, which are equipped with our exclusive Music in Incubators system to mimic the natural vibrations of the womb.
In this guide, we walk you through this fascinating journey—from zygote to blastocyst—so you understand exactly what is happening in the laboratory while you await your transfer, potentially watching it in real-time via our Embryomobile app.
What is an Oocyte? The oocyte (commonly called the egg) is the female reproductive cell. It acts as the starting point which, once mature (MII), has the potential to be fertilized by a sperm to create an embryo.
At Institut Marquès, we leave nothing to chance. Depending on sperm quality, we use conventional IVF (natural penetration) or ICSI (injecting a selected sperm directly into the egg). Not all retrieved eggs will be mature, so selection is key.
What is a Zygote? The zygote is the very first stage of an embryo, formed immediately after successful fertilization. It is a single cell containing the complete DNA of both parents.
Between 16 and 18 hours after insemination, our biologists look for a specific sign:
Correct Fertilization: We see 2 pronuclei (2PN)—one from the mother, one from the father.
Abnormal Fertilization: If we see 1 or 3 pronuclei, the embryo is genetically non-viable and is discarded.
The zygote begins to divide. It doesn't get bigger in size yet, but the number of cells (blastomeres) multiplies.
Day 2: We expect to see 4 cells.
Day 3: We expect between 7 and 8 cells.
At this stage, we evaluate symmetry and fragmentation (small cellular debris). While slight fragmentation (<10%) is normal, high levels can impact quality.
The Morula Stage (Day 4) On the fourth day, the embryo compacts. The cells merge so tightly they can no longer be distinguished individually, resembling a mulberry (hence the name Morula). This compaction is vital for the final leap.
Reaching Day 5 is the first major survival test. Only the strongest embryos reach this stage.
What is a Blastocyst? A blastocyst is an embryo with a complex structure differentiated into two parts:
Why do we prefer Blastocyst Transfer? At Institut Marquès, we aim for long culture to Day 5 whenever possible.
Natural Selection: Many embryos stop developing before Day 5. Waiting prevents the transfer of embryos that wouldn't result in pregnancy.
Synchronization: The uterus is naturally more receptive to a Day 5 embryo.
Higher Success Rates: It allows us to transfer a Single Embryo (SET) while maintaining high success rates, avoiding twin risks.
To implant, the blastocyst must "break" its outer shell (zona pellucida). Sometimes, they need help.
What is Assisted Hatching? It is a precise laser technique where we make a tiny opening in the shell. This helps the embryo "hatch" and implant in the endometrium. We typically use this for thawed embryos, patients with advanced maternal age, or thickened shells.
Is Day 3 or Day 5 transfer better? Day 5 (Blastocyst) offers superior selection and implantation rates. However, in cases with few embryos, your doctor might personalize the strategy to Day 3.
What do the grades (e.g., 4AA, 3BB) mean? The number (1-6) is expansion size. The first letter grades the Inner Cell Mass (baby), and the second grades the Trophectoderm (placenta). While "AA" is top tier, "BB" embryos also result in healthy babies.
