Does the success rate of IVF depend on the number of days? Embryologist reveals: The key to successful blastocyst transfer is not "speed", but this sole criterion

In the journey of in vitro fertilization, as the embryo grows quietly in the laboratory, the hearts of expectant parents often hang in suspense. A common question arises: "Is it better for my embryo to be cultured until day 5 or transferred on day 7? Is the success rate on day 5 the highest?" To answer this question, we first need to understand what the embryo goes through in the laboratory. After egg retrieval, the egg combines with sperm (usually through ICSI technology), and the initial form of life after successful fertilization is called the "fertilized egg". It begins to divide and typically develops into a "fresh embryo" consisting of 6-8 cells by the third day. At this point, it can choose to be transplanted back into the mother's uterus.

But in order to improve the success rate, modern reproductive medicine often adopts the "blastocyst culture" strategy. The embryologist will continue to carefully cultivate the "fresh embryo" on the third day. From the the fourth day, the embryo undergoes key changes, cells begin to differentiate and form cavities, until the fifth, sixth or seventh day, a cell mass with a blastocyst cavity develops - this is the "blastocyst". This process is commonly known as "nourishing the bladder".

So, the core question is: What is the best day to cultivate blastocysts?

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The traditional view may lean towards the notion that the fifth day blastocyst, which grows faster, is more energetic and therefore superior. But in recent years, authoritative clinical research and laboratory data have provided us with a more scientific and detailed perspective.

A study covering over 2500 IVF cycles from 2017 to 2018 specifically evaluated the transplantation outcomes of day 5, 6, and 7 blastocysts after screening with next-generation genetic sequencing (NGS). The conclusion is very clear: for "diploid" blastocysts confirmed to have normal chromosomes through genetic screening, regardless of whether they are formed on the 5th, 6th, or 7th day, their final transplantation success rate is similar and there is no statistically significant difference.

This conclusion breaks the myth of the "only theory of days". The logic behind it is actually easy to understand:

Individual differences in embryonic development speed: Just like some children walk early and some speak early, each embryo has its own unique developmental rhythm. Some embryos' run fast 'and become fully formed on the 5th day, meeting the standards for freezing or biopsy; Some embryos belong to the "steady and steady type", which takes until the 6th or even 7th day to develop and mature. As long as it can ultimately meet the standards of high-quality blastocysts, it has the same developmental potential.

The core of screening is "health" rather than "speed": modern embryology places more emphasis on the intrinsic quality of the embryo, especially whether the chromosomes are normal. Laboratory cultivation and extended observation time are themselves powerful natural screening processes. Embryos that can survive tenaciously and form blastocysts, especially those that can pass the PGS/PGT-A gene screening, have demonstrated their vitality. The healthy blastocysts formed on the 7th day and the "same age" healthy blastocysts formed on the 5th day can be equally evaluated in terms of implantation ability.

A valuable opportunity in special circumstances: For patients who are older and have fewer retrieved eggs, every embryo is precious. If the embryo appears to have developmental potential on the fifth day but has not fully reached the standard, persisting in culturing it until the sixth or seventh day may provide patients with additional, transplantable opportunities for healthy embryos. Arbitrarily stopping cultivation may mean the loss of potential life.

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Therefore, the role of an embryologist in the laboratory is more like a patient observer and precise referee. They do not simply 'harvest' by day, but dynamically evaluate multiple indicators such as embryo morphology, expansion degree, and cell mass quality on a daily basis. Their core goal is to identify the most vibrant and healthy seed of life, no matter how many days it blooms.

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