In recent years, there has been a clear trend in assisted reproduction to limit the number of embryos transferred to the uterus, without a negative impact on success rates. This can only be achieved by increasing the implantation rate of a single embryo, that is, by maximizing the chance that the embryo we choose for transfer is the one most likely to result in a pregnancy.
In assisted reproduction laboratories, embryo assessment has for years been based mainly on morphology. This assessment is quick and non-invasive, but it has the drawback that observation is subjective and therefore sometimes not enough to help the doctor limit the number of embryos transferred to the uterus. This led scientists working in assisted reproduction to find other ways to assess embryos created through IVF methods, based on their reproductive potential.
The preimplantation genetic testing (PGT-A, formerly known as PGS) approach can help us choose the embryos to be transferred to the uterus based on their chromosomal integrity, that is, only the embryos that are chromosomally normal. It is indicated for specific groups of patients, such as older women undergoing IVF and women with a history of recurrent miscarriage. Besides not being suitable for all patients, this method requires specialized scientific staff and appropriate equipment. Today the biopsy is usually done at the blastocyst stage, on day 5 or 6, and the embryos are vitrified until the results are ready. The transfer takes place in a later cycle, so there is no longer any time pressure on the laboratory.
Monitoring the morphokinetic development of embryos (Time-lapse microscopy – Embryo morphokinetics) takes place while the embryos are in the laboratory, inside special incubators where they are assessed through frame-by-frame photographs, without disturbing their incubation environment, which is very important for their proper development. This continuous recording gives very important information about which embryo is dividing at the right pace, that is, neither too fast nor too slow, which is considered a disadvantage both in the laboratory and in nature. It also gives information about the developmental pattern of each embryo, based on growth models considered ideal for embryos that will lead to a viable pregnancy. So, even if a couple has many embryos with excellent morphology, we can rely on time-lapse microscopy criteria to choose, among the morphologically excellent embryos, only the one or two that meet these criteria and transfer only those to the uterus.
Embryo metabolic profile (Metabolomics). This is another non-invasive way to assess embryos and identify the best one for embryo transfer. It is based on measuring certain molecules that embryos secrete or consume while they are cultured in the laboratory, such as amino acids, sugars, oxygen and the toxic by-products of embryo metabolism. These measurements are also non-invasive, meaning they do not affect embryo development or disturb the embryos. The metabolic profile of each embryo, combined with its morphology, is related to that embryo’s potential to result in a pregnancy. However, randomized studies did not show an improvement in pregnancy rates, so the method is not used in clinical practice today.
The vitrification technique. This is a method for freezing eggs and embryos at all stages of development, from the two-cell stage up to the blastocyst stage, when the embryo has more than 200 – 300 cells. The introduction of this method is very important for several reasons. First, because eggs, which are cells quite sensitive to cryopreservation, can now be frozen with post-thaw survival rates that usually exceed 80 to 90%, unlike in the past, when the method was considered experimental and had more limited success. This gives women the opportunity to successfully preserve their fertility by freezing their eggs and postponing pregnancy for several years. This can of course be done for social as well as medical reasons. Second, donor eggs are successfully cryopreserved and can be made available right away to couples who need them, without a waiting period until a suitable donor is found. Third, blastocysts are now cryopreserved with this method with very high survival rates, and this is very important, because we can transfer a single blastocyst to the uterus while successfully freezing the rest, and we can also perform preimplantation testing on the blastocyst of interest, then cryopreserve it and transfer it to the uterus in a later cycle, when the conditions of the woman’s endometrium will be ideal to receive it.
New embryo culture systems. In recent years, embryo culture systems in the laboratory have improved dramatically with the introduction of new-generation culture media. These media try to mimic as closely as possible the environment of the uterus and fallopian tubes, where fertilization and embryo development take place in nature, an environment rich in free amino acids, sugars and trace elements that protects, nourishes and metabolically supports the developing embryo.
The new culture systems are designed to support the entire preimplantation development of the embryo, as it passes through the first stages just before and after the egg is fertilized by the sperm, until it reaches the blastocyst and hatching stage, when it is ready to implant in the endometrium. The new culture media fall into sequential media, which are specially formulated for each stage of the embryo, and single step media, in which the embryo is not disturbed by human intervention at all during the days it is cultured in the IVF laboratory.
Intracytoplasmic morphologically selected sperm injection (IMSI). This is the ICSI technique, in which the woman’s eggs are fertilized with her husband’s sperm after the sperm are selected under a microscope at high magnification, 6000 times, whereas the magnification used for classic ICSI reaches 400 times. This allows us to choose the sperm with the best morphology by better detecting changes in its fine structure, with the aim of increasing implantation and pregnancy rates. However, large systematic reviews have not shown a clear increase in births compared with classic ICSI, so the technique is not recommended routinely.
It is indicated for couples with an unusually low sperm count or sperm with a high rate of abnormal morphology, as well as for couples with a small number of eggs or multiple repeated IVF failures. Most laboratory studies on this technique have been carried out in laboratories in Israel, France and Italy, and by 2008 about 1000 babies had been born in Asia and Europe using IMSI.
Mitochondrial DNA donation. In 2015, the United Kingdom became the first country in the world to legalize mitochondrial DNA donation to prevent mitochondrial diseases from being passed from mother to child. The first births within the British program have already been recorded. The new method concerns women with mutated genetic material in the mitochondria of their cells and eggs who wish to have genetically related children, while replacing their mitochondria with mitochondria from the eggs of a healthy egg donor. Mitochondria are organelles that provide energy to the cell, and they have their own genetic material.
Many children are born each year with mutations in their mitochondrial DNA that are not severe enough to cause disease. However, one in 6000 children with more serious types of mitochondrial DNA mutation will suffer from heart and muscle diseases, blindness and other conditions. This new technique involves transferring the paternal and maternal nuclei (which carry the genetic material of the father and mother, respectively) into a donor egg that carries healthy mitochondria (with the donor’s genetic material). The nucleus with the donor’s own genetic material has first been removed from the donor egg, so technically the donor’s only “genetic” contribution is the DNA in her mitochondria, which is very small in quantity.
This is a scientific innovation with many pros and cons, and with the scientific and ethical dilemmas that all innovations involve, at least those applied to humans.
Which new embryology techniques have strong evidence
The list of IVF “add-ons” grows every year. Not all of them have the same scientific basis. The European Society of Human Reproduction and Embryology (ESHRE) has published recommendations that help separate what has a proven benefit from what does not:
- Established practices: vitrification of eggs and embryos, culture to the blastocyst stage in suitable cases, single embryo transfer, and ICSI when there is a severe male factor.
- Useful with a specific indication: preimplantation genetic testing for a known inherited disease (PGT-M) or a chromosomal rearrangement in a parent (PGT-SR).
- No clear benefit on live birth rates when used routinely: PGT-A for all patients, assisted hatching, IMSI, embryo “glue” and embryo selection based only on time-lapse algorithms.
This does not mean that the techniques in the third group are dangerous. It means they should be used with a specific indication and honest information about what they can and cannot offer.
Single embryo transfer: why it is now the rule
From the start, new embryology techniques had one goal: to allow a single embryo to be transferred without lowering the chances of pregnancy. The reason is simple. A twin pregnancy significantly increases the risk of preterm birth, preeclampsia, gestational diabetes and complications for the newborns.
With culture to the blastocyst stage and reliable vitrification, the remaining good embryos are stored for future transfers. This way, the cumulative chance of having a baby from one egg retrieval remains high, while the risks of a multiple pregnancy are reduced.
What is coming: techniques still under research
- Artificial intelligence in embryo assessment: algorithms analyze time-lapse images to predict which embryo has the best chance of implantation. Early results are interesting, but they have not yet shown an advantage over an experienced embryologist.
- Non-invasive genetic testing (niPGT): analysis of the DNA released by the embryo into the culture medium, without a biopsy. Its accuracy is not yet sufficient for routine clinical use.
- New sperm selection methods: microfluidic devices and other selection techniques are being tested, mainly when there is high sperm DNA fragmentation, without clear proof yet that they increase births.
- Nuclear transfer: apart from preventing mitochondrial disease, maternal spindle transfer is also being studied in research settings for repeated failures due to poor egg quality.
Questions worth asking before an additional embryology technique
- What is the specific indication in our case?
- Does the technique increase the chance of having a baby, or only an intermediate marker, such as implantation?
- Are there risks for the embryos, for example with a biopsy?
- How much does it cost and what will it mean for the number of cycles we can do?
- Are there alternatives, such as egg donation when egg quality is the main limitation?
Video: Dr. Polyzos on new embryology techniques
For couples who prefer fewer medications, there is also Mini IVF, while for the genetic assessment of embryos we use preimplantation genetic testing (PGT).
For a personalized assessment of your case, book an appointment with Dr. Panagiotis Polyzos at his office in Athens or Volos.
Frequently Asked Questions
What is ICSI?
It is the technique in which the embryologist injects a single sperm directly into the egg. It is used mainly for male infertility or when there has been a previous fertilization failure.
Why are embryos cultured to the blastocyst stage?
Culturing to day 5 or 6 helps identify the embryos with the greatest implantation potential, so that a single embryo with good chances can be transferred.
What is assisted hatching?
A small opening is made in the outer shell of the embryo with a laser, to make it easier for the embryo to hatch and implant. It is recommended in selected cases, not in all.
Is vitrification of embryos and eggs safe?
Yes. Vitrification is rapid freezing with survival rates above 90 to 95% for embryos. See more about embryo cryopreservation.
Does every couple need all the techniques?
No. Each technique has specific indications. The doctor and the embryologist choose what offers a real benefit in your case, such as time-lapse or preimplantation genetic testing (PGT).
Does embryo biopsy for preimplantation testing cause harm?
A biopsy at the blastocyst stage removes a few cells from the part that will form the placenta and is considered safe in experienced laboratories. However, it is not completely without effect, so testing is recommended when there is a real indication.
Are children born from frozen embryos healthy?
Data from a large number of births are reassuring. Children from frozen embryos do not show an increased risk of birth defects. Frozen embryo transfer is linked to slightly higher birth weight and, in some protocols, an increased risk of high blood pressure in pregnancy, which is taken into account when planning.
Can new techniques make up for a woman’s age?
Not fully. No embryology technique improves the quality of the eggs that already exist. The techniques help select the best available embryo, but age remains the most important factor for success.