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Fertility – embryo development

Why don’t my eggs or embryos grow?

This is a common problem occurring in IVF patients. It is commonly misconceived that all eggs will fertilise, and all those fertilise will go on to continue splitting into day 5 embryos ready for implantation. It’s not until after a great IVF cycle where 12 eggs are collected but only 3 embryos are made, – that the couple starts to understand  the attrition rates even with IVF. In fact, the standardised ‘blastulation’, or ‘utilisation’ rate – that being the rate at which fertilised eggs should go onto day 5 – is quoted somewhere between 40-50%. For example, 10 fertilised eggs, should result in 4 or 5 embryos on day 5.

There is a lot that goes behind the scenes as we all know. The DNA from sperm combines with the DNA from the egg after fertilisation. Thereafter, genetic replication resumes. The egg bears the greatest responsibility in producing all the necessary proteins required for this process – until day 3 when up such a time the sperm DNA also becomes active. Of course, the embryo will have 50% contribution from each parent’s genome.

It is therefore posturised that in cases of really poor sperm quality (which is usually correlated with DNA damage as well) that a lot of embryos will arrest in development after day 3. But by far, the vast majority of times it is still the females’ eggs that are chromosomally abnormal. Those eggs are arrested in a stage of semi-maturation – ready to replicate again after fertilisation. And unlike sperm which live only for 90 days – the egg would have been in a womans body – at least 20 years – and in some cases, more than 40 years. Chromosomal testing in past, conducted on arrested embryos confirms this finding.

There are other reasons of course – but much rarer. Maternal conditions, such as PCOS – that leads to multiple follicular development and OHSS – often produce poor quality eggs. In such cases, despite large amount of eggs – very few go on to blastulate. In fact, even in the general population, the rate of embryo formation more or less stagnants after 20 eggs.

The lab also plays an important role in this. The embryo has significantly different nutritional requirements as it passes from day 3 (when it’s in the fallopian tubes) to day 5 (when it’s in the uterus). The best labs in the world, work to mimic this environment. However, due to differences in culture media, handling, environmental controls – this can have an impact on the blastulation rate. Very rarely, infections that affect culture media can lead to failed embryo development.

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