How embryos develop – from egg retrieval to blastocyst

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After retrieval, the egg and sperm are combined, and if fertilisation is successful, your embryo spends the first few days growing in the lab under the expert care of our team. This blog will help guide you through these early embryo development steps before transfer or freezing.

Factors affecting embryo development

Embryo development is a complex process that requires a combination of genetic, environmental and physiological factors to progress successfully. To achieve good embryo development, we require:

Healthy egg and sperm

The egg and sperm provide the genetic blueprint for development. Each embryo needs a complete set of 46 chromosomes – 23 from the egg and 23 from the sperm. Some embryos inherit incorrect genetic instructions that can impact embryo development and make it harder for them to divide and grow as expected.

Mitochondrial energy

Embryos need energy to divide and grow, which is provided by mitochondria (tiny energy-producing structures in cells). Poor mitochondrial function can slow or stop embryo development.
Efficient metabolic function:

An embryo’s metabolic function provides both energy and the building blocks needed for development. This includes the creation of protein and fats, and the removal of waste products that can be toxic to the embryo. Together, these provide what the embryo needs for growth, cell division and viability.

Timely cell division

Embryos should ideally divide at a regular pace (2-cell, 4-cell, 8-cell, etc.). Uneven and/or delayed division can mean the embryo is of suboptimal quality and may not develop as expected.

Embryonic genome activation

Around Day 3 (see diagram below), embryos start using their own DNA instead of relying on maternal DNA (genetic code from the egg). If this DNA transfer is inadequate, interrupted or missing, this can slow or stop the development of an embryo.

Stable conditions

Embryos need the right temperature, oxygen and pH balance to grow. These factors are carefully assessed and monitored continuously in the laboratory environment.

Difficulties or inaccuracies in any of these processes can affect the way an embryo grows and can impact whether an embryo will reach the blastocyst stage (where it has divided into many cells), making it suitable for transfer or freezing.

Safeguarding your embryos is our highest priority

We understand how important every embryo is to your journey. That’s why we use the most advanced technology and scientifically proven methods to create the ideal environment for embryo development. From carefully performing every procedure to closely monitoring each embryo’s progress, our highly trained embryologists work tirelessly to give each embryo the best possible chance to grow and thrive.

We maintain strict laboratory conditions, including precise temperature, humidity and air quality control to mimic the natural environment as closely as possible. Our team carefully observes each embryo’s development at every stage. Even though not all embryos will reach the blastocyst stage, please know that we do everything in our power to maximise their potential. Your dream of building a family is at the heart of everything we do, and we are committed to providing the best possible care every step of the way.

If you have any questions about embryo development or your fertility journey, please reach out to Newlife IVF. In the meantime, let’s walk through the different stages of embryo development to help you better understand the process

Comparing the merits of fresh versus frozen embryo transfer: is fresh really best?

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Once here, it must ‘implant’ in the wall of the womb and grow before we can say that IVF has resulted in a successful pregnancy. The timing of embryo transfer can vary, depending on whether the embryos being transferred are ‘fresh’ or ‘frozen’. Fresh embryo transfer refers to embryos that are transferred to the uterus 3–5 days after a woman’s eggs have been collected and fertilised by sperm. Frozen embryo transfer refers to embryos that have first been frozen before being thawed at a later date for transfer into the womb.

In the early days of IVF, fresh embryo transfer was the favoured approach. However, the techniques used for freezing and thawing of embryos have since improved to a point where more than 90% of embryos will survive the process. Consequently, a ‘freeze-all’ strategy has become more common, whereby all embryos are frozen following successful growth. They generally remain frozen for at least a month before the best embryos are thawed and transferred into the womb.

Understandably, people undergoing IVF are often eager to get pregnant as quickly as possible – and may assume that fresh embryo transfer is both faster and more effective. But is one approach better than the other?

To appreciate the pros and cons of fresh versus frozen embryo transfer, you first need to understand the so-called ‘window of uterine receptivity’.

The window of uterine receptivity

The success of embryo transfer depends on a number of factors, one of which is uterine receptivity – that is, how ready the uterus is to ‘receive’ the embryo. Outside this window of receptivity, the embryo may fail to implant in the wall of the uterus.

To receive the embryo successfully, the uterus must be ‘primed’ by the hormones oestrogen and progesterone. Under natural conditions, the priming of the uterus is perfectly timed with a woman’s monthly cycle, such that if an egg is released from the ovary and fertilised by sperm, the uterus is ready to receive the embryo. In the IVF setting, however, this timing may be less than perfect.

During IVF, the ovaries are stimulated via self-injectable medication so that the highest possible number of eggs can be collected. By artificially driving the release of so many eggs, the levels of oestrogen and progesterone skyrocket – they can rise to 10 times higher than normal peak levels. This may cause the uterus to prematurely prepare itself for embryo implantation, bringing forward the time frame in which the uterus is receptive. The problem with this is that by the time a fresh embryo is grown and ready for transfer, the window of uterine receptivity may have passed.

Frozen embryo transfer overcomes this problem by delaying the transfer process. This gives the hormone levels time to return to normal and the embryo is then transferred at a later date, when the uterus is receptive again.

When is frozen transfer best?

There are some situations where frozen embryo transfer may be considered the best option, including:

  • High levels of progesterone: When progesterone levels are high at the time of egg retrieval, there is a higher chance that the window of uterine receptivity will shift forward. In these cases, it is generally better to freeze the embryos and transfer them later when the uterus is receptive again.
  • Polycystic ovary syndrome (PCOS): Studies have found that women with PCOS tend to have better results from frozen versus fresh transfer. Frozen transfer reduces the risk of ovarian hyperstimulation syndrome (a complication of egg retrieval), and is also associated with a higher chance of ongoing pregnancy.
  • Embryos that require genetic testing: The genetic testing of embryos takes time. Consequently, by the time a tested embryo is ready for transfer, the window of uterine receptivity is likely to be over. In this scenario, frozen embryo transfer is usually more appropriate.

What are the arguments for fresh transfer?

On the other side of the coin, fresh transfer avoids the need for the freeze-thaw process. While current technology enables a greater than 90% survival rate for frozen embryos, this level of risk may not be acceptable for some patients – especially if they already have a low number of embryos. Fresh transfer potentially also results in a shorter time to pregnancy.

Which is the best option for you?

When choosing between fresh and frozen embryo transfer, there is no one-size-fits-all approach. As with all aspects of fertility care, the decisions need to be personalised to your individual circumstances. Our fertility specialists will assess the specifics of your situation and tailor their advice accordingly.

If you would like professional advice about the next steps to take on your fertility journey, you can make an appointment with one of our fertility specialists by calling Newlife IVF on (03) 8080 8933. Alternatively, you can book online via our appointments page.