Embryology at Boston IVF

The highly skilled embryology teams at Boston IVF in New England and New York play an essential, behind-the-scenes role in every IVF journey. They are the dedicated professionals responsible for managing eggs and embryos from the moment of egg retrieval to embryo transfer or cryopreservation, ensuring the highest standards of care at every step.

During egg retrieval, follicular fluid containing eggs is sent to the lab, where an embryologist meticulously identifies and isolates the eggs. If the patient is undergoing egg freezing, the team carefully freezes them. For patients creating embryos, the team expertly prepares the eggs for insemination.

Embryologists perform conventional insemination by combining eggs with sperm from a partner or donor, allowing fertilization to occur naturally over several hours. For the 30% of patients requiring intracytoplasmic sperm injection (ICSI), they precisely inject a single sperm directly into a single egg to achieve fertilization.

Once fertilized, embryos are closely monitored, graded, and cared for as they develop. The embryology team evaluates each embryo to identify the most promising ones for transfer or freezing. When it’s time for an embryo transfer, an embryologist delicately prepares and loads the selected embryo into a catheter, handing it to the doctor for placement into the uterus.

Though their work often goes unseen, the impact of the embryology team is profound. They are at the heart of every critical step in the IVF process, working tirelessly to give patients the best chance of success and helping to turn dreams of parenthood into reality.

What is embryo grading?

Embryo grading refers to the appearance of an embryo at a certain point in time, most often at day 3 and day 5 (and day 7 if applicable) of development after fertilization, which occurs at day 0.

Embryo grading assesses each embryo at its current stage of development. 

Day 3

At day 3, embryologists are looking at growth in terms of the number of cells and degree of fragmentation. Every embryo receives a letter grade, which is a kind of shorthand for where they are at in their development.

Healthy embryos at day 3 are allowed to continue developing naturally for two additional days, so that they can hopefully grow more viable for implantation. In cases where an embryo is deemed unlikely to grow stronger, the embryo will then be transferred on Day 3.

Day 5

At day 5, we expect to see fewer embryos but each will have better odds of implanting than on day 3. A day 5 embryo is also called a blastocyst and it contains hundreds of cells with distinctive cell types: the cells that end up creating the baby (inner cell mass) and those that end up creating a placenta (trophectoderm). 

Only one-third of all embryos grow to this stage, and each of these embryos receives both a number grade, indicating how far the embryo is in expanding into a blastocyst, and two letter grades.  The first letter grade relates to the quality of the inner cell mass, a group of cells that will develop into the fetus, and the second letter grade relates to the trophectoderm, a layer of cells that helps the embryo implant. Studies show that the quality of the trophectoderm is related to the likelihood of an embryo to successfully implant.

What do the embryo grading letters and numbers mean?

This is a big topic that our embryologists discuss in detail in a recent video:

Here are the key points:

At day 3, embryos get a letter grade based on the number of cells and degree of fragmentation. Fragmentation is waste from the cell division process, so it can indicate overall health of the embryo and tell us whether the cells are dividing abnormally. 

✓  Embryos graded A are exceeding expectations for this stage of development and have no more than 5% fragmentation.

✓  Embryos graded B may have up to 20% fragmentation but are meeting expectations and there is no cause for concern.

✓  Embryos graded C at this point are still developing and may have more fragmentation than we like to see, but they may still catch up.

✓  Embryos graded D at day 3 would be considered struggling to develop.

At day 5, embryos are becoming blastocysts and start to look like a ball with a sack of fluid inside it. The larger and more expanded the fluid, the better. The grading scale reflects expansion and cell differentiation of each embryo at this stage.

A number system is used to indicate the degree of expansion an embryo has. It is not a ‘good or bad’ scale but merely indicates the extent to which an embryo has developed so far.

✓ 1 and 2 grades are for embryos that do not yet show full differentiation in the cells that will become the baby and those that will become the placenta.

✓ Blastocysts will be graded as follows:

• • 3 - Early blastocysts where the embryo is fully expanded and the cells are differentiated
  • 4 - Fully expanded blastocysts with a thinned out ‘shell’ called the zona pellucida
  • 5 - Blastocysts which have a cracked open zona pellucida in preparation for implantation
  • 6 - An implantable embryo

There is also an A, B or C grade given to the trophectoderm, which is a layer of cells on the outer edge of the blastocyst that aid in uterine implantation. Grades A and B are both great.

It’s important to understand that ‘low’ grades simply indicate that an embryo or blastocyst is still developing. Low grades do not mean that an embryo is not viable or diseased.  Equally, it is important to recognize that grading systems vary between clinics, even if similar number and letter formats are used.

How do you choose which embryo to transfer?

In order to decide which embryos to transfer, close observation of all embryos is key. At Boston IVF, our doctors, scientists, and laboratory embryologists closely observe embryo development in the lab and communicate with each other in order to best determine which embryo to select for transfer.

Your fertility team knows that all groups of embryos are unique. Sometimes the best embryo is immediately apparent; at other times it takes a few days to see which is best suited for transfer.