All embryos need to progress through this pivotal stage of compaction to form a morula. Despite an increasing number of people having an embryo transfer on day 4, the morula gets very little attention compared to other more glamorous stages of embryo development.

morula development

A morula usually forms 4 days after fertilisation when the embryo has divided from a single cell into around 16-32 cells. At this point, the cells merge together to form one compacted mass - hence why these are also referred to as compaction-stage embryos.

The word 'morula' comes from the Latin meaning for mulberry, as the compacted cells can resemble a berry or bunch of grapes. It is important that the embryo completes this process of compaction correctly as it can be an indictor of the embryo's potential to go on create a successful pregnancy.

morula 3

When compaction is complete, it should be difficult to distinguish individual cells from one another as the cell boundaries should be almost entirely gone.  A nicely compacted embryo on day 4 shows that the cells are communicating well with each other as they reshuffle cellular components and decide which part of the blastocyst they will develop into.


Morula Grading

The grade of the morula is based mostly on how compact the cells are. Ideally, every cell in the embryo should be included in the mass but it is very common to have some cells or small bits of cells excluded. It is thought that well-compacted morulas with all their cells included, have a greater chance of implanting and creating a pregnancy than morulas with cells missing from the mass.

Accurately grading morulas can be very difficult so they are usually allowed to develop for another day until they reach the blastocyst stage where the best embryo is selected for transfer or freezing.


Day 4 Embryo Transfer

If you have a day 4 embryo transfer it is likely that a morula or early blastocyst will be selected to be replaced into the uterus. Most clinics will only arrange a day 4 transfer due to scheduling of staff and availability of procedure rooms. For example, if you have your egg collection on a Tuesday, your embryos will be day 5 on a Sunday and many clinics do not offer embryo transfer procedures on a Sunday. This means that a day 4 transfer may be done instead.

If you only have a couple of reasonable quality embryos this is fine because it is clear which embryo should be transferred. However, if you have a lot of good quality embryos, a day 5 transfer is preferable to a day 4 transfer because most good quality morulas look very similar so we cannot say for sure which embryo is the highest quality. Another day of development means that the best embryo can shine through to be selected correctly. This is something you should speak to your embryologist and doctor about if you are concerned.