An IVF embryo starts as a single round cell barely visible to the naked eye and transforms over five or six days into a fluid-filled sphere of more than 100 cells. At every stage, it looks like a tiny translucent ball enclosed in a clear protective shell. The whole thing is smaller than a grain of sand, so everything you see in photos or on a clinic screen has been magnified hundreds of times under a microscope.
Day 1: The Fertilized Egg
Within 16 to 18 hours of fertilization, embryologists check the egg for two small circles inside it called pronuclei, one carrying DNA from the egg and one from the sperm. At this point the embryo is still a single cell, roughly 120 to 130 micrometers across (about the width of a human hair). It sits inside the zona pellucida, a translucent shell made of glycoproteins that is 13 to 15 micrometers thick. Under the microscope it looks like a pale, slightly grainy circle with a clearly visible outer ring.
Days 2 and 3: The Cleavage Stage
Over the next two days the single cell divides, but the embryo itself does not grow larger. The cells simply split into smaller and smaller units within the same shell. By day 2 you typically see two to four cells; by day 3, a healthy embryo contains six to ten cells. Each cell (called a blastomere) looks like a translucent bubble packed tightly against its neighbors, giving the whole embryo the appearance of a tiny cluster of soap bubbles inside a clear casing.
Embryologists pay close attention to how symmetrical these cells are. In a good-quality cleavage-stage embryo, the cells are roughly equal in size with smooth, even edges and minimal fragmentation. Fragments are tiny membrane-bound pieces that break off during division. A small amount is normal, but heavy fragmentation makes the embryo look cluttered and uneven, and it can signal lower quality.
Day 4: The Morula
Around day 4 the individual cells compact together so tightly that you can no longer distinguish separate boundaries. The embryo is now called a morula, and it resembles a solid, slightly bumpy ball, like a microscopic mulberry. This compaction is a critical transition: the cells are beginning to communicate and organize themselves for the next stage.
Day 5 and 6: The Blastocyst
By day 5, the embryo has reorganized into a hollow, fluid-filled sphere called a blastocyst. This is the stage most commonly transferred or frozen in modern IVF. An early blastocyst measures about 140 micrometers in diameter. As it expands, it reaches roughly 190 micrometers, still far too small to see without magnification.
Under the microscope a blastocyst has three distinct features. The first is a fluid-filled cavity that takes up most of the interior, giving the embryo a balloon-like look. The second is a clump of tightly packed cells gathered at one pole, called the inner cell mass. This cluster eventually becomes the fetus. The third is a thin outer layer of cells that wraps around the entire sphere, forming a smooth, continuous sheet. These outer cells will go on to form the placenta and other pregnancy support tissues.
A top-quality blastocyst looks like a swollen, almost perfectly round sphere with a compact, bright cluster of cells clearly visible on one side and a smooth, even outer layer. The shell has stretched thin from the expansion. Lower-quality blastocysts may have a small, loosely grouped inner cell mass that is harder to spot, or an outer layer with fewer, larger cells that appear patchy rather than uniform.
How Embryos Are Graded
Most clinics use a three-part grading system. The first part is a number from 1 to 6 describing how expanded the blastocyst is: 1 means the cavity is just starting to form, 3 means it fills the embryo completely, 4 means it has expanded beyond the original shell size and the shell is thinning, and 5 or 6 means the embryo is hatching or has fully hatched out of the shell. The second part is a letter (A, B, or C) for the inner cell mass: A means many tightly packed cells, B means several loosely grouped cells, C means very few cells. The third part is another letter grading the outer cell layer by the same logic.
So a grade like “4AA” describes an expanded blastocyst with a dense, well-organized inner cell mass and a smooth, cohesive outer layer. A “3BB” is a full blastocyst with moderately grouped cells in both areas. These grades are entirely based on what the embryo looks like under a microscope, and while higher grades correlate with better pregnancy rates, plenty of “average-looking” embryos result in healthy babies.
Hatching: Breaking Out of the Shell
Before an embryo can implant, it has to escape its protective shell. As the blastocyst expands, the shell stretches and thins until it becomes almost invisible. The embryo goes through cycles of contraction and re-expansion, eventually squeezing through a small opening. Under the microscope a hatching blastocyst looks like a partially deflated balloon with a cluster of cells bulging out through a crack in the shell. In some IVF cycles, embryologists use a laser to create a small opening in the shell beforehand (assisted hatching), especially when the shell appears unusually thick or dark.
What a Biopsy Looks Like
If your embryos undergo genetic testing, a small sample of cells is removed from the outer layer at the blastocyst stage. The embryologist uses a laser to make an opening in the shell, then gently suctions five to eight outer cells as they push through the gap. These cells are stretched into a narrow bridge between the pipette and the embryo, and a few targeted laser pulses sever the connection at the junctions between cells. The inner cell mass is not touched. Afterward, the embryo looks essentially the same, just with a visible gap in the outer cell layer near the opening where the sample was taken.
Visual Red Flags Embryologists Watch For
Not every embryo looks textbook-perfect, and certain visual features signal potential problems. Vacuoles, which appear as clear, bubble-like inclusions inside the cell, are one concern. Small vacuoles are relatively common, but large ones (bigger than about 14 micrometers) can interfere with fertilization and healthy development. Very large vacuoles, above 25 micrometers, can distort the internal structure of the cell enough to impair its ability to function.
Color matters too. Healthy human embryos are pale and relatively translucent. Unusually dark or brown-toned cells can indicate problems with the internal structure, though this finding sometimes appears alongside other abnormalities like excessive granularity. The shell itself can also look abnormal: a dark, dense-looking shell with reduced translucency has been linked to lower implantation and pregnancy rates. Irregular shape is another marker. About 6 percent of eggs have noticeably non-spherical shapes, and this asymmetry has been associated with delayed development and reduced blastocyst formation.
Time-Lapse Monitoring
Many clinics now use specialized incubators with built-in cameras that photograph embryos every few minutes, creating a continuous time-lapse movie of development. Instead of removing the embryo from the incubator for a single daily check, embryologists can watch the entire developmental sequence on screen. The system tracks when each cell division happens, how long each stage lasts, and whether any divisions are abnormal (such as a cell splitting directly from one into three instead of two). These timing patterns, combined with the visual appearance at each checkpoint, give a more complete picture of embryo quality than a single snapshot ever could.