An embryo is the earliest stage of a developing baby, spanning from fertilization (when sperm meets egg) through the first eight weeks of development. After those eight weeks, it’s called a fetus. During this short window, a cluster of cells smaller than a grain of rice transforms into a recognizable form with a beating heart, a developing brain, and the beginnings of every major organ system.
From Fertilized Egg to Embryo
Development doesn’t start as an embryo. When a sperm penetrates an egg, the result is a single cell called a zygote. That zygote travels down the fallopian tube toward the uterus over the next three to five days, dividing as it goes. By the time it arrives, it has become a hollow ball of cells called a blastocyst.
About six days after fertilization, the blastocyst attaches to the wall of the uterus, usually near the top. This process, called implantation, is completed by day nine or ten. The inner group of cells within the blastocyst is what will become the embryo itself, while the outer cells begin forming the placenta and other supporting structures.
One note on timing that often causes confusion: pregnancy is dated from the last menstrual period, which is roughly two weeks before fertilization actually happens. So when someone is “four weeks pregnant,” the embryo is closer to two weeks old.
Three Cell Layers That Build Everything
One of the most important events in early embryonic life is gastrulation, when cells organize themselves into three distinct layers. Each layer is responsible for building different parts of the body, and this sorting process sets the entire blueprint for organ development.
The innermost layer becomes the gastrointestinal tract, liver, pancreas, thyroid, and the lining of the lungs. The middle layer gives rise to an enormous range of structures: muscles, bones, cartilage, the heart and blood vessels, kidneys, reproductive organs, and connective tissue. The outermost layer produces the skin, hair, nails, the lens of the eye, the inner ear, and the entire nervous system, including the brain and spinal cord.
All three layers are established within the first few weeks. From that point on, the embryonic period is about those layers differentiating into increasingly specialized tissues and organs.
Week-by-Week Milestones
Development during the embryonic stage is fast. More structural change happens in these eight weeks than in any comparable period of life.
The heart is the first functional organ. It begins beating around 21 to 23 days after fertilization, roughly the start of the fourth week. At this point, the embryo is tiny, but it already has the beginnings of a circulatory system pumping blood. The neural tube, which will become the brain and spinal cord, is closing around this same time. Disruptions to neural tube closure during this window are what lead to conditions like spina bifida.
By week five, limb buds appear. By week six, fingers are beginning to form, and the eyes and ears are taking shape. By the end of week eight, all major organ systems have at least a rudimentary form. The embryo at this point is roughly the size of a kidney bean. After eight completed weeks of development, it officially transitions to the fetal stage, where the focus shifts from building new structures to growing and maturing the ones already in place.
How the Embryo Gets Nutrients
Before the placenta is fully functional, the embryo depends on a structure called the yolk sac. Despite its name, the human yolk sac contains no yolk. Instead, it serves as the embryo’s first supply line, handling nutrition and gas exchange between the mother and the developing embryo during those earliest weeks.
The yolk sac also plays several other critical roles. It’s where the first blood cells are produced, a process that later shifts to the liver and eventually to bone marrow. It generates the precursor cells that will become either ovaries or testes. And it contributes to the formation of the umbilical cord. By around the 14th week of pregnancy, the yolk sac has been fully replaced by the placenta and becomes undetectable on ultrasound.
Embryos in Fertility Treatment
The term “embryo” also comes up frequently in the context of in vitro fertilization (IVF), where eggs are fertilized in a laboratory and allowed to develop for several days before being transferred to the uterus or frozen for later use.
In an IVF lab, embryos are typically grown to the blastocyst stage, around five or six days after fertilization. Embryologists evaluate them using a grading system that scores three things: how expanded the blastocyst is (on a scale of one to six), the quality of the inner cell mass that would become the baby (graded A through C, with A being tightly packed cells), and the quality of the outer cell layer that would become the placenta (also graded A through C). A blastocyst graded 4AA, for example, is an expanded embryo with top marks on both cell groups.
These grades help fertility specialists select which embryos have the best chance of implanting successfully, though grading is just one piece of the picture. Some average-looking embryos implant and develop perfectly, while some high-grade embryos do not.
Embryo vs. Fetus: Why the Distinction Matters
The shift from embryo to fetus at eight weeks after fertilization isn’t arbitrary. It marks the transition from a period of organ formation to a period of growth and refinement. During the embryonic stage, the developing organism is most vulnerable to disruptions, whether from certain medications, infections, or environmental exposures. That’s because organs are actively being built from scratch. Once the fetal stage begins, the basic architecture is in place, and the risks shift accordingly.
This is why the first trimester is considered the most sensitive period of pregnancy, and why prenatal vitamins with folic acid (which supports neural tube closure) are recommended even before conception. Many of the most critical developmental events happen before most people even know they’re pregnant.