An embryo is the earliest stage of a developing organism after a fertilized egg begins dividing. In humans, the embryonic stage spans from roughly the third week of pregnancy through the eighth week. During this surprisingly short window, a cluster of cells transforms into a recognizable form with a beating heart, a developing brain, and the beginnings of every major organ system. After the eighth week, the embryo is reclassified as a fetus.
From Fertilized Egg to Embryo
The journey starts at fertilization, when a sperm and egg combine to form a single cell called a zygote. Within 24 hours, that cell starts dividing rapidly in a process called cleavage, splitting the original large cell into many smaller ones without growing in overall size. By about day four, the dividing cells form a solid ball, then hollow out into a fluid-filled structure called a blastocyst.
The blastocyst implants into the wall of the uterus roughly 8 to 10 days after ovulation. Once it attaches and begins taking on distinct characteristics, it earns the name “embryo.” This marks the beginning of the embryonic stage, which is the second of three developmental phases: the germinal stage (fertilization through implantation), the embryonic stage (weeks three through eight), and the fetal stage (week nine onward).
Three Cell Layers Build Everything
Around day 15 or 16, the embryo undergoes a dramatic reorganization called gastrulation. Cells rearrange themselves into three distinct layers, and each layer is responsible for building different parts of the body. The outer layer eventually forms the skin and nervous system. The middle layer gives rise to muscles, bones, the heart, and blood vessels. The inner layer develops into the lining of the digestive tract, lungs, and organs like the liver and pancreas.
This three-layer blueprint is the foundation for every tissue and organ in the human body. Damage or disruption during gastrulation can have profound effects on development, which is one reason the embryonic period is considered the most sensitive phase of pregnancy.
Week-by-Week Milestones
Development during the embryonic stage is remarkably fast. Here’s what happens in the major weeks:
Week 3: The heart begins forming, and the earliest version of the brain and spinal cord (the neural tube) starts to take shape. The heart begins beating around day 22 or 23, making it the first functioning organ in the embryo.
Week 4: The neural tube closes by day 28. The heart establishes its four chambers. The first neurons appear. At this point the embryo is still tiny, barely visible to the naked eye.
Weeks 5 and 6: The heart’s chambers begin separating further, and the heart and lungs descend into the chest area. Limb buds appear, and facial features like the eyes and mouth start to form. The brain begins developing distinct regions.
Weeks 7 and 8: Arms and legs continue growing. The trunk straightens. External ears, eyelids, and eyes become recognizable. By the end of week eight, the embryo measures roughly half an inch (11 to 14 millimeters) from crown to rump. Every major organ system has at least begun to form.
When an Embryo Becomes a Fetus
After the eighth week of pregnancy, healthcare providers stop using the term “embryo” and begin calling it a fetus. This isn’t an arbitrary label change. By this point, the process of organ formation (organogenesis) is largely complete. The fetal stage is about growth, maturation, and refinement of structures that already exist in basic form. The shift from embryo to fetus reflects a real biological transition: from building the blueprint to filling it in.
Embryos in Fertility Treatment
The term “embryo” also comes up frequently in the context of in vitro fertilization (IVF). During IVF, eggs are fertilized in a laboratory, and the resulting embryos are grown for several days before being evaluated for transfer to the uterus. Most clinics grow embryos to the blastocyst stage (around day five) before transferring or freezing them.
Embryo quality is assessed using a grading system with three components. The first score rates how expanded the blastocyst is, on a scale of 1 to 6. The second grades the inner cell mass (the cluster of cells that will become the baby) from A (many tightly packed cells) to C (very few cells). The third grades the outer cell layer (which becomes the placenta) on the same A-to-C scale. A score like “4AB” means a well-expanded blastocyst with a strong inner cell mass and a slightly less organized outer layer. Higher grades generally correlate with better chances of successful implantation, though lower-graded embryos can and do result in healthy pregnancies.
Why Embryos Matter in Stem Cell Science
Embryos are also central to stem cell research. Cells from the inner cell mass of a blastocyst are called embryonic stem cells, and they have two properties that make them uniquely powerful. First, they can divide indefinitely, producing identical copies of themselves. Second, they are pluripotent, meaning they can develop into virtually any cell type in the body: nerve cells, heart muscle, blood cells, skin, or anything else derived from those three foundational cell layers.
This is different from adult stem cells, which exist throughout the body but are more limited in what they can become. A blood-forming stem cell in bone marrow, for instance, can regenerate blood cells but not brain tissue. Embryonic stem cells don’t have that restriction, which is why they hold significant interest for understanding disease and potentially repairing damaged tissues. Mouse embryonic stem cells were first isolated in 1981, and the ability to maintain and study them in the lab has been a cornerstone of developmental biology ever since.