After a zygote forms, it immediately begins dividing into smaller and smaller cells as it travels down the fallopian tube toward the uterus. This journey takes roughly five to six days, and the single-celled zygote transforms through several distinct stages along the way: first a morula, then a blastocyst, and eventually an embryo that implants in the uterine lining.
The First Divisions: Morula Stage
Fertilization happens in one of the fallopian tubes. About 24 hours later, the zygote makes its first division, splitting from one cell into two. From there, each new division takes roughly 12 to 24 hours. The cells don’t grow between divisions. They just keep halving in size, so the overall cluster stays about the same diameter as the original zygote.
By around day three or four after fertilization, the dividing cells form a solid, tightly packed ball called a morula (named after the Latin word for mulberry, which it resembles). At this point the morula is still traveling through the fallopian tube, pushed along by tiny hair-like structures lining the tube walls. It hasn’t reached the uterus yet.
Blastocyst: The First Specialization
By day five or six, the morula reaches the uterus and undergoes a critical change. Fluid collects inside the cell cluster, forming a hollow cavity, and the cells begin organizing into two distinct groups. This hollow ball of cells is called a blastocyst, and it’s the first time the cells take on different roles.
The outer ring of cells, called the trophoblast, will never become part of the baby. Instead, these cells go on to form the placenta and the membranes that support the pregnancy. The inner cluster, known as the inner cell mass, is the group that eventually develops into every structure in the adult body. These are the cells referred to as embryonic stem cells in research contexts, because they retain the ability to become any tissue type.
Implantation in the Uterine Lining
The blastocyst floats freely in the uterus for one to three days. During that time, it “hatches” out of a protective outer shell it carried since the zygote stage. Once exposed, the trophoblast cells on the outside of the blastocyst make direct contact with the uterine lining and begin burrowing in. This process is called implantation, and it typically occurs around six to ten days after fertilization.
Almost immediately after implantation, the trophoblast cells start producing a hormone called hCG. This is the hormone that pregnancy tests detect. The initial amounts are tiny, but by three to four days after implantation, a sensitive blood test can pick it up. Home urine tests usually need a few more days to register a positive result.
Embryo Stage and Gastrulation
Once implanted, the inner cell mass begins a dramatic reorganization. The cells rearrange themselves into three distinct layers in a process called gastrulation. Each layer is destined to build different parts of the body:
- Ectoderm (outer layer): becomes the skin, the nervous system, and structures like the cornea and lens of the eye.
- Mesoderm (middle layer): forms the skeleton, muscles, circulatory system, and the muscular walls of internal organs like the stomach and intestines.
- Endoderm (inner layer): lines the digestive and respiratory tracts and gives rise to organs like the liver and pancreas.
This three-layered structure is the blueprint for the entire body. Every organ and tissue traces back to one of these layers. The organism is officially called an embryo from implantation onward, and this label applies for the first eight weeks after fertilization.
From Embryo to Fetus
During the embryo stage, development is extraordinarily fast. The heart begins beating, limb buds appear, and the basic architecture of the brain, spinal cord, and major organs takes shape. By the end of eight weeks after fertilization, all the major organ systems have at least begun forming.
At nine weeks after fertilization, the terminology changes. The developing organism is now called a fetus, a label it keeps until birth. The fetal period is less about building new structures and more about growth and maturation of what’s already in place.
The Full Sequence at a Glance
- Day 0: Fertilization creates the zygote (single cell) in the fallopian tube.
- Day 1 to 3: Cell divisions produce a solid ball of cells, the morula, still traveling through the fallopian tube.
- Day 5 to 6: The morula reaches the uterus and becomes a hollow blastocyst with two cell types.
- Day 6 to 10: The blastocyst implants in the uterine lining.
- Weeks 2 to 8: The embryo forms three germ layers and develops all major organ systems.
- Week 9 to birth: Now called a fetus, the organism grows and matures.