During the first week of human development, a single fertilized cell transforms into a hollow ball of roughly 100 cells and begins embedding itself in the uterine wall. This entire sequence, from fertilization to the start of implantation, unfolds over about six to seven days as the embryo travels down the fallopian tube and into the uterus.
Fertilization and the First Cell
After ovulation, an egg is viable for only 12 to 24 hours. It travels partway down the fallopian tube and pauses at a junction point, where it rests for about 30 hours. If sperm are present, fertilization happens here. The moment a sperm fuses with the egg, the resulting single cell is called a zygote. It contains a complete set of 46 chromosomes, half from each parent, and all the genetic instructions needed to build a human body.
The zygote doesn’t stay a single cell for long. Within hours, it begins dividing.
Days 1 Through 4: Rapid Cell Division
The zygote splits in a process called cleavage, doubling its cell count in roughly daily intervals. By day one after fertilization it has two cells, by day two it has four, by day three around twelve, and by day four it reaches sixteen or more. These divisions happen without the embryo actually growing in size. The original cell simply partitions itself into smaller and smaller units, all still contained within a protective shell called the zona pellucida (the same tough coating that surrounded the original egg).
At the 16- to 32-cell stage, around day four, the embryo is called a morula. It looks like a tiny solid ball, roughly the size of the period at the end of this sentence. At this point, it’s still drifting through the fallopian tube, slowly being pushed toward the uterus by muscular contractions and tiny hair-like structures lining the tube walls.
During these early days, the embryo’s cells are remarkably flexible. Each cell in the very early stages has the potential to become any type of tissue in the body, or even the placenta. But at the morula stage, a critical first decision occurs: cells on the outer surface begin to specialize differently from cells on the inside. This is the embryo’s first step toward organizing itself into distinct tissues.
Day 5: The Blastocyst Forms
By day five, the solid ball of cells reorganizes into a hollow sphere called a blastocyst. Fluid fills the interior, creating a cavity, and two distinct cell groups emerge. The outer layer, a single cell thick over most of its surface, will eventually form the placenta and other support structures. A small clump of cells bunched together on one side, three to four cells thick, will become the embryo itself and ultimately every tissue in the fetal body.
This inner cluster contains what scientists call pluripotent cells, meaning they can develop into all the different cell types of the human body: muscle, bone, nerve, skin, and everything else. Key molecular signals that maintain this flexibility are active in all cells of the early blastocyst on day five, then gradually become restricted to just the inner cluster by day seven. A second decision within the inner group further separates the cells that will form the fetus from those that will form the yolk sac, an early support structure.
Hatching From the Protective Shell
Before the blastocyst can attach to the uterine wall, it must escape the zona pellucida that has surrounded it since it was an unfertilized egg. This protective shell served important functions during the trip through the fallopian tube, preventing the embryo from sticking to the tube walls and holding the early cells together. But now it’s an obstacle.
At the blastocyst stage, the embryo breaks through this shell in a process sometimes called “hatching.” The blastocyst actively pushes through a gap in the zona, squeezing out so its outer cells can make direct contact with the uterine lining. Without this step, implantation cannot occur.
Days 6 Through 7: Implantation Begins
Around day six after fertilization, the now-hatched blastocyst arrives in the uterus and attaches to the uterine wall, typically near the top of the cavity. The outer cells of the blastocyst begin burrowing into the uterine lining, anchoring the embryo in place. This process, called implantation, isn’t instantaneous. It begins around day six and isn’t fully complete until day nine or ten.
As the outer cells invade the uterine wall, they start producing a hormone called hCG. This is the same hormone that pregnancy tests detect. Although the genetic instructions for making hCG are activated as early as the eight-cell stage, the hormone doesn’t reach detectable levels in a mother’s blood or urine until somewhere between six and fourteen days after fertilization. This is why pregnancy tests taken too early often come back negative even when an embryo is present.
How the Embryo Gets Its Nutrients
One common question is how the embryo survives before a placenta is established. The answer is that during the first week and well beyond, the embryo relies on secretions produced by glands in the uterine lining. These secretions, sometimes called “uterine milk” or histiotroph, are a rich mixture of carbohydrates, proteins, and fats that provide energy and building materials.
This form of nutrition turns out to be far more important than scientists once assumed. A full maternal blood supply to the placenta isn’t established until 10 to 12 weeks of pregnancy. During the entire first trimester, the space around the developing placenta fills with a clear fluid derived partly from these glandular secretions. So for the first week and months beyond it, the embryo essentially absorbs nutrients from the surrounding tissue rather than receiving them through a blood supply the way a later-stage fetus does.
By the end of the first week, what started as a single cell has become a structured, implanting organism with its first specialized cell types, an emerging hormonal signal to the mother’s body, and a foothold in the uterine wall that will support the next eight and a half months of development.