A human embryo is the earliest stage of human development after fertilization, spanning the first eight weeks. During this brief window, a single fertilized cell transforms into a recognizable human form with a beating heart, a developing brain, and the beginnings of every major organ. After the eighth week, the embryo is reclassified as a fetus, a designation it keeps until birth.
From Fertilized Egg to Embryo
Development doesn’t start as an embryo. It begins when a sperm cell fertilizes an egg, creating a single cell called a zygote. Over the next several days, that zygote divides repeatedly as it travels down the fallopian tube toward the uterus. By about day five, it has become a hollow ball of roughly 100 cells called a blastocyst. The blastocyst then implants into the uterine lining, usually around days six through ten after fertilization. Once implantation is secure and the cells begin organizing into distinct layers, the structure is considered an embryo.
The Three Cell Layers That Build Everything
One of the most important events in early embryonic life is the formation of three primary germ layers. These layers are the blueprint for every tissue and organ in the body, and they form within the first few weeks after fertilization.
The outer layer gives rise to the skin, nails, hair, all nerve tissue, salivary glands, and the mucous glands of the nose and mouth. Essentially, anything that interfaces with the outside world or processes information traces back to this layer.
The middle layer produces muscles, bone, cartilage, fat tissue, blood vessels, and blood cells. The linings of the heart and the body’s internal cavities also come from this layer, along with parts of the urinary and reproductive systems.
The inner layer builds the digestive and respiratory systems from the inside out. It forms the lining of the digestive tract, the lungs and airways, the liver, the bladder, and the glands that produce digestive secretions. By the time these three layers are established, the embryo has a complete molecular plan for assembling a human body.
What Happens Week by Week
The embryonic period packs an extraordinary amount of development into just eight weeks. Scientists have mapped it into 23 distinct Carnegie stages, each defined by specific structural changes rather than by size or age alone.
The heart is one of the first organs to function. Researchers long believed the heart muscle first contracted around day 21 after conception. More recent work funded by the British Heart Foundation at the University of Oxford suggests the heart may start beating even earlier, possibly as soon as 16 days after conception. Either way, a primitive heartbeat is present well before most people confirm a pregnancy.
The neural tube, which becomes the brain and spinal cord, forms and closes during the first several weeks. This is the reason folic acid is so important before and during early pregnancy: the tube needs to seal properly for the brain and spine to develop normally, and this happens before many women know they’re pregnant.
By around weeks four and five, tiny limb buds appear. Facial features start taking shape, with the beginnings of eyes, ears, and a mouth. By weeks seven and eight, fingers and toes become distinguishable, and the major internal organs have at least a rudimentary form. The skeleton, initially made of soft cartilage, begins the slow process of hardening into bone. In 1949, embryologist George Streeter proposed that bone marrow replacing cartilage in the upper arm bone could serve as a practical marker of the transition from embryo to fetus.
Size at the End of the Embryonic Period
Despite all this complexity, an eight-week embryo is remarkably small. Measurements are taken from the top of the head to the bottom of the torso (crown to rump) because the legs are curled tightly. At eight weeks, the embryo is roughly the size of a kidney bean. If you’re less than seven weeks pregnant, the embryo is so small that standard crown-to-rump measurement isn’t possible, and clinicians instead measure the greatest overall length.
When an Embryo Becomes a Fetus
The transition happens at the start of week nine after fertilization. The American College of Obstetricians and Gynecologists defines the embryo as lasting for eight weeks after fertilization, with the fetus designation beginning at week nine and continuing until birth. This isn’t an arbitrary calendar cutoff. It reflects a genuine shift in what the developing organism is doing: the embryonic period is about building organs from scratch (a process called organogenesis), while the fetal period is about growing, maturing, and refining structures that already exist.
One source of confusion is how pregnancy weeks are counted. Clinicians typically date pregnancy from the first day of the last menstrual period, which adds roughly two weeks to the actual developmental age. So an embryo at eight weeks post-fertilization is in what doctors call “week 10 of pregnancy.” If your ultrasound report or pregnancy app seems two weeks ahead of what you’d expect, that’s why.
Embryos in Fertility Treatment
The term “embryo” also comes up frequently in the context of in vitro fertilization (IVF). In IVF, eggs are fertilized in a lab, and the resulting embryos develop for several days before being transferred to the uterus or frozen for later use. The age of the embryo at the time of transfer is used to calculate the estimated due date. A frozen embryo that has been stored for years is still classified by its developmental stage at the time it was frozen, not by how long it has been in storage.
Whether developing naturally or through IVF, the embryo follows the same biological sequence: a single cell divides, organizes into layers, builds a heart, a brain, a spine, and the scaffolding of every organ system, all within the first eight weeks of life.