A baby is called an embryo for the first 8 weeks after fertilization. Once 8 complete weeks have passed, the embryo is reclassified as a fetus, a label it keeps from week 9 until birth. That 8-week window is short, but it’s when nearly every major organ and body system begins to form.
Why the Week Count Can Be Confusing
There are two different ways to count how far along a pregnancy is, and they don’t line up. This is where most of the confusion comes from.
Fertilization age starts from the actual day the egg was fertilized. By this clock, the embryonic period lasts 8 weeks. Gestational age starts from the first day of the last menstrual period, which is roughly two weeks before fertilization even happens. So the same milestone, the transition from embryo to fetus, falls at 8 weeks by fertilization age but around 10 weeks by gestational age.
Doctors and pregnancy apps almost always use gestational age. That means when your provider says you’re “10 weeks pregnant,” the embryo has actually been developing for about 8 weeks and is just crossing into fetus territory. If you’ve seen conflicting numbers online (some sources say 8 weeks, others say 10), this two-week gap is the reason. Both are describing the same moment in development.
What Happens During Those 8 Weeks
The embryonic period is sometimes called organogenesis because it’s the stretch when all the major organs begin taking shape. Scientists divide it into 23 distinct stages, known as Carnegie stages, spanning roughly 56 to 60 days after fertilization. Each stage marks the appearance of specific structures, from the earliest cell divisions to the formation of recognizable limbs, eyes, and internal organs.
In the first few weeks, the fertilized egg divides rapidly and implants in the uterine wall. The cells begin sorting themselves into three layers that will eventually become different body systems: the outer layer forms skin and the nervous system, the middle layer forms muscles, bones, and the circulatory system, and the inner layer forms the digestive tract and lungs. By the end of week 4 (fertilization age), a primitive heart tube has formed and started beating.
Weeks 5 through 8 are when things move fast. Arm and leg buds appear and elongate into recognizable limbs with fingers and toes. The brain divides into distinct regions. The eyes, ears, and nose begin to develop. The liver, kidneys, and intestines start forming. By the end of week 8, the embryo has the basic blueprint of nearly every organ system it will need, even though none of them are anywhere close to fully functional yet.
How Big Is an Embryo at 8 Weeks
At the end of the embryonic period, the crown-to-rump length (measured from the top of the head to the bottom of the torso, since the legs are curled up) is roughly 22 millimeters. That’s a little under an inch, or about the size of a kidney bean. Despite being tiny, the embryo at this point looks distinctly human compared to the formless cluster of cells it was just weeks earlier.
What Changes When It Becomes a Fetus
The shift from embryo to fetus at week 9 (fertilization age) isn’t a sudden physical transformation. It marks a change in what the body is doing. During the embryonic stage, the primary job is building new structures from scratch. During the fetal stage, those structures grow, mature, and begin to function. The heart refines its four-chamber structure. The bones harden. The lungs develop air sacs. The brain adds billions of neurons.
This distinction matters medically because the embryonic period is when the developing baby is most vulnerable to disruptions. Exposure to certain medications, infections, or toxins during organogenesis can interfere with organs as they first form, potentially causing structural birth defects. Once the fetal period begins, the organs are established and the risks shift more toward growth and functional development rather than missing or malformed structures.
A Quick Reference
- Embryonic period by fertilization age: weeks 1 through 8 after conception
- Embryonic period by gestational age: approximately weeks 3 through 10 (since gestational age adds about 2 weeks)
- Fetal period: from week 9 after fertilization (around gestational week 11) until birth
- Size at the transition: roughly 22 mm, or just under 1 inch