A baby in the womb goes by different names depending on its stage of development. In the earliest days after conception, it’s called a zygote. From conception through the eighth week of pregnancy, the medical term is an embryo. From the ninth week until birth, it’s called a fetus. In everyday conversation, people simply say “baby,” “unborn baby,” or “unborn child” at any point during pregnancy.
The Three Stages of Prenatal Development
The terminology follows biology. Each name marks a distinct phase of growth, and the shifts between them reflect real changes in what’s happening inside the womb.
Zygote (conception to about 2 weeks): The moment a sperm fertilizes an egg, the resulting single cell is called a zygote. Over the next few days it divides rapidly as it travels down the fallopian tube toward the uterus. Once it implants in the uterine wall, it’s sometimes called a blastocyst, though most people outside a fertility clinic will never hear that term.
Embryo (weeks 3 through 8): This is the stage when the body’s basic blueprint takes shape. Organs, limbs, and major systems begin forming in a process doctors call organogenesis. By the end of week 8, nearly all organs and systems have at least a rough structure in place.
Fetus (week 9 through birth): Once the major structures exist, the developing baby is reclassified as a fetus. The fetal stage is about growth, maturation, and fine-tuning. Bones harden, organs become functional, and the baby gains the size and strength needed for life outside the womb.
What Happens During the Embryonic Stage
The embryonic period packs an enormous amount of development into just a few weeks. By week 5, the neural tube forms. This structure eventually becomes the brain, spinal cord, and the rest of the central nervous system. A tiny tube that will become the heart begins pulsing around this same time, beating roughly 110 times per minute by the end of week 5.
By week 6, small buds that will grow into arms and legs appear. Blood cells start forming and circulation begins. The earliest structures for the ears, eyes, and mouth also take shape during this week. By the end of week 8, most of the embryo’s organs and body systems have a basic form, even though the entire structure is still remarkably small, roughly the size of a kidney bean.
What Changes When It Becomes a Fetus
The shift from embryo to fetus at week 9 isn’t just a name change. It reflects a real transition in what the body is doing. During the embryonic stage, the priority is building new structures from scratch. During the fetal stage, those structures grow larger, more complex, and increasingly functional. Soft cartilage gradually hardens into bone. The brain develops more intricate wiring. Fingers and toes separate. Facial features become more defined.
The fetal period lasts about 30 weeks, far longer than any other stage, because growing a full-sized, functioning human body takes time. By the middle of pregnancy, the fetus can move, swallow, and respond to sound. By the final trimester, the lungs are maturing, fat is building up under the skin for warmth, and the brain is developing at a rapid pace to prepare for life after birth.
Medical Terms vs. Everyday Language
In clinical settings, doctors and midwives use “embryo” and “fetus” because the terms carry specific medical meaning. They tell other providers exactly how far along development is. You’ll see these words on ultrasound reports, in medical charts, and in prenatal literature.
In everyday life, most people just say “baby” regardless of the week. That’s perfectly normal and understood by everyone, including your healthcare provider. You might also see the term “unborn baby” or “unborn child” in legal or informational contexts. After birth, the medical term switches to “neonate” for the first four weeks of life, though nearly everyone simply says “newborn.”
Why the Terminology Matters
Knowing these terms can make prenatal appointments and pregnancy resources easier to follow. When your provider mentions the embryonic stage, you’ll know they’re talking about the critical first eight weeks when organs are forming, a period when the developing baby is most sensitive to disruptions like alcohol, certain medications, or infections. When they refer to the fetus, you’ll know the conversation has shifted to growth and maturation rather than the initial construction of body systems.
The names also help make sense of pregnancy tracking apps, ultrasound results, and the week-by-week guides many parents follow. Whether you call it an embryo, a fetus, or simply your baby, you’re describing the same remarkable process of a single fertilized cell developing into a fully formed human being over roughly 40 weeks.