A baby in the womb develops across three major prenatal stages: the germinal stage (weeks 1–2), the embryonic stage (weeks 3–8), and the fetal stage (weeks 9–40). These stages track the transformation from a single fertilized cell into a fully formed infant ready for birth. Pregnancy is also commonly divided into three trimesters, which overlap with these developmental stages but are organized around the 40-week calendar instead.
The Germinal Stage: Weeks 1–2
Development begins the moment a sperm fertilizes an egg, creating a single cell called a zygote. Over the next five days, that cell divides rapidly until it becomes a hollow ball of about 100 cells known as a blastocyst. This tiny cluster already has two distinct groups of cells with different jobs: an inner group that will become the embryo itself, and an outer group that will form the support system, including the placenta, to nourish it.
The germinal stage lasts about 14 days and ends when the blastocyst fully implants into the wall of the uterus. Implantation is a critical step. Many fertilized eggs fail to implant successfully, ending the pregnancy before most people even know they’re pregnant.
The Embryonic Stage: Weeks 3–8
Once implanted, the developing organism is called an embryo, and the next six weeks are some of the most dramatic in all of human development. This is when all major organs and body systems begin to form, a process called organogenesis.
By week 5, the neural tube, which becomes the brain and spinal cord, takes shape. A tiny tube of heart cells also forms and begins pulsing around 110 times per minute by the end of that week. By week 6, small buds appear that will eventually become arms and legs. The face starts to take shape with early structures for the eyes, ears, and jaw. By the end of week 8, virtually every organ system has at least begun forming, even though the embryo is only about an inch long.
Because so much foundational development happens during this window, the embryonic stage is when a developing baby is most vulnerable to disruption from alcohol, certain medications, infections, or environmental toxins.
The Fetal Stage: Weeks 9–40
Starting at week 9, the embryo is reclassified as a fetus. From this point on, the organs and structures that formed during the embryonic stage grow, mature, and become functional. This stage spans the remaining seven-plus months of pregnancy and covers an enormous range of development.
Weeks 9–12 (Late First Trimester)
The fetus grows rapidly, developing fingernails, toenails, and tooth buds beneath the gums. Reproductive organs begin to differentiate, though they’re not yet visible on ultrasound. The fetus starts making small movements, but it’s far too early for you to feel them.
Weeks 13–28 (Second Trimester)
This trimester brings the most noticeable changes for both the baby and the pregnant person. The fetus grows from a few inches to roughly 14 inches long. Bones harden, muscles strengthen, and the skin develops a waxy coating that protects it from the surrounding fluid.
One of the biggest milestones of this period is “quickening,” the moment you first feel the baby move. This typically happens between 16 and 20 weeks. If you’ve been pregnant before, you may notice it closer to 16 weeks. First-time pregnancies more commonly reach 20 weeks before movement is felt.
The lungs also begin early preparation during this trimester. A substance called surfactant, which keeps the air sacs in the lungs from collapsing, starts being produced in small amounts between 20 and 24 weeks. However, the lungs are far from ready. Babies born before 26 weeks often lack enough surfactant to breathe effectively on their own.
Weeks 29–40 (Third Trimester)
The final trimester is largely about growth, fat accumulation, and organ maturation. The fetus gains the body fat it will need to regulate temperature after birth, and the brain undergoes rapid development. Sensory abilities sharpen considerably. Research has shown that babies in the third trimester respond to patterns of light shone into the womb, and studies dating back to the 1980s established that babies can hear and distinguish voice patterns before birth.
Lung development continues through multiple stages, with the final phase beginning around 36 weeks and extending into life after birth. This is one reason babies born even a few weeks early can have breathing difficulties.
Viability: When Survival Outside the Womb Becomes Possible
Viability refers to the point at which a baby could potentially survive outside the womb with medical support. This threshold falls around 22 to 24 weeks, though survival rates at these early ages are sobering. Babies delivered before 23 weeks survive only about 5 to 6 percent of the time. At 23 weeks, survival rises to roughly 23 to 27 percent. By 24 weeks, the range is 42 to 59 percent. Every additional week in the womb significantly improves a baby’s chances and reduces the risk of long-term complications.
These numbers reflect what’s possible in hospitals with specialized neonatal intensive care. Outcomes vary based on birth weight, overall health, and the resources available at the delivering hospital.
How the Three Trimesters Map to Developmental Stages
You’ll often hear pregnancy described in trimesters, which is how most prenatal care is organized. The first trimester (weeks 1–12) encompasses the entire germinal and embryonic stages plus the beginning of the fetal stage. The second trimester runs from week 13 through week 28, and the third trimester covers weeks 29 through 40.
The trimester framework is useful for tracking your own symptoms, scheduling prenatal appointments, and understanding screening timelines. The developmental stages (germinal, embryonic, fetal) are more useful for understanding what’s actually happening inside the womb at any given point. Both frameworks describe the same pregnancy; they just slice it differently.
Key Milestones at a Glance
- Week 1–2: Fertilized egg divides and implants in the uterine wall
- Week 5: Brain and spinal cord structures form; heart cells begin pulsing
- Week 6: Limb buds appear
- Week 8: All major organ systems have begun forming
- Week 9: Embryo becomes a fetus; growth and maturation take over
- Weeks 16–20: First fetal movements felt (quickening)
- Weeks 20–24: Lungs begin producing surfactant in small amounts
- Weeks 22–24: Earliest viability with intensive medical care
- Third trimester: Baby responds to sound and light; gains body fat; lungs and brain mature rapidly
- Week 36+: Final stage of lung development begins
- Week 40: Full-term birth