During the first trimester, a baby transforms from a barely visible speck the size of a poppy seed into a recognizably human form about the length of a lime. At four weeks, it’s roughly 2 millimeters long. By week 12, it measures 2 to 2.5 inches from head to rump, with a formed face, tiny fingers and toes, and the ability to move on its own. The changes week to week are dramatic, and most of them happen faster than you might expect.
Weeks 4 and 5: Smaller Than a Sesame Seed
At four weeks, the fertilized egg has burrowed into the uterine lining as a ball of cells called a blastocyst. It’s about 2 millimeters long, roughly the size of a poppy seed, and invisible to the naked eye in any meaningful way. There’s nothing that looks like a baby yet. A fluid-filled sac has already begun forming around it for cushioning, and the earliest version of the placenta is taking shape. By week five, the cluster has grown to about the size of a sesame seed, and the cells are organizing into layers that will become different organ systems.
Weeks 6 and 7: A Heartbeat and a Tail
Around the sixth gestational week, the embryo’s heart begins its first pumping action. These early contractions start as small, irregular movements within developing heart cells and gradually become coordinated enough to push fluid through the tiny body. On an ultrasound at this stage, many people see this flickering motion for the first time. The embryo is now about the size of a single pea.
One of the more surprising features at this stage is a small tail. Between weeks four and five, the embryo develops 10 to 12 tail vertebrae, and the tail measures about one-sixth of the embryo’s total size. It’s a normal part of development and will be gone within a few weeks. The structures needed to form eyes and ears are also taking shape now, though they don’t yet look like anything recognizable. By week seven, small depressions appear where the nostrils will be. The embryo is roughly blueberry-sized.
Week 8: Limbs Take Shape, the Tail Disappears
Week eight marks a turning point in appearance. The tail vertebrae undergo cell death and are reabsorbed, so the embryo starts to lose its curved, tadpole-like profile. Small swellings appear where the outer ears will eventually sit. Limb formation, which began around week four, is now well underway. Arm and leg buds are visible and starting to lengthen. The fingers and toes are still webbed at this point, fused together in paddle-like shapes. Between the sixth and seventh weeks after conception, those digits begin separating into individual fingers and toes, a process that continues through this period. The embryo is about the size of a raspberry.
Weeks 9 and 10: A Recognizable Face Emerges
By week nine, eyelids have formed, and the face is starting to look distinctly human. The embryo is now officially called a fetus. It’s about the size of a grape, and spontaneous movements are already happening. Ultrasound can pick up these first movements as early as week seven, though they’re far too small for you to feel. You won’t notice any movement until well into the second trimester.
At week ten, the eyelids and outer ears continue developing. The fingers and toes have fully separated. All the long bones in the limbs are beginning to harden as primary bone-forming centers appear, a process that completes around week 12. The fetus is roughly the size of a kumquat.
Weeks 11 and 12: Fully Formed in Miniature
By week 11, the eyelids have fused shut, and they’ll stay that way until well into the second trimester. The ears are set low on the sides of the head and will gradually migrate upward. The fetus is about the size of a fig and has all its major organs and structures in place, though they’ll continue maturing for months.
At week 12, the fetus measures 2 to 2.5 inches from crown to rump. Its legs are curled up tightly against the body, so length is measured only from head to bottom. The face has nostrils, a formed upper lip, and the beginning of a chin. Fingernails are starting to develop. The body is still proportionally unusual by newborn standards, with an oversized head making up roughly a third of the total length, but the profile is unmistakably human.
What You Can’t See Yet
Despite all this development, some features remain impossible to determine during the first trimester. External genitalia begin forming around weeks eight and nine, but at this stage they look identical regardless of sex. The structures that will become a penis or clitoris are in an “indifferent stage,” meaning they haven’t yet differentiated. Full formation of external genitalia doesn’t happen until 17 to 18 weeks, which is why sex is typically identified on ultrasound during the second trimester anatomy scan, not before.
The fetus is also moving regularly by the end of the first trimester, flexing its newly formed limbs and shifting position. These movements coincide with the development of connections between nerves and muscles throughout the body. But the fetus is still so small that you won’t feel any of it. Most people first notice fetal movement, called quickening, toward the end of the second trimester.
Size at a Glance
- Week 4: Poppy seed (2 mm)
- Week 5: Sesame seed
- Week 6: Pea
- Week 7: Blueberry
- Week 8: Raspberry
- Week 9: Grape
- Week 10: Kumquat
- Week 11: Fig
- Week 12: Lime (2 to 2.5 inches)
The first trimester covers the most rapid transformation of the entire pregnancy. In just 12 weeks, a single-celled egg becomes a moving, recognizably human form with a beating heart, individual fingers, and a face. Nearly every major organ system has at least begun to form, even though the fetus is still small enough to fit in the palm of your hand.