At 5 weeks of pregnancy, your baby is a tiny curved structure about the size of a sesame seed, roughly 2 millimeters long. It doesn’t look like a baby yet. The embryo has a distinct C-shape, a tail-like extension, and is just beginning to form the earliest foundations of a brain, heart, and limbs.
The Embryo’s Shape and Size
The 5-week embryo is shaped like a comma or the letter C, curled tightly with a visible head end and a tapered tail. That tail is a real structure made up of a secondary neural tube, notochord, and developing tissue segments called somites. It’s not a leftover from evolution in any simple sense; it plays a role in spinal development and will gradually regress over the next two weeks.
At this stage, the embryo has three layers of cells that are actively specializing. The outer layer is building the foundation for the brain, spinal cord, and skin. The middle layer is forming the earliest blood vessels, muscles, and bones. The inner layer will eventually become the digestive and respiratory systems. None of these organs are functional yet, but the blueprint is being laid down rapidly.
The Heart Begins to Pulse
One of the most significant milestones at 5 weeks is the start of cardiac activity. A small cluster of heart cells begins to contract rhythmically, even though the heart itself hasn’t fully formed into four chambers. This pulsing cardiac tissue can sometimes be detected on a transvaginal ultrasound, though it’s more reliably seen closer to 6 weeks. The rate and rhythm are irregular at this point, which is completely normal for tissue that is still organizing itself into a functioning organ.
Early Limbs, Eyes, and Ears
By the end of week 5, tiny bumps called limb buds appear on both sides of the embryo’s body. Upper limb buds show up first, with lower ones following shortly after. These don’t look like arms or legs; they’re small, paddle-shaped swellings that will elongate and differentiate over the coming weeks.
The head region is the most developed part of the embryo. Small thickenings on the surface mark the spots where the eyes and ears will eventually form. The lens placode (the precursor to the eye’s lens) and the otic placode (the precursor to the inner ear) are both present. By days 35 to 37, the very first traces of retinal pigment appear, giving the earliest hint of future eye color, though functional vision is still months away. The inner ear structure that will eventually spiral into the cochlea is already beginning to elongate.
What You See on Ultrasound
If you have an ultrasound at 5 weeks, don’t expect to see anything that looks like a baby. What appears on screen is a small, dark, fluid-filled circle called the gestational sac, which becomes visible around weeks 4 to 5. Inside or near it, you may see a smaller bright ring called the yolk sac, which provides nutrients to the embryo before the placenta takes over. Seeing a yolk sac within the gestational sac is the earliest definitive confirmation of a pregnancy located inside the uterus.
The embryo itself is often too small to distinguish clearly at this point. A transvaginal ultrasound (an internal scan) offers much better resolution than an abdominal one at this early stage. If cardiac activity is detected, it will appear as a tiny flicker on the screen. Many providers prefer to wait until closer to 6 or 7 weeks for a first scan, simply because there’s more to see and measure by then. A scan at 5 weeks that shows only a gestational sac doesn’t necessarily mean something is wrong; it may just be too early.
What’s Happening in Your Body
At 5 weeks, your body is producing rising levels of a hormone called hCG, which is what pregnancy tests detect. Blood levels at this stage typically range from 200 to 7,000 units per liter, a wide spread that reflects normal variation between pregnancies. The number itself matters less than the trend: in a healthy early pregnancy, hCG roughly doubles every 48 to 72 hours.
Physically, you may already be experiencing some of the classic first-trimester symptoms. Extreme tiredness is one of the most common. Other signs include sore or swollen breasts, nausea (which can strike at any time of day, not just mornings), a metallic taste in your mouth, bloating, and needing to urinate more often. Some women notice a heightened sense of smell, sudden food aversions, or mood swings. You might also see light spotting, which can be implantation bleeding from the embryo embedding into the uterine wall. Skin changes like darkened patches on the face can appear this early as well, driven by the same hormonal shifts.
Not everyone experiences all of these symptoms, and some women feel almost nothing at 5 weeks. Symptom intensity doesn’t reliably predict how the pregnancy is progressing.