At 5 weeks pregnant, the embryo is roughly the size of a sesame seed, measuring about 2 millimeters long. It doesn’t look like a baby yet. What you’d see, if you could look closely enough, is a tiny curved structure with a distinct head end and tail end, surrounded by a fluid-filled sac inside your uterus.
What’s Happening Inside the Embryo
This week marks a major shift in how the embryo is organized. It transforms from a flat, two-layered disc into three distinct cell layers, each of which will become different parts of your baby’s body. The outer layer will form the skin and nervous system. The middle layer will become the cardiovascular system, bones, and muscles. The inner layer will develop into the digestive and respiratory tracts.
The neural tube, which eventually becomes the brain and spinal cord, has already closed by this point. That closure happened during weeks three and four, which is one reason folic acid in early pregnancy is so important. By week 5, the front end of the neural tube is beginning to develop into three primitive sections that will later become distinct parts of the brain.
The Earliest Heart Activity
A primitive heart tube has formed and is beginning to develop, but it doesn’t look or function like a heart you’d recognize. It’s essentially a tube of cells that will eventually fold into four chambers. At 5 weeks, it’s often too early to detect any cardiac activity on ultrasound. Most providers recommend waiting until six or seven weeks, when detection is more reliable.
Tiny Buds Where Arms and Legs Will Be
Around day 32, which falls near the end of week 5, small bumps called limb buds start to appear on the embryo. These are the earliest beginnings of arms and legs, though at this stage they look like tiny paddles rather than anything resembling limbs. The upper limb buds typically appear slightly before the lower ones.
Early structures for the eyes and ears are also taking shape. The tissue that will become the lens of the eye has started to thicken, and the inner ear structure has begun to form. None of these features are visible to the naked eye, but the groundwork is being laid at a cellular level.
What You’d See on an Ultrasound
If you have a transvaginal ultrasound at 5 weeks, don’t expect to see anything that looks like a baby. What typically shows up is a small, dark, circular area inside the uterus: the gestational sac. Inside that sac, your provider may be able to spot a yolk sac, which is a small round structure that provides nutrients to the embryo before the placenta takes over.
An early form of the embryo, sometimes called a fetal pole, may appear as a tiny thickening next to the yolk sac. On screen, it looks like a small white line or dot. At this stage, it’s common for the embryo to be barely visible or not visible at all, which doesn’t necessarily mean anything is wrong. The timing of ovulation and implantation can shift things by several days, so what’s visible at exactly 5 weeks varies quite a bit from one pregnancy to another.
If you’re pregnant with twins, a 5-week scan can sometimes reveal two separate gestational sacs, which would indicate a dichorionic twin pregnancy. However, many twin pregnancies aren’t clearly identifiable this early. Determining the number of sacs, yolk sacs, and embryos becomes more accurate in the weeks that follow.
How Growth Is Fueled
The hormone hCG, which is what pregnancy tests detect, is rising rapidly this week. At 5 weeks, levels typically range from 200 to 7,000 µ/L, a wide span that reflects how quickly this hormone doubles every couple of days. That rapid rise is what drives many early pregnancy symptoms like nausea and fatigue, and it supports the environment the embryo needs to keep developing.
The yolk sac is doing most of the heavy lifting for nutrition right now. It delivers essential nutrients and even produces early blood cells for the embryo. Over the coming weeks, the placenta will gradually take over this role, but at 5 weeks the yolk sac is the embryo’s lifeline.
Putting the Size in Perspective
It helps to remember just how small everything is at this point. The gestational sac is roughly the size of a blueberry, and the embryo inside it is barely visible without magnification. The entire structure, sac and all, would fit on the tip of your finger. Despite that tiny size, the pace of development this week is extraordinary. Cells are dividing, migrating, and specializing into the tissues that will form every organ system in your baby’s body. By next week, the embryo will have roughly doubled in size and cardiac activity will likely be detectable for the first time.