What Does a 5 Week Fetus Look Like?

Being pregnant is a time of wonder. Both parents can become curious to what their developing baby looks like inside the womb. Luckily, the development of a fetus follows a relatively predictable course of growth 1. A 5-week-old fetus is developing rapidly, although not as quickly as in earlier weeks. Its appearance resembles that of a tadpole, but human features are beginning to form.

Three Layers

If you were able to look at a 5-week-old fetus, you would see it composed of three layers of cells. MayoClinic.com advises that the top layer, or ectoderm, is what will become the skin, central nervous system, eyes and inner ear. The middle layer is called the mesoderm and is where the heart, bones, muscles, kidneys and reproductive organs will develop. The innermost layer of cells, the endoderm, is where the intestines, lungs and bladder will form.


According to the American Pregnancy Association, a 5-week-old fetus is very small. It measures only 0.118 inches in length. At this age, a baby’s height is measured from the top of the head to the rear end area. He will be measured this way until he reaches 20 weeks, when measurements will be taken from head to heel. The fetus looks about the size of a sesame seed.

Baby Gaga explains that much of the growth during week 5 is focused on the head. If you were able to peer into the womb, you would see the brain beneath a layer of translucent cells developing rapidly. The brain is already working to regulate the heart and circulation of blood. The neural tube, which will grow to become the spinal cord and brain, is busy developing. A layer of cells lies where the eyes and ears are beginning to form.

Heart and Circulatory System

By 5 weeks old, BabyCenter says a fetus’ heart starts to beat. This is the first organ that begins to work. There are two chambers to the heart at 5 weeks old, each one pumping separately. The heart will then divide into four chambers and begin to function, helping to develop the circulatory system. Blood pumps and travels through the entire body at a regular rhythm, making all additional developments of organs, bones and muscles possible.


Rudimentary forms of the lungs, pancreas, stomach, and liver are beginning to appear at 5 weeks old. In addition, Baby Gaga says that the buds that will become the arms and legs begin to develop. The legs, which grow slower than the hands and arms, resemble a tail.