K12 Fetal Development

From Embryology

Fetal Development

After week 8 the embryonic period of human development ends and the fetal period begins and continues through to birth (weeks 9 to 38). This is also called the second and third trimester (3 months) of development.

From now on we call the baby a fetus.

This time is about continued organ development, but the most obvious feature is the growth in overall size and weight of the fetus.

The fetus size is measured during this period by the distance beween "crown to rump" (top of the head to bottom) this is because the fetus is initialy bent into a tight "C" shape and slowly straightens as it grows in length.


At week 9 half the fetus oveall size is its head, but by 12 weeks the body size has doubled. In the 12 week ultrasound below you can see the fetus has also begun to make body movements.


Another measure to guess fetal age of increase in size are the long bones (in the limbs) and head diameter. These are usually measured by special sound waves (ultrasound) through the wall of the mothers tummy.

  • Fetus begins making urine during this period
  • Male and female fetus begin by looking the same but slowly over time look different on the outside
  • Limbs may move around during this period (kicking or elbows can be felt)


K12 Links: Start Here | Week 1 | Week 2 | Week 3 | Week 4 | Week 5 to 8 | Arms and Legs | Heart | Fetus | Brain Growth | Eyes and Ears | Animal Development Times | Humans and Animal Embryology | Comparative Embryology | Thalidomide

Words Used

brain- the thing inside your head that allows you to think, talk, see, hear and move. Even though it appears in the embryo very early, it is not fully made until many years after you are born. It is protected by the skull (not yet there).

bone - the skeleton of the body. This gives your body a shape so that you are not a blob lying on the ground.

embryo - the name given to the baby during the early time of growth.

heart - the thing in your chest that pumps blood around your body.

limb buds - these are the early arms and legs of the embryo. At this time they look like paddles either side ou the embryo.

liver - this is below the heart and has an important role in blood and later for digesting food. It is an organ that works as part of the digestive system (stomach....) and part of the blood system (heart, blood vessels...).

muscle - part of the body that moves the skeleton and allows you to stand up. There are two other types of muscle we do not control, the heart and muscle in our digestive system.

organ - this is a name given to the different parts of our body doing different jobs. It saves you writing a long list: brain, spinal cord, heart, liver, eye, ear, nose.........

placenta - the thing that provides support for the embryo growth through the placenta. It connects the embryo with the mother. It carries food and oxygen to the embryo and takes waste and carbon dioxide from the embryo.

special sensory - name given to parts of the eye, ear and nose.

spinal cord - part of the brain like a long rope running down your back. The spinal cord does what the brain tells it and allows you to move your muscles. The spinal cord is also the way you can to feel things with your skin (touch, hot, cold). It acts like a telephone cable passing messages from the brain to the body and from the body to the brain. If the spinal cord is damaged you cannot move your body.

trimester - dividing a time into three parts. Because humans take 9 months to develop each trimester (first, second, third) is 3 months.

uterus- the space within the mother where the embryo grows. It also grows when the embryo does and has lots of blood to feed the embryo and strong muscles in its walls for birth.

Teachers Note

Lengths and weights are metric (cm and grams).

Try and get a ruler and show how over time the size of the baby increases, draw lines the length of the baby at different times.

You can also do this with the fetal weights, either get items that correspond to the graph weights at different times or a small grocery tin and keep adding to show the weight increase.


Cite this page: Hill, M.A. (2019, September 18) Embryology K12 Fetal Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/K12_Fetal_Development

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G