UNSW Embryology

Embryology for K12 Students

© Dr Mark Hill (2008)

Acknowledgements

Week 9 to 12

After week 8 the embryonic period of human development ends and the fetal period begins and continues through to birth (weeks 9 to 38). 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.

Fetal Length Increase

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.

Facts

• 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)

Fetal Weight and Length

Fetal Weight Increase

How heavy is this to carry around all the time?

Fetal Length Increase

How does something this big fit inside your mother's tummy?

Ultrasound

Movie of 12 week fetus ultrasound showing the head, body and spine.

Quicktime movie 800 Kb

Face

During this period the face of the fetus grows and features on the face are begin to be placed in their correct positions. Initially many features are on the side of the head rather than at the front.

Detailed Notes

More detailed notes can be found in the section Fetus Notes

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.
• organ- this is a name given to the different parts of our body. It saves you writing a long list: brain, spinal cord, heart, liver, eye, ear, nose.........
• placenta- all mammals provide 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- 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.
• 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 childbirth.

Quick Links

Week 1 | Week 2 | Week 3 | Week 4 | Week 5 to 8 | Week 9 to 12 | Week 13 to 15 | Week 16 to 20 |

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Genes/DNA Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Urogenital Musculoskeletal Integumentary

Movies ----------------- Embryo Growth Ultrasound Week 1 Week 2 Week 3 Week 4 Stage 13/14 Stage 22 Neural Head and Neck Heart Vasculature Gastrointestinal Urogenital Other Embryos Larsen Internet

Other Embryos ----------------- Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G