Human Development - Weeks 9 - 40

At week nine, the embryo is called a fetus. Tthe transition is gradual, as by the end of the eighth week organogenesis is mostly complete. As such, the fetal period is characterized by rapid body growth.

 

9 to 12 Weeks

Primary ossification centres appear in the skeleton

External genetalia develop and become distinguishable at the end of the twelfth week.

Blood cell formation decreases in the liver and increases in the spleen.

Urine formation begins between the 9th and 12th weeks.

 

13 to 16 Weeks

Growth is very rapid.

Ossification of skeleton progresses

 

17 to 20 Weeks

Fetal movements, known as quickening, are first felt by mother

Ovaries and testes begin to descend

 

21 to 25 Weeks

Substantial weight gain occurs

Lungs begin to secrete surfactant

Fetus becomes viable

 

26 to 29 Weeks

Lungs are capable of breathing air

CNS can direct rhythmic breathing movements and body temperature

Eyelids open

Erythropoiesis moves from the spleen to the bone marrow

 

30 to 34 Weeks

I quit.

 

Factors Influencing Fetal Growth

The fetus requires glucose and amino acids. The fetal pancreas secretes insulin.

Intrauterine growth retardation (IUGR) can result from many factors, including cigarettes, alcohol, poor maternal nutrition, multiple pregnancy, social drugs, impaired uterine blood flow, and genetic factors.