Chapter 29. The Fetal Period: Weeks 21 to Term - Review of Medical Embryology Book

There is much weight gain; the body is proportioned; the skin is wrinkled but is very translucent and is pink to red in color, with blood in the capillaries visible
If born at this time, the fetus usually dies due to an immature respiratory system

The fetal eyes reopen, and the head and lanugo hair are well developed
There is much subcutaneous fat formed, and the wrinkles of skin smooth out
White fat increases to 5%
The fetus is viable and can survive if born prematurely, but the mortality is high as a result of respiratory problems, even though the lungs and pulmonary vasculature are developed for gas exchange
The CNS is mature and can control breathing and body temperature

Usually takes place about 266 days or 38 weeks after fertilization, or 280 days (about 40 weeks) from the onset of the last menstrual perio Variations are due to irregular menstrual periods and difficulty of counting and may be from 2 to 3 weeks
Most fetuses are born within 10 to 15 days of the above times but may go to 276 to 286 days after fertilization
Postmature infants are thin and have dry, parchmentlike skin
For calculations of birth date: in the typical 28-day menstrual cycle, you count back 3 calendar months from the first day of the last menstrual period and then add 1 year and 1 week

Amino acids and glucose (primary source of energy): derived from mother via placenta
Insulin: for glucose metabolism and stimulation of fetal growth. Secreted by the fetal pancreas; little of the mother's insulin reaches the fetus normally

Maternal malnutrition can cause reduced fetal growth
Smoking can reduce fetal growth rate if in excess
Multiple pregnancies: each fetus tends to be smaller than normal single births
Impaired uteroplacental blood flow can cause fetal starvation and even fetal growth may decrease
Placental dysfunction or defects may result in a growth decrease
Genetic factors and chromosomal aberrations may lead to retarded fetal growth