Embryo and fetus

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Chapter 4 Embryo and fetus

CHAPTER CONTENTS

Nutrition 27

Cardiovascular system 28

Gastrointestinal tract 30

Renal system 30

Immune system 30

Muscular system 30

Endocrine activity 30

Once implantation has occurred the embryo develops rapidly. A neural primitive streak develops in the second week after fertilization; during the third week the fetal heart develops and links up with the primitive vascular system; during the fourth week the gut has formed; and by the sixth week the urogenital sinus has formed.

By the seventh week after fertilization most of the organs have formed and the embryo becomes a fetus. The early growth of the fetus is shown in Table 4.1.

Table 4.1 Characteristics indicating maturity of the fetus

NUTRITION

Fetal growth is determined by many factors, both genetic and environmental. Of the latter, adequate placental perfusion and placental function are crucial. Maternal nutrition is not a limiting factor except in cases of extreme starvation, although chronic undernutrition may be associated with anaemia and may lead to a low-birthweight baby.

The fetus, insulated in its protective amniotic sac and relatively weightless, directs most of the energy supplied to it to growth. The energy is derived mainly from glucose. Only small amounts of lipids, as free fatty acids, cross the placenta until the fourth quarter of pregnancy. Any excess carbohydrate, after the growth and metabolic energy needs of the fetus have been met, is converted into lipids, and this conversion increases as term approaches.

From the 30th gestational week the fetal liver becomes increasingly efficient and converts glucose into glycogen, which is stored in the fetal heart muscle, the skeletal muscle and the placenta. Should fetal hypoxia occur the fetus is able to obtain energy from the heart muscle and placenta for anaerobic glycolysis (see Ch. 20).

Free fatty acids are formed and stored in brown and white adipose tissue. Brown fat is deposited around the fetal neck and behind the scapulae and the sternum and around the kidneys. It is metabolized to provide energy to maintain the infant’s body temperature after birth. White adipose tissue forms the subcutaneous cover of the body of a term fetus, but in preterm babies the layer may be thin. It acts as an insulator and as a lipid store.

The fat stores of an 800 g fetus (24–26 weeks’ gestation) constitute 1% of its body weight; by the 35th week fat constitutes 15% of fetal body weight.

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