Chapter 5 The Fetus

Knowledge of fetal development is needed to estimate the approximate age of a baby born before term and appreciate the ways in which developmental abnormalities arise.

Time scale of development

For the first 3 weeks following conception the term zygote is used.

From 3–8 weeks after conception it is known as the embryo.

Following this it is called the fetus until birth. Development within the uterus is summarised in Box 5.1.

Box 5.1 Summary of development

0–4 weeks after conception

• Rapid growth

• Formation of the embryonic plate

• Primitive central nervous system forms

• Heart develops and begins to beat

• Limb buds form

4–8 weeks

• Very rapid cell division

• Head and facial features develop

• All major organs laid down in primitive form

• External genitalia present but sex not distinguishable

• Early movements

• Visible on ultrasound from 6 weeks

8–12 weeks

• Eyelids fuse

• Kidneys begin to function and fetus passes urine from 10 weeks

• Fetal circulation functioning properly

• Sucking and swallowing begin

• Sex apparent

• Moves freely (not felt by mother)

• Some primitive reflexes present

12–16 weeks

• Rapid skeletal development

• Meconium present in gut

• Lanugo appears

• Nasal septum and palate fuse

16–20 weeks

• ‘Quickening’ – mother feels fetal movements

• Fetal heart heard on auscultation

• Vernix caseosa appears

• Fingernails can be seen

• Skin cells begin to be renewed

20–24 weeks

• Most organs become capable of functioning

• Periods of sleep and activity

• Responds to sound

• Skin red and wrinkled

24–28 weeks

• Survival may be expected if born

• Eyelids reopen

• Respiratory movements

28–32 weeks

• Begins to store fat and iron

• Testes descend into scrotum

• Lanugo disappears from face

• Skin becomes paler and less wrinkled

32–36 weeks

• Increased fat makes body more rounded

• Lanugo disappears from body

• Head hair lengthens

• Nails reach tips of fingers

• Ear cartilage soft

• Plantar creases visible

36–40 weeks after conception (38–42 weeks after last menstrual period, LMP)

• Term is reached and birth is due

• Contours rounded

• Skull firm

Fetal organs

Blood

The fetal haemoglobin is termed HbF. It has a much greater affinity for oxygen and is found in greater concentration (18–20 g/dl at term). Towards the end of pregnancy the fetus begins to make adult-type haemoglobin (HbA).

In utero, the red blood cells have a shorter lifespan, this being about 90 days by the time the baby is born.

The renal tract

The kidneys begin to function and the fetus passes dilute urine from 10 weeks’ gestation.

The adrenal glands

The fetal adrenal glands produce the precursors for placental formation of oestriols. They are also thought to play a part in the initiation of labour.

The liver

The fetal liver is comparatively large in size. From the third to the sixth month of intrauterine life, it is responsible for the formation of red blood cells, after which they are mainly produced in the red bone marrow and the spleen.

Towards the end of pregnancy, iron stores are laid down in the liver.

The alimentary tract

The digestive tract is mainly non-functional before birth. Sucking and swallowing of amniotic fluid containing shed skin cells and other debris begin about 12 weeks after conception. Digestive juices are present before birth and they act on the swallowed substances and discarded intestinal cells to form meconium. This is normally retained in the gut until after birth, when it is passed as the first stool of the newborn.

The lungs

It is mainly the immaturity of the lungs that reduces the chance of survival of infants born before 24 weeks’ gestation, owing to the limited alveolar surface area, the immaturity of the capillary system in the lungs and the lack of adequate surfactant. Surfactant is a lipoprotein that reduces the surface tension in the alveoli and assists gaseous exchange. It is first produced from about 20 weeks’ gestation and the amount increases until the lungs are mature at about 30–34 weeks. At term the lungs contain about 100 ml of lung fluid. This is expelled during birth but mostly absorbed and carried away by the lymphatics and blood vessels.

There is some movement of the thorax from the third month of fetal life and more definite diaphragmatic movements from the sixth month.

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Related posts:

1. The Placenta
2. Trauma During Birth, Haemorrhage and Convulsions
3. Obstetric Emergencies
4. Problems of Pregnancy

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