Fetal intrauterine development

Formation of rudiments and the nervous system

Three germ layers differentiate from the germ disc: ectoderm, endoderm, and mesoderm. They serve as the starting material for all embryonic tissues. The establishment of the nervous system is among the first to occur. The lumen of the closed neural tube forms the brain ventricles and the spinal canal, while the development of the cerebellum proceeds in parallel.

Myelination of the central nervous system begins in the second half of pregnancy and spreads in a cranial direction: from the medulla oblongata to the hemispheres. Functional maturation of the CNS is clearly visualized on ultrasound through fetal reflexes:

• 7–8 weeks: first motor reactions appear.
• 16 weeks: movements become coordinated.
• 21 weeks: spontaneous sucking movements emerge.
• 32 week: the «activity–rest» cycle is established (the main indicator of CNS maturation).

Despite morphological maturity by the end of pregnancy, the final functional maturity of the nervous system is achieved only after birth.

Features of fetal circulation

Fetal circulation sequentially goes through three stages: yolk sac (up to the 2nd week), allantoic (up to the 15th–16th week), and placental (from the 2nd trimester). Gas exchange takes place in the intervillous space of the terminal villi of the placenta.

A single umbilical vein (v. umbilicalis) runs from the placenta to the fetus, carrying arterial blood 80% saturated with oxygen. A major feature of fetal hemodynamics is the presence of three physiological shunts that direct oxygenated blood to bypass the non-functioning lungs and liver:

1. Venous (Arantius) duct (ductus venosus) — shunts about 50% of arterial blood from the umbilical vein directly into the inferior vena cava.
2. Foramen ovale (foramen ovale) — reroutes blood from the right atrium directly to the left, bypassing the pulmonary circulation.
3. Arterial (Botallo) duct (ductus arteriosus) — shunts venous blood from the pulmonary trunk into the descending aorta.

As a result, all fetal organs receive mixed blood. However, the head, brain, and upper half of the torso are supplied with blood better than the lower body and lungs. The return of venous blood to the placenta for purification is carried out via two umbilical arteries (aa. umbilicales).

Endocrine and respiratory systems

The fetal endocrine system operates autonomously. For example, maternal thyroid hormones do not cross the placenta. The synthesis of its own thyroid hormones, which are critically important for neuronal development and skeletal ossification, begins at the 12th week. Fetal insulin (produced starting at the 3–4th week) acts as a growth hormone, while maternal insulin does not cross the placental barrier due to its high molecular weight. Adrenocorticotropic hormone stimulates the production of cortisol, which is necessary for the maturation of lung tissue.

The respiratory system is laid down at the 4th week. In the 3rd trimester, the alveolar epithelium differentiates: type II cells begin to produce surfactant—a lipoprotein that facilitates the expansion of the lungs at birth. Intrauterine respiratory movements (30–70 per minute) are performed with a closed glottis, which prevents the aspiration of amniotic fluid but promotes blood flow to the heart.

Excretory and immune systems

The kidneys develop from the metanephros. From the 14th week, the loop of Henle becomes functionally developed and capable of glomerular filtration. The fetus actively produces urine (the rate increases from 10 ml/h at 30 weeks to 650 ml/day by the end of pregnancy). Urine is excreted into the amniotic fluid and then enters the maternal circulation.

Gonadal development starts at the 5th week, and at the 6th–9th week (gonadal sex stage), the indifferent gonad transforms into ovaries or testes.

The immune system forms in stages: from the 5th week, lymphoid cells appear in the liver; from the 8th–9th week, the thymus produces T lymphocytes; and from the 10th–12th week, the synthesis of its own immunoglobulins begins. At the same time, the phagocytic activity of leukocytes remains low throughout the entire intrauterine period.

Class and exam questions

What else to study on this topic

• Chronology of the synthesis of fetal pituitary hormones (GH, FSH, LH, prolactin) and their clinical significance.
• Placental growth dynamics and the anatomical structure of its mature structures.
• Changes in newborn hemodynamics in the first minutes after birth (closure of shunts and initiation of pulmonary circulation).
• Shift of the oxyhemoglobin dissociation curve during the intrauterine development period.

Other topics in the "Obstetrics" section

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