Fetal and neonatal physiology differ. Prenatally, nutrient and gas exchange occur via the fetoplacental unit. Blood flows from the placenta to the fetus through the umbilical vein, while deoxygenated blood is removed through the umbilical arteries and directed back into the maternal circulation. The fetal circulation has three bypass pathways. Oxygenated blood bypasses both the liver (through the ) and the lungs (via the ). The allows the deoxygenated blood to bypass the lungs by connecting the aorta and . Postnatally, the and fetal organs must adapt to the new environment. All three bypass pathways, as well as the and the , obliterate. The obliterated vessels form , while the foramen ovale forms the . Interruption of placental blood supply at birth (i.e., cutting of the ) initiates changes in metabolism as well as . Neonatal temperature regulation occurs via lipolysis of brown adipose tissue and peripheral vasoconstriction.
The fetal period begins at week 9 of gestation and continues until birth. After the rudimentary structure of the organs is established during the embryonic period (weeks 1–8), the organs begin to grow and differentiate. In many cases, the fine structure and function of individual organs develop slowly. Some organs do not develop completely until after birth. Fetal circulation and fetal organ function differ considerably from that of a child or adult. Intrauterine conditions ensure that nutrient and gas exchange occurs via the fetoplacental unit. The metabolism and hormones are also provided by the mother, especially at the beginning of development.
Fetal length can be determined on ultrasound starting from week 9 of development by measuring the crown-rump length (CRL). The CRL in centimeters is roughly equal to the square of the month of gestation.
All of the tissues and organs that develop during the embryonic period grow and differentiate during the fetal period (week 9 of development until birth). This period is initially characterized by an increase in fetal size and, from the sixth month onwards, an increase in fetal weight. Fetal body parts do not all grow at the same rate. Head growth, in particular, lags behind the growth of the rest of the body. For more detailed information on embryonic development (first eight weeks after fertilization), see “Embryogenesis”.
|Overview of the fetal period|
|Weeks of development||Characteristics|
|Weeks 9–12|| |
|Weeks 26–29|| |
Fetal circulation must meet the needs of the fetus with the maternal placental supply, as it cannot rely on pulmonary respiration. It must also adapt rapidly to postnatal conditions. While the heart begins contracting in a coordinated manner at the end of week 4, resulting in directed blood flow, development of the fetal circulation extends up to week 9. In fetal circulation, delivery of oxygenated blood and clearance of deoxygenated blood follows the route described below.
- Direction: placenta (highest O2 saturation) → umbilical vein (O2 saturation ∼ 80%; PO2 ∼ 30 mmHg) → liver → inferior vena cava → right atrium → → left atrium → left ventricle → ascending aorta → vessels of the head, neck, and arm
- First bypass pathway: 50% of the oxygenated blood bypasses the liver and flows from the umbilical vein to the inferior vena cava via ductus venosus.
- Second bypass pathway: Oxygenated blood bypasses the nonventilated lungs and enters the systemic circulation directly (right atrium → → left atrium).
- Direction: vessels of the head, neck, and arm → superior vena cava → right atrium→ right ventricle → pulmonary trunk → ductus arteriosus → aortic arch and descending aorta → common iliac artery
- Third bypass pathway: The ductus arteriosus connects the pulmonary trunk with the aorta and conducts most of the blood directly from the right ventricle to the aorta, bypassing the lungs because of the increased pulmonary artery resistance.
Ductus arteriosus Directs Deoxygenated blood to the Descending aorta.
Because of high resistance in the pulmonary trunk, pressure on the right side of the circulation is on average higher than that on the left side.
Postnatal adaptation of the circulatory system
|Postnatal adaptation of fetal circulation|
|Closure of the ductus arteriosus|| |
|Closure of the foramen ovale|
|Closure of the umbilical arteries|| |
|Closure of the umbilical vein|| |
|Closure of the ductus venosus|| |
Fetal circulation and organ function differ considerably from that of a child or adult. Nutrient and gas exchange takes place in the fetoplacental unit. The lungs are not ventilated and are poorly perfused. Other organ functions also develop gradually during the course of prenatal development, some even after birth. The table below provides an overview of the differences between fetal and postnatal organ function. Organ development is not discussed, but can be found in the articles on the individual organs.
|System/Organ||Overview of fetal functional development |
|Endocrine system|| |
|Blood and immune system|| |
After birth, the bypass pathways of the fetal circulation close to accommodate pulmonary respiration and the cutting of the umbilical cord. After placental circulation is interrupted at birth, the newborn takes its first breath of air. The neonate must now regulate their own circulation (see “ ” above), respiration, metabolism, and temperature, which require a series of adaptations. For more information on neonatal care, see “ ”.
Respiratory adaptation of the newborn infant
- Initiation: start of pulmonary respiration (after cutting the umbilical cord)
- Lung ventilation: first breaths → alveoli are filled with air → lungs inflate → change in pressure conditions
- Changes: Once the placenta can no longer ensure a continuous supply of glucose, blood sugar levels decrease in the newborn.
- Adaptation: Infants born at term meet their glucose requirements through gluconeogenesis and breastfeeding (or formula).
Normal body temperature of the newborn infant: 36.5–37.5 °C (97.7–99.5°F)
- Changes: Large body surface area to weight ratio and loss of intrauterine warmth contribute to heat loss.