During gestation, the fetus undergoes numerous physiological changes. Nutrients and oxygen must be consistently supplied to the fetus to support organ development. In the fetus, gas exchange occurs in the placenta, not in the lungs. In the fetal circulation, blood and nutrients from the placenta are delivered through the umbilical vein and deoxygenated blood is removed through the umbilical arteries and directed back into the maternal circulation. The oxygenated blood in the umbilical vein flows to the right atrium and is shunted to the left atrium through an interatrial opening (foramen ovale), bypassing the non-functional lung. In the newborn infant, closure of the right to left shunt occurs after pulmonary respiration begins and the umbilical cord is removed, enabling the neonate to cope with the extrauterine environment.
The fetal period begins in week 9 of gestation and continues until birth. After the rudimentary structure of the organs is established in 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 occur 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 created during the embryonic period grow and differentiate during the fetal period (week 9 of development until birth). This period is initially characterized by a dramatic increase in fetal size and, from the sixth month onwards, a sharp 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.
|Weeks of development||Characteristics|
|Weeks 9–12|| |
|Weeks 13–16|| |
|Weeks 26–29|| |
|Weeks 35–38|| |
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.
Placenta (highest O2 saturation) → umbilical vein → inferior vena cava → right atrium
- 50%: umbilical vein → liver → inferior vena cava → right atrium
- First bypass pathway: 50%: umbilical vein → ductus venosus (bypassing the liver) → inferior vena cava → right atrium
Second bypass pathway: The non-ventilated lungs are bypassed and oxygenated blood enters the systemic circulation directly.
- Right atrium → foramen ovale (heart) → left atrium → left ventricle → ascending aorta →vessels of the head, neck, and arm
- Vessels of the head, neck, and arm → superior vena cava → right atrium
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.
- Right atrium → right ventricle → pulmonary trunk → ductus arteriosus → aortic arch and descending aorta → common iliac artery
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.
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 learning cards on the individual organs.
|Organ||Overview of functional development|
|Endocrine system|| |
|Blood and immune system|| |
|Gastrointestinal tract|| |
In the event of complications during pregnancy with the risk of premature birth, lung maturity of the newborn can be induced to avoid or reduce postnatal regulation disorders. In such cases, the mother receives two intramuscular injections of glucocorticoids within 24 hours. As a result, the maturation of pneumocytes, which produce surfactant, is accelerated. This procedure is indicated in preterm infants whose lungs are not yet fully mature (weeks 33–34).References:
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 their first breath of air. The neonate must now regulate their own circulation, respiration, metabolism, and temperature, which require a series of adaptations. These processes progress smoothly in ∼ 95% of newborns, while the remaining ∼ 5% have regulation disorders. For this reason, vital signs should be monitored and assessed with the Apgar score. The newborn's glucose levels and body temperature should be monitored as well.
For signs of maturity in the newborn infant, see .
Circulatory and respiratory adaptation
- 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
- Results in:
- Closure of the ductus arteriosus: : ↑ pO2 in the pulmonary arteries; (due to respiration) and ↓ prostaglandins; (due to placental separation), which has a vasodilatory effect → ↓ pulmonary vascular resistance and ↑ pulmonary blood flow → reverse flow in the ductus arteriosus → functional closure of the duct through smooth muscle contraction within a few days → intimal proliferation and obliteration to become the ligamentum arteriosum
- Closure of the foramen ovale: ↑ pulmonary blood flow → ↑ pressure in the pulmonary veins and left atrium → simultaneous ↓ resistance in the pulmonary arteries → ↓ pressure in the right atrium → functional closure of the foramen ovale → adhesion of both septa → The fossa ovalis remains as a remnant of the septum primum.
- Closure of the umbilical arteries: contraction of the umbilical arteries → prevents blood loss → obliteration within 2–3 months
- Closure of the umbilical vein: : initially remains open after birth → later closes to form the round ligament of the liver
- Closure of the ductus venosus: : obliterates after the ductus arteriosus and becomes the ligamentum venosum (extends from the porta hepatis to the inferior vena cava).
Other postnatal structures that derive from fetal structures include the urachus, which becomes the mediam umbilical ligament, and the notochord, the center of development of the axial skeleton, which becomes the nucleus pulposus of intervertebral discs in adults.
- Changes: : Once the placenta can no longer ensure a continuous supply of glucose, blood sugar levels fall drastically in the newborn → risk of hypoglycemia
- Adaptation: Infants born at term meet their energy 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.
- Lipolysis in brown adipose tissue produces heat.
- Peripheral vasoconstriction induced by a catecholamine surge reduces peripheral heat loss.
Newborn infants with regulation disorders especially depend on exogenous protection against cooling!References: