Prenatal and postnatal physiology

Fetal physiology

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.

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!

The fetal period

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.

For more detailed information on embryonic development (first eight weeks after fertilization), see “Embryogenesis”.

Weeks of development Characteristics
Weeks 9–12
  • Intestinal loops return to the abdominal cavity in week 10.
  • First breathing movements (week 11)
  • The liver is replaced by the spleen as the primary site of hematopoiesis (week 12).
  • Fetal sex can be identified (week 12).
  • Despite a decline in head growth and doubling of the crown-rump length by the end of week 12, the head remains disproportionately large.
Weeks 13–16
  • Start of coordinated movement of the arms and legs (week 14)
  • As a result of active ossification, fetal bones are visible on ultrasound (week 16).
Weeks 17–20
  • Fetal movements become noticeable.
Weeks 21–25
  • Premature infants can theoretically survive from week 22.
  • Surfactant secretion, which keeps the alveoli open, begins (week 20–22).
  • Ductal development of the alveoli begins.
Weeks 26–29
  • Many premature infants survive at this stage with intensive medical care.
    • Lung tissue is sufficiently mature to enable gas exchange.
    • The CNS is mature enough to provide central respiratory drive and regulate body temperature.
  • Hematopoiesis begins in the bone marrow (week 28).
Weeks 30–34
Weeks 35–38
  • The fetus is fully mature at 37 weeks of gestation.
  • Septation of air sacs

Fetal circulation

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.

Oxygenated blood

Deoxygenated blood

The umbilical vein transports oxygenated blood from the placenta towards the fetal heart, whereas the umbilical arteries direct deoxygenated blood from the fetus to the placenta.

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 organ function

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
  • Fetal breathing movement, despite not being involved in gas exchange, is important for pulmonary development.
  • Distal pulmonary epithelial cells produce chloride-rich fluid that distends the airway.
  • Surfactant lipoproteins produced by type II pneumocytes decrease alveolar surface tension.
Blood and immune system
  • Hematopoiesis: in the mesenchyme of the yolk sac wall from week 3 of development; later in the liver and spleen
  • Gas exchange: O2 affinity in fetal hemoglobin (HbF) is especially high to ensure oxygen diffusion from maternal to fetal hemoglobin. This makes the release of oxygen into fetal tissue more difficult.
  • Hemoglobin concentration
  • Immunoglobulins: The fetus does not produce immunoglobulins, maternal IgG crosses the placenta providing passive immunity.

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:[1]

Neonatal physiology

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 assessment of the newborn.

Circulatory and respiratory adaptation

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 newbornrisk 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.
  • Adaptation
    • 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:[1]

  • 1. Morton SU, Brodsky D. Fetal physiology and the transition to extrauterine life. Clin Perinatol. 2016; 43(3): pp. 395–407. doi: 10.1016/j.clp.2016.04.001.
last updated 10/11/2019
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