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Prenatal and postnatal physiology

Last updated: February 2, 2021

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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 ductus venosus) and the lungs (via the foramen ovale of the fetal heart). The ductus arteriosus allows the deoxygenated blood to bypass the lungs by connecting the aorta and pulmonary trunk. Postnatally, the circulatory system and fetal organs must adapt to the new environment. All three bypass pathways, as well as the umbilical arteries and the umbilical vein, obliterate. The obliterated vessels form ligaments, while the foramen ovale forms the fossa ovalis. Interruption of placental blood supply at birth (i.e., cutting of the umbilical cord) initiates changes in metabolism as well as respiratory adaptation of the neonate. 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
  • Intestinal loops return to the abdominal cavity (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).
  • Start of urine production, which is excreted into the amniotic fluid
  • Fetal waste products are delivered to the maternal circulation via the placenta.
  • Despite a decline in head growth and doubling of the crown-rump length by the end of week 12, the head remains disproportionately large.
  • Limbs have almost reached their final proportions.
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).
  • Rapid growth
  • Head and body proportions adapt to one another.
Weeks 17–20
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.
  • Fingernails start to form.
  • Start of fat deposition
  • Pronounced weight gain
Weeks 26–29
Weeks 30–34
  • The fetus responds to light.
  • As a rule, infants survive if born at this stage of pregnancy.
Weeks 35–38
  • The fetus is fully mature at 37 weeks of gestation (term infant).
  • Septation of air sacs

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

Ductus arteriosus Directs Deoxygentaed blood to the Descending aorta.

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.

Postnatal adaptation of the circulatory system

Postnatal adaptation of fetal circulation
Process Physiology Postnatal remnant
Closure of the ductus arteriosus
Closure of the foramen ovale
Closure of the umbilical arteries
  • Contraction of the umbilical arteries to prevent blood loss
  • Obliteration within 2–3 months
Closure of the umbilical vein
  • Initially remains open after birth
Closure of the ductus venosus

FEEtal patency of the ductus arteriosus is maintained by prostaglandins E1 and E2.

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 [1]
Endocrine system
Lungs
Liver
Blood and immune system
Gastrointestinal tract
Kidney
Brain
  • Myelination begins in the late fetal period and continues for at least another 10–12 years of life.

Lung maturity in preterm newborns can be achieved via intramuscular injections of glucocorticoids to the mother which induce pneumocytes maturation and thus the production of surfactant.

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 “Postnatal adaptation of the circulatory system” above), respiration, metabolism, and temperature, which require a series of adaptations. For more information on neonatal care, see “Assessment of the newborn”.

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

Metabolism

Thermoregulation

Normal body temperature of the newborn infant: 36.5–37.5 °C (97.7–99.5°F)

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): p.395-407. doi: 10.1016/j.clp.2016.04.001 . | Open in Read by QxMD
  2. Desoye G, Nolan CJ. The fetal glucose steal: an underappreciated phenomenon in diabetic pregnancy. Diabetologia. 2016; 59 (6): p.1089-1094. doi: 10.1007/s00125-016-3931-6 . | Open in Read by QxMD
  3. Lautt WW. Hepatic Circulation. Morgan & Claypool Publishers ; 2009