At birth, an infant's body undergoes a number of changes that allow it to survive outside the womb and adapt to life in a new environment.

LUNGS AND CIRCULATORY SYSTEM

While the fetus is in the womb, it "breathes" by exchanging oxygen and carbon dioxide through the mother's circulation via the placenta. Most of the blood does not go through the developing baby's lungs. Instead, it travels through the heart and flows throughout the baby's body.

At birth, the baby's lungs are filled with amniotic fluid and are not inflated. The baby takes the first breath within about 10 seconds after delivery. It sounds like a gasp, as the newborn's central nervous system reacts to the sudden change in temperature and environment.

Once the umbilical cord is cut and the baby takes the first breath, a number of changes occur in the infant's lungs and circulatory system:

• Increased oxygen in the lungs causes a decrease in blood flow resistance to the lungs.
• Blood flow resistance of the baby's blood vessels also increases.
• Amniotic fluid drains or is absorbed from the respiratory system.
• The lungs inflate and begin working on their own, moving oxygen into the bloodstream and removing carbon dioxide through exhalation.

TEMPERATURE REGULATION

A developing baby produces about twice as much heat as an adult. That heat dissipates as blood flows into the mother's circulation via the placenta and is cooled. A small amount of heat is removed through the developing baby's skin, the amniotic fluid, and the uterine wall.

After delivery, the newborn begins to lose heat. Receptors on the baby's skin send messages to the brain that the baby's body is cold. The baby's body then creates heat by shivering and by burning stores of brown fat , a type of fat found only in fetuses and newborns.

LIVER

In the fetus, the liver acts as a storage site for sugar (glycogen) and iron. When the baby is born, the liver has various functions:

• It produces substances that help the blood to clot.
• It begins breaking down waste products such as excess red blood cells.
• It produces a protein that helps break down bilirubin. If the baby's body does not properly break down bilirubin, it can lead to newborn jaundice.

GASTROINTESTINAL TRACT

A baby's gastrointestinal system doesn't fully function until after birth. However, in the womb, it does allows the baby to absorb nutrients from the placenta.

In late pregnancy, the fetus produces a tarry green or black waste substance called meconium. Meconium is the medical term for the newborn infant's first stools. Meconium is composed of amniotic fluid, mucous, lanugo (the fine hair that covers the baby's body), bile, and cells that have been shed from the skin and the intestinal tract. In some cases, the baby passes stools (meconium) while still inside the uterus.

URINARY SYSTEM

The developing baby's kidneys begin producing urine by 9 - 12 weeks into the pregnancy. After birth, the newborn will usually urinate within the first 24 hours of life. The kidneys become able to maintain the body's fluid and electrolyte balance.

The rate at which blood filters through the kidneys (glomerular filtration rate) increases sharply after birth and in the first 2 weeks of life. Still, it takes some time for the kidneys to get up to speed. Newborns have less ability to remove excess salt (sodium) or to concentrate or dilute the urine compared to adults. This ability improves over time.

IMMUNE SYSTEM

The immune system begins to develop in the fetus, and continues to mature through the child's first few years of life. The womb is a relatively sterile environment. But as soon as the baby is born, he or she is exposed to a variety of bacteria and other potential disease-causing substances. Although newborn infants are more vulnerable to infection, their immune system can respond to infectious organisms.

Newborns do carry some antibodies from their mother, which provide protection against infection. Breastfeeding also helps improve a newborn's immunity.

SKIN

Newborn skin will vary depending on the length of the pregnancy. Premature infants have thin, transparent skin. The skin of a full-term infant is thicker.

Characteristics of newborn skin:

• A fine hair called lanugo might cover the newborn's skin, especially preterm babies. The hair should disappear within the first few weeks of the baby's life.
• A thick, waxy substance called vernix may cover the skin. This substance protects the fetus from constant exposure to amniotic fluid in the womb. Vernix should wash off during the baby's first bath.
• The skin might be cracking, peeling, or blotchy, but this should improve over time.

Other possible skin changes in the newborn:

• Acne may be seen from birth, but it should go away within a few months.
• Milia are small white spots on the cheeks and nose. They are filled with a waxy protein called keratin. They should disappear within the first few weeks.
• Mongolian spots are blue-gray or brown spots can emerge on the skin of the buttocks or back, mainly in dark-skinned babies. They should fade within a year.
• Stork bites are small red patches on the baby's forehead, eyelids, back of the neck, or upper lip are caused by stretching of the blood vessels. They often go away within 18 months.

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Rakel RE. Textbook of Family Medicine. 7th ed. Philadelphia, Pa: Saunders, 2007.

Rudolph CD, Rudolph AM, Hostetter MK, Lister G, Siegel NJ, eds. Rudolph's Pediatrics. 21st ed. New York, Ny: McGraw Hill Medical; 2003.