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 in 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 while floating in 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.

See: Skin characteristics in newborns for other skin changes in the newborn.

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Review Date: 11/02/2009
Reviewed By: Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)