Premature (or preterm, also called “preemies”) babies are those born before the 37th week of pregnancy. Pregnancy is calculated from the first day of the mother's last menstrual period * before conception (kon-SEP-shun). Fullterm babies are born in the 38th to 42nd week of pregnancy.

Premature babies often are put in incubators to keep them warm and protected while they develop and mature.

Premature babies often are put in incubators to keep them warm and protected while they develop and mature.

What Causes Premature Birth?

In the third trimester * of pregnancy (months seven, eight, and nine), the fetus * adds ounces of body weight and is better prepared for living outside the mother's body. However, sometimes the mother goes into labor before the full term of the pregnancy. A baby born from this early birth is referred to as premature. Many premature babies are not ready to live on their own.

There are a variety of reasons why a mother goes into labor early. In up to half of the occurrences, the exact cause is never determined. Histories of women giving premature birth show several risk factors. These risk factors include poverty, poor prenatal * care, poor nutrition, smoking, alcohol abuse, and drug abuse. Other risk factors include high blood pressure, preeclampsia (high blood pressure and excess protein in the urine after 20 weeks of pregnancy), and anxiety. Premature births happen more often to mothers under the age of 18 or over 35.

Physical abnormalities, such as a misshapen uterus * , may cause early labor. The cervix * , the opening to the uterus, may loosen and open too soon. In fact, a mother having a short cervix is the strongest predictor of premature birth. If the mother has a condition (such as high blood pressure) or the baby is at risk, the doctor may induce labor. Inducing labor means the mother is given a drug to start contractions.

Left to right: Embryo and fetus at 3 weeks; 7 weeks; 15 weeks; and 22 weeks.

Left to right: Embryo and fetus at 3 weeks; 7 weeks; 15 weeks; and 22 weeks.
Illustration by Frank Forney. © 2016 Cengage Learning®.

What Does a Preterm Baby Look Like?

Preterm infants tend to look very frail, with thin pink skin with underlying veins showing through and very little body fat. Yet they are active. Even those born 12 weeks early can open their eyes and respond to sound, light, and being touched. Their slender arms and legs are wrinkled and covered with fine hair, called lanugo (la-NOO-go).

Small babies must be kept warm. The surface area of any baby's skin is large relative to the mass of the body. Premature babies lose heat more quickly and need a climate-controlled incubator (ing-kyoo-BAY-tor).


Some premature babies are not ready to breathe on their own. Apnea * , when breathing stops for several seconds, is common. A special apnea mattress assists breathing, and the heartbeat is monitored. If these episodes begin to last a long time, a ventilator (VEN-tih-lay-ter) or breathing machine is used.

Premature babies may experience respiratory distress syndrome (RDS) because the babies' lungs do not have the slippery lubricant called surfactant (sir-FAK-tant) that allows the lungs' tiny air sacs, called alveoli (al-VEE-o-lye), to open and close smoothly. Without surfactant, babies have to struggle to transfer oxygen * and carbon dioxide * . Infants may become exhausted and may not get enough oxygen. This lack of oxygen causes major changes in the body's chemistry. Artificial surfactant and breathing assistance may be needed temporarily, until babies begin to make natural surfactant on their own.

Feeding Jaundice

In preterm babies a yellow pigment called bilirubin (bihlih-ROO-bin) may accumulate in the blood and skin, causing jaundice (JON-dis). The liver * is not mature enough to break down bilirubin, and high levels may cause brain damage. Treatment involves phototherapy (fo-to-THER-uh-pee), where the baby is exposed to fluorescent lights, which can help reduce the bilirubin level.

What Is Very Low Birth Weight?

A very low birth weight infant weighs less than about three pounds and is more than eight weeks early. Babies this small frequently need special care, but any newborn baby who is ill or has obvious problems may be looked after in special care units called neonatal * intensive care units (NICUs).

Low birth weight is sometimes caused by the lifestyle choices of the mother, who ignores prenatal care or does not see a physician. Smoking, alcohol, and drugs may also contribute to the baby's low weight.

In the 21st century, medical science is more able than ever before to help very tiny, very sick newborns survive. Specialized NICUs are available throughout the United States. It is costly to take care of these very sick babies, and the number of premature births is increasing. About 1 out of every 10 births in the United States is premature. Many babies survive, but some have lifelong learning and physical development problems.

See also Birth Defects: Overview • Fetal Alcohol Spectrum Disorders (FASD) • Jaundice • Preeclampsia/Eclampsia • Pregnancy


Books and Articles

Belluck, Pam. “Premature Babies May Survive at 22 Weeks If Treated, Study Finds.” The New York Times. May 6, 2015. http://www (accessed August 12, 2015).

Gunter, Jennifer. The Preemie Primer: A Complete Guide for Parents of Premature Babies—From Birth through the Toddler Years and Beyond. Cambridge, MA: De Capo Lifelong, 2010.

Saigal, Saroj. Preemie Voices: Young Men and Women Born Very Prematurely Describe Their Lives, Challenges, and Achievements. Victoria, BC: Friesen Press, 2014.


Healthy Children. “Caring for a Premature Baby: What Parents Need to Know.” American Academy of Pediatrics. (accessed November 13, 2015).

Eunice Kennedy Shriver National Institute of Child Health and Human Development. “Preterm Labor and Birth: Overview.” (accessed August 8, 2016).

MedlinePlus. “Premature Babies.” U.S. National Library of Medicine, National Institutes of Health. (accessed August 12, 2015).


American Academy of Pediatrics. 141 Northwest Point Blvd., Elk Grove Village, IL 60007-1098. Telephone: 847-434-4000. Website: (accessed August 26, 2015).

Eunice Kennedy Shriver National Institute of Child Health and Human Development. 31 Center Dr., Building 31, Room 2A32, MSC 2425, Bethesda, MD 20892-2425. Toll-free: 800-370-2943. Website: (accessed August 3, 2016).

Global Alliance to Prevent Prematurity and Stillbirth. 1100 Olive Way, Suite 1000, Seattle, WA 98101. Telephone: 206-884-2777. Website: (accessed August 12, 2015).

March of Dimes. 1275 Mamaroneck Ave., White Plains, NY 10605. Telephone: 914-997-4488. Website: (accessed August 12, 2015).

* menstrual period (MEN-strooal PE-re-od) is the discharging through the vagina (vah-JY-nah) of blood, secretions, and tissue debris from the uterus (YOO-terus) that recurs at approximately monthly intervals in females of breeding age.

* trimester (tri-MES-ter) is any of three periods of approximately three months each into which a human pregnancy is divided.

* fetus (FEE-tus) is the term for an unborn human after it is an embryo, from nine weeks after fertilization until childbirth.

* prenatal (pre-NAY-tal) means existing or occurring before birth, with reference to the fetus.

* uterus (YOO-teh-rus) is the muscular, pear-shaped internal organ in a woman where a baby develops until birth.

* cervix (SIR-viks) is the lower, narrow end of the uterus that opens into the vagina.

* apnea (AP-nee-uh) is a temporary stopping of breathing.

* oxygen (OK-si-jen) is an odorless, colorless gas essential for the human body. It is taken in through the lungs and delivered to the body by the bloodstream.

* carbon dioxide (CAR-bon dyOK-side) is an odorless, colorless gas that is formed in the tissues and breathed out through the lungs.

* liver is a large organ located beneath the ribs on the right side of the body. The liver performs numerous digestive and chemical functions essential for health.

* neonatal (ne-o-NAY-tal) means pertaining to the first 4 weeks after birth.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)