An infant is considered preterm if delivered at less than 37 weeks gestational age. An infant's gestational age is calculated from the first day of his or her mother's last menstrual period. There are various methods used to classify the degree of prematurity. Infants born before 31 weeks gestation are considered very premature and appear quite different from full term infants. Depending on gestational age, these infants may weigh as little as 450 grams - they have not had time to store much body fat and typically look very thin and frail. Infants lacking sufficient body fat typically must be kept in an incubator to prevent hypothermia. Very premature infants are born with a white coating on their skin which offers protection in utero and is easily removed. Fine body hair, called lanugo, is also present, which falls out as the infant matures. The skin will appear red and wrinkly and may be somewhat transparent. Very premature infants have soft bones which are easily molded - thus their heads may become flattened by a constant back-lying position. Additionally, very preterm infants have poor muscle tone and exhibit uncoordinated movements. Infants do not typically develop the suck reflex until 29-30 weeks gestational age. Thus, intravenous lines or feeding tubes are often required for nourishment.

Of extreme importance is the level of lung maturity an infant has developed at birth. Usually an infant's lungs are not fully developed until nearly full-term (40 weeks) gestation. Respiratory dysfunction is one of the most life-threatening problems which present in the preterm infant. The lungs do not produce sufficient surfactant to insure adequate inflation of air sacs until about 35 weeks; thus infants born before this time will require extensive ventilatory support.

Moderately Preterm Infants

Infants delivered at 31-36 weeks gestation are considered moderately premature. These infants typically have less body hair (lanugo), more body fat, and thicker skin. The bones and cartilage are somewhat firmer yet more moldable than those of a full-term infant. Moderately preterm infants usually weigh between 1500 and 2500 grams. Those weighing less than 1750 grams may require incubation to keep warm. Infants in the 31-36 weeks gestational age range exhibit improved muscle tone and tend to rest with flexed, rather than floppy, limbs. Preterm infants born in this range most often have a suck reflex but may require nutritional assistance through intravenous lines or supplementation of formula or breast milk. Most infants born moderately premature will require some degree of respiratory support.

Causes of Preterm Delivery

There are a variety of reasons why an infant may be born preterm. Premature rupture of the membranes may occur (often secondary to urinary tract or vaginal infection, excess amniotic fluid, or multiple pregnancy), and necessitate preterm delivery. Another cause of preterm birth may be an incompetent cervix which opens as gestation increases. Atypical uterine shape may also result in preterm birth, as well as problems with placental attachment. The placenta may be only partially attached to the uterine wall or detach, resulting in bleeding which threatens the infant's and mother's life. Eclampsia (toxemia) may develop in the mother and necessitate emergency preterm delivery. Other risk factors for preterm birth include maternal diabetes, kidney disease, high blood pressure, pneumonia, genetic disorders and influenza. Women under 18 or over 35 years of age, those with multiple pregnancies, abortions, or miscarriages - as well as those who smoke or abuse drugs - also experience a higher rate of preterm delivery.

The previous material was taken in part from the Baby Awareness and Support through Interactive Computer Systems (BASICS); a project supported by the US Department of Education and the University of Kentucky.