Premature infants

Premature infants are babies born before full-term gestation of 9 months (37 weeks). Prematurity is an important topic in both pediatrics and obstetrics because the health of a baby at birth is strongly dependent on full or substantial completion of a term pregnancy. When babies are born before about 24-25 weeks of gestation,there are no survivors. When infants are born after that period of gestation in the womb, then, depending on the level of sophicticated medical care available, many will live while suffering a variety of illnesses and crises that are not the usual experience of full term infants. Premature babies account for such a disproportionately high amount of the morbidity and mortality of all babies, that preventing premature labor and delivery is a major concern in obstetrics, and the care of premature neonates is an important part of the hospital care of infants.

Prematurity is qualitatively different according to the extent of in utero (in the womb) development that the baby has undergone, the closer that the baby is to term, generally, the better his or her chances of survival and a normal life after survival.The "youngest" of premature newborns, those who have undergone the least amount of time in the womb, are generally worse off than the infants who have come closer to term gestation. One of the most major differences between the usual very preterm an the usual term or near term baby is the ability to breathe without the assistance of machines, and to oxygenate the blood through the pulmonary circulation by breathing air that has not been supplemented with extra oxygen. Most of the severe problems associated with prematurity, that is, death, disability, and illness, affects “very preterm” infants (those born before 32 weeks' gestation), and especially “extremely preterm” infants (those born before 28 weeks of gestation). (Tucker J. McGuire W. Epidemiology of preterm birth.[see comment]. [Review] [0 refs] [Journal Article. Review] BMJ. 329(7467):675-8, 2004 Sep 18. UI: 15374920). About 1-2% of babies in Europe and North America are "very preterm". There are other characteristics that are important in physicians' view of the health and care of these babies, but, as in all populations defined by medical criteria, the specific way that this term is applied is important in reviewing the known literature on prematurity. Just how is prematurity defined?

Prediction of due date and estimation of gestational age at birth

A fair percentage of babies who are pre-term by estimation of a calendar due date have characteristics of full term babies. In fact, at least some of these babies are not really premature but arrived sooner than expected simply because the estimation of their birth date was incorrect. One study 3 comparing sonographic with menstrual dates from a large database found that only 78% of pregnancies designated preterm (less than 37 completed weeks) by menstrual dates were actually preterm, according to a confirming sonogram. (Klebanoff MA. Gestational age: not always what it seems.[comment]. [Comment. Editorial] Obstetrics & Gynecology. 109(4):798-9, 2007 Apr. UI: 17400838). That due date is determined by the reported date of the last menstrual period, and in women who do not keep records, and for those who have irregular periods, the date given may be inaccurate. In other words, many of the babies who are born long before they are expected, but are born with the appearance and size of full term, or near full term infants are likely not premature but, instead, their mothers had been given the wrong estimated date of confinement.

In the embryo and fetus, development proceeds in a predictable fashion. In general, in all vertebrate embryos, stages occur first at the head, and later the "tail", and first towards the midline and later at the distal areas of the body. There are changes in the fetal physiology as the gestational age increases and approaches 37 weeks that are usual for independant life. (Give examples of hormones and inflammatory proteins)

The average full term infant is about 7 pounds (3500 grams) at birth. Babies gain weight as they approach term, and sometimes the birth weight is taken as a general indication of the degree of prematurity. There are other factors, such as the degree of nutrition that the fetuis receives through the placenta, that influence size. However, very low birthweight infants share many of the same aspects of the babies of earliest viable gestational age. babies who are at the highest risk for serious complications are 28 weeks or less in gestational age, and/or 1500 grams or less in weight.

Some of the features that are typical at varying gestational ages are summarized below: (Please note that these features are included for educational purposes, and not because they specifically "make" a diagnosis of a particular gestational age.)

25-26 weeks: This is about the limit for prematurity in which the baby has a reasonable chance at survival.

27-28 weeks:

29 weeks: "The pupils are normally relatively dilated until 29 weeks of gestation, at which time the pupillary light response first becomes apparent." (reference for quote: Douglas R. Fredrick, MD:Chapter 17. Special Subjects of Pediatric Interest in Paul Riordan-Eva and John P. Whitcher (Eds) Vaughan & Asbury's General Ophthalmology 16th Edition, Copyright © 2004 by The McGraw-Hill Companies, Inc.)

Additional hazards associated with prematurity

Simply being born before the end of gestation yields a baby whose anatomy and physiology are not yet ready for the transition from being nurtured in the womb to independent life. However, sometimes the condition that provoked premature birth causes more problems for the baby than simply being expelled out of the womb too soon.

Premature neonates around the world

"Approximately 4 million children are born in the United States each year, of which approximately 11% are premature... 1% of these low birth weight (LBW) infants are less than 1,500 grams at birth and more than 80% of these very-low-birth-weight (VLBW) infants survive to discharge. A significant minority, 20-40%, of the very-low-birthweight babies have complex medical problems." Verma RP. Sridhar S. Spitzer AR. Continuing care of NICU graduates. Clinical Pediatrics. 42(4):299-315, 2003 May. UI: 12800725

Neonatal intensive care unit

In countries offering technilogically advanced medical care, special units for the care of premature and other ill newborns called neonatal instensive care units exist. If a baby requires support of breathing with mechanical ventilation, then admission to the neonatal ICU is usual.

Discharge from the hospital

The criteria used to determine if a baby is ready to be discharged vary from hospital to hospital, but there are some generalizations that may be made. The baby who is ready to go home to his parent's care is usually able to feed by a nipple (bottle or breast), and is gaining weight adequately with such feeding. He no longer requires an "incubator"- that is a temperature-controlled environment, but can maintain his body temperature in a crib or bassinet. The baby is not having periods of abnormal heart rhythyms that are potentially lifethreatening, or lifethreatening periods of apnea.

Even once the premature baby is no longer obviously small, the immaturity of his organ systems puts him at greater risk than a full term infant. When epidemiologic statistics on reported illness and deaths among newborns and infants are compiled, premature infants are overrepresented.

Continuing care of premature infants after discharge

Premature babies who require care in the neonatal intensive care unit are often offered specialized care after discharge.

Lung problems:bonchopulmonary dysplasia (BPD)

When a newborn baby inspires, air displaces fluid in the lungs. With the change of fluid for air, the pressure of the vessels in the pulmonary bed drops, and blood perfuses the lungs. After several breaths, if the lungs are suitably mature, the baby's lungs are normally aerated and gas exchange can take place. One of the most important factors in allowing this process of transition to occur without resulting in distress is whether the lungs have an adequate amount of surfactant. "The presence of surfactant, synthesized by type II pneumocytes, lowers surface tension in the alveoli and thereby prevents the collapse of the lung with each expiration." (Williams Obstetrics - 22nd Ed. (2005))

Lack of sufficient surfactant, common in preterm infants, leads to the prompt development of respiratory distress syndrome

Retinopathy (visual problems)

"The American Academy of Pediatrics, the American Association for Pediatric Ophthalmology and Strabismus, and the American Academy of Ophthalmology produced a joint statement recommending the initial eye examination be performed between 4-6 weeks of chronologic age or between 31-33 weeks postconceptional age (gestational age at birth plus chronologic age)."Richard W. Allinson, MD:RETINOPATHY OF PREMATURITY (ROP) in Frank J. Domino, MD (Ed) The 5-Minute Clinical Consult 2007- 15th Ed.© 2007 by LIPPINCOTT WILLIAMS & WILKINS, a Wolters Kluwer business.ISBN 9780781763349

Central nervous system

The brain continues to grow and develop after birth even in term babies, the central nervous system of all infants shows plasticity. However, there are known differences in the brains of premature infants and term babies. Some of these differences are in the reaction to the potentially catastrophic stress of poor blood perfusion (ischemia) or inadequate oxygenation (hypoxia) of the brain. Such stress is a potential hazard of the birth process for all babies.

When the insult (stress) is the same, for example if a baby has a period of hypoxia, ..."the preterm brain exhibits a higher degree of susceptibility of the white matter while the term brain exhibits primarily gray matter injury."(Jensen FE. Developmental factors regulating susceptibility to perinatal brain injury and seizures. Current Opinion in Pediatrics. 18(6):628-33, 2006 Dec.

References

American Academy Of Pediatrics Committee on Practice and Ambulatory Medicine and Committee on Fetus and Newborn. The role of primary care pediatrician in the management of high-risk newborns. Pediatrics. 1996;98:786-788

Ballard JL, Khoury JC, Wedig K, et al: New Ballard score, expanded to include extremely premature infants. J Pediatr 119:417, 1991

Inder TE, Wells SJ, Mogridge NB, et al. Defining the nature of the cerebral abnormalities in the premature infant: a qualitative magnetic resonance imaging study. J Pediatr 2003; 143:171–179.

Edwards AD. Tan S. Perinatal infections, prematurity and brain injury. [Review] [90 refs] [Journal Article. Review] Current Opinion in Pediatrics. 18(2):119-24, 2006 Apr. UI: 16601489

Verma RP. Sridhar S. Spitzer AR. Continuing care of NICU graduates. [Review] [102 refs] [Journal Article. Review] Clinical Pediatrics. 42(4):299-315, 2003 May. UI: 12800725