Preterm birth remains a major cause of newborn and infant illness and death. If its causes were better understood and prevention or treatment more effective, fewer babies would die every year. For a pregnant woman, prevention of preterm birth requires knowing the early signs of impending labor and seeking prompt care should any occur.

There is no exact physical or physiologic point at which a fetus ceases to be premature and becomes mature, no matter how precisely the pregnancy is dated. All the changes that occur are gradual, especially in the latter two-thirds of the third trimester. There are no great leaps forward in baby behavior from one day to the next. In general, however, 37 weeks completed gestation is considered mature. Any labor occurring before 37 weeks is considered preterm labor and any baby born before 37 weeks is considered preterm.

Today, a distinction is made between premature babies and preterm babies. "Maturity" refers to how well the baby is able to function outside the womb, with the most emphasis on the respiratory system, as breathing is a crucial function the baby must assume. A baby whose lungs cannot function is "premature." A "preterm" baby is one whose gestational age is less than 37 completed weeks gestation. Some "preterm" babies may not be "premature" from the perspective of their ability to breathe.

Acknowledging the difficulty in precisely dating a pregnancy, particularly prior to ultrasound, experts in the early part of the twentieth century suggested defining prematurity by weight, not gestational age. Any baby weighing less than 2,500 grams (2.5 kilograms), or approximately 5½ pounds, was considered premature. However, using weight imposes its own imprecision, as weight varies greatly with many other factors. We now make a distinction between birth weight and gestational age.

A baby's birth weight may be said to be appropriate for gestational age(AGA) or small for gestational age(SGA). Small babies may be small because they are preterm or small because they are "growth retarded," or "growth restricted." A baby may be both preterm and small for gestational age.

If two babies are the same birth weight, but one is born at 33 weeks and is the appropriate weight for this gestational age and the other is a growth restricted baby born at 38 weeks, in general, the older infant will have an easier time adapting to life outside the uterus. If a baby is large, but born early, it may still have the problems that accompany early birth. The baby who will have the most trouble is the one who is both premature and small for gestational age.

Prior to delivery, when the baby can physically be examined to determine its gestational age, the date of the first day of the last menstrual period is the best single piece of information regarding the duration of the pregnancy. When a woman does not remember this date, when she has irregular periods or gets pregnant after discontinuing a birth control method that made her periods stop, such as Depo-Provera(the "shot"), an ultrasound determines the duration of pregnancy. Ultrasound may also be relied upon when the uterine size does not match the dates. The ultrasound may need to be repeated to ascertain if the difference in uterine size and menstrual dates(called a size-dates discrepancy) is due to the baby growing excessively or not growing adequately, or if in fact the baby is growing as expected, just older or younger than the dates suggest. Serial ultrasounds, done several weeks apart, can measure the fetus's rate of growth. Proper growth may also be determined by using a variety of ultrasound measurements to assess the relationship of the size of the head, for example, to the size of the abdomen. If there is a big discrepancy between these two, the baby may be growing improperly.

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