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Preterm birth, also known as premature birth, is the birth of a baby at less than 37 weeks gestational age. These babies are known as preemies or premmies.〔 Symptoms of preterm labor include uterine contractions which occur more often than every ten minutes or the leaking of fluid from the vagina. Premature infants are at greater risk for cerebral palsy, delays in development, hearing problems, and problems seeing. These risks are greater the earlier a baby is born.〔 The cause of preterm birth is often not known.〔 Risk factors include diabetes, high blood pressure, being pregnant with more than one baby, being either obese or underweight, a number of vaginal infections, tobacco smoking, and psychological stress, among others.〔 It is recommended that labor not be medically induced before 39 weeks unless required for other medical reasons. The same recommendation applies to cesarean section.〔 Medical reasons for early delivery include preeclampsia. In those at risk, the hormone progesterone, if taken during pregnancy, may prevent preterm birth. Evidence does not support the usefulness of bed rest. It is estimated that at least 75% of preterm infants would survive with appropriate treatment.〔 In women who might deliver between 24 and 34 weeks corticosteroids improve outcomes. A number of medications including nifedipine may delay delivery so that a mother can be moved to where more medical care is available and the corticosteroids have a greater chance to work. Once the baby is born care includes keeping the baby warm through skin to skin contact, supporting breastfeeding, treating infections, and supporting breathing.〔 Preterm birth is the most common cause of death among infants worldwide.〔 About 15 million babies are preterm each year (5% to 18% of all deliveries). In many countries rates of premature births have increased between the 1990s and 2010s. Complications from preterm births resulted in 0.74 million deaths in 2013 down from 1.57 million in 1990. The chance of survival at less than 23 weeks is close to zero, while at 23 weeks it is 15%, 24 weeks 55% and 25 weeks about 80%. The chances of survival without long term difficulties is less. ==Classification== In humans the usual definition of preterm birth is birth before a gestational age of 37 complete weeks.〔 〕 In the normal human fetus, several organ systems mature between 34 and 37 weeks, and the fetus reaches adequate maturity by the end of this period. One of the main organs greatly affected by premature birth is the lungs. The lungs are one of the last organs to mature in the womb; because of this, many premature babies spend the first days/weeks of their life on a ventilator. Therefore, a significant overlap exists between preterm birth and prematurity. Generally, preterm babies are premature and term babies are mature. Preterm babies born near 37 weeks often have no problems relating to prematurity if their lungs have developed adequate surfactant, which allows the lungs to remain expanded between breaths. Sequelae of prematurity can be reduced to a small extent by using drugs to accelerate maturation of the fetus, and to a greater extent by preventing preterm birth. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Preterm birth」の詳細全文を読む スポンサード リンク
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