Preterm birth is defined as delivery of a baby before 37 completed weeks of pregnancy. Legally, in the United Kingdom, the 1992 Amendment to the Infant Life Preservation Act, defined the limit of viability as 24 weeks. A small number of infants born at 23 weeks will, however, survive. Mortality and morbidity in preterm babies born after 32 weeks gestation is similar to that of babies born at term. The risk of neonatal mortality of survival with handicap becomes significant in 'Very Preterm Infants' defined as those born between 28 and 32 weeks but is most significant in 'Extremely Preterm Infants' defined as those born before 28 weeks. In modern obstetric practice assessment of gestational age is based both upon the date of the last menstrual period and ultrasound fetal biometry. In the past, however, assessment of gestational age was not always accurate and paediatric statistics may be based upon birthweight rather than gestational age data. 'Low Birthweight' is defined as less than 2.25 kg, 'Very Low Birthweight' as less than 1.5 kg and 'Extremely Low Birthweight' as less than 1 kg. Using these definitions to describe outcome data leads to blurring of the distinction between preterm babies and small for gestational age babies, particularly in the low birthweight category and also fails to differentiate the normally grown preterm neonate from the neonate who is both preterm and small for gestational age (Table 21.1).
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.