The traditional method of denoting a small baby as being below 2500 g, or 1500 g, does not distinguish between smallness due to short gestation and smallness due to intrauterine growth restriction (IUGR).
The terms small for gestational age (SGA), average for gestational age (AGA) and large for gestational age (LGA) are therefore preferred, which adjust the limit for the average at the respective gestational age. Traditionally, the 10th and 90th centiles respectively are used, although the 5th and 95th, or the 3rd and 97th (equivalent to ±2 standard deviations) can also be applied. However, SGA is not synonymous with IUGR as it includes pathological as well as constitutional smallness.
Increasingly, it has become apparent that birthweight and fetal growth vary with a number of factors, apart from gestational age. These factors can be physiological (constitutional) or pathological.
Physiological factors include birth order (parity), maternal characteristics such as height, weight and ethnic origin and fetal gender . Coefficients have been derived to allow for the normal birthweight ranges to be adjusted, from which then growth curves can be drawn (see below) .
Pathological factors affecting growth include smoking, alcohol, social class and deprivation, multiple pregnancy, and pregnancy complications such as placental failure and related underlying conditions associated with hypertensive diseases in pregnancy, antepartum haemorrhage and diabetes (see Chapter 27). However, such variables should
Fetal and neonatal weights at 32 weeks
Fig. 4.2 Ultrasound versus birth weight standard at 32 weeks gestation. The line shows ultrasound weight estimations derived from pregnancies which have proceeded to normal term delivery. The curve is characterized by a relatively narrow, normal distribution. The histogram shows birth weights of babies born at this same, preterm gestation in dataset of approximately 40,000 cases in the Midlands. The distribution shows a lower median, a wider range and negative skewness.
not be adjusted for, as the standard should reflect the optimal growth potential of the fetus. For example, it is well established that maternal smoking adversely affects fetal growth; however, the standard or norm should not be adjusted downwards if a mother smokes, but it should be 'optimized' as if the mother did not smoke, to allow better detection of the fetus that is affected.
In practice, well-dated birthweight databases with sufficient details about maternal characteristics and pregnancy outcome are used to derive the coefficients needed to adjust for constitutional variation.
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Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.