Estimation of fetal maturity is the most accurate method of estimating gestation age by ultrasonographic measurements of crown-rump length during the first trimester. From the first trimester through 34 weeks, the biparietal diameter is accurate to within 10 days. Other measurements used in the 2nd and 3rd trimesters include fetal abdominal diameter and femur length.

Low birth weight (LBW) is a worldwide problem usually defined as birth weight <2,500 g, irrespective of gestational age. It is associated with increased perinatal morbidity and is used as a marker of increased neonatal risk. It is not an ideal marker of fetal growth and development and combines both prematurity and various degrees of growth retardation.Morbitity is associated with LBW and growth retardation. Twenty-one million LBW infants are born each year internationally, 90% in developing countries.

Insulin growth factor (IGF) II is essential for organogenesis and early fetal growth. IGF-I is essential for late fetal growth. IGF-I is low in intrauterine growth retardation (IUGR) infants.

Proportion of LBWs varies by type of society:

1. ■ 3–12% rates of LBW infants occur in developed countries, 60% premature, 40% growth retarded

2. ■ 12–40% rates of LBW in developing countries, 20% premature, 80% growth retarded

There is a high rate of perinatal morbidity in growth-retarded infants:

1. ■ perinatal depression is 3 times more likely

2. ■ hypoglycemia is 4–6 times more likely

3. ■ hypothermia is 5 times more likely

4. ■ meconium aspiration is 13 times more likely

5. ■ fetal distress in labor is 6 times more likely