[Fetal biometry: review and application of sex determination]

Diagnostic ultrasound was introduced in the practice of obstetrics through its ability to measure accurately the biparietal (DBP) diameter of the foetus. The development of bidimensional and grey tone images expanded the possibility of fetal biometry. One of its most important uses is the estimation of gestational age. A knowledge of foetal age is necessary to interpret the results of most of the investigations currently performed during pregnancy. Different fetal dimensions increase with age at different rates according to the period of gestation. During the embryonic period, the crown-rump length gives a very good estimation of age within 4-6 days for twice the standard deviation. Later on, because of the curvature of the fetus, the age is estimated with the DBP. The accuracy diminishes with the slowing down of fetal growth, from 12 weeks onwards, when twice the standard deviation of the age estimate may be as long as 2 weeks. When DBP is no longer reliable in determining age, femur length is measured being more reliable for that purpose at least till 24 weeks. A very early assessment of foetal age reduces the risk of errors in estimating the duration of pregnancy and is important to detect early growth retardation (IUGR); this is usually symmetric, i.e. affecting all fetal parts (versus asymmetrical or late IUGR where trunk growth is first involved). Fetal weight can be estimated by fetal parts measurements. A formula was derived as long as twenty years ago but not all authorities agree on any measuring technique as evidenced by the many different formulas proposed--a sign that no very satisfactory one yet exists.(ABSTRACT TRUNCATED AT 250 WORDS)