In the absence of any prenatal screening, some two percent of babies will be born with a structural anomaly; a further 1 in 700–800 will be born with Down's syndrome, with similar numbers having other chromosomal and serious genetic disorders. The prevalence of abnormalities in early pregnancy is higher because abnormal fetuses are more likely to miscarry than normal ones. A small number of women enter pregnancy at increased risk of conceiving a baby with an abnormality. They may have a maternal condition such as diabetes, need medication for conditions such as epilepsy, or have a family history of a genetic disorder (www3.ncbi.nlm.nih.gov/Omim/searchomim.html). Most abnormalities, however, occur in healthy, low-risk women.

With the ascendancy of biophysical assessment with Doppler velocimetry, and the establishment of routine scan dating in early pregnancy, there needs to be a radical re-think of the role of ultrasound biometry in the definition and assessment of fetal growth restriction (FGR).

Doppler flow velocimetry of the umbilical artery has proven its value in defining the FGR fetus. It is more useful than cardiotocography (CTG) or biophysical profile scoring. However, the sensitivity of any test depends on the prevalence of the condition being looked for. While Doppler is of value in fetuses which are small-for-gestational age (SGA), it is less useful in predicting growth restriction or adverse outcome in the general population. The question is therefore, how to detect those pregnancies for which further fetal assessment is indicated.

Placenta praevia is an uncommon but dangerous and occasionally fatal condition. It increases maternal, fetal and neonatal morbidity and mortality. Placenta praevia is one of the leading causes of third-trimester bleeding and the incidence of both placenta praevia and placenta accreta is steadily increasing due to the rising caesarean section rate. At delivery, women with placenta praevia have an increased risk of abruptio placenta, caesarean delivery, fetal malpresentation, postpartum haemorrhage and disseminated intravascular coagulation. Massive obstetric haemorrhage is still a leading cause of pregnancy-related deaths, and placenta praevia/accreta remains one of the major predisposing factors.

With the advent of nuchal translucency as a screening test for trisomy 21, increasing numbers of women are having an ultrasound scan between 11 and 14 weeks. The RCOG Working Party (July 2000) recommendations for ultrasound in pregnancy do not mention a first trimester scan for nuchal translucency or fetal anatomy. However, this scan is not just an opportunity to perform nuchal translucency; many structural abnormalities can also be seen at this stage.