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Genetics of Alzheimer's disease.

differences did not result in a discrepancy of more than a week in any patient scanned between eight and 14 weeks. When realtime equipment is used the fetal movements are readily appreciated, and with electronic callipers several measurements can quickly be made, thus minimising errors. In practice, however, most patients do not attend this early, so that other schemes are needed. At King's College Hospital7 a plan of routine scanning between 16 and 18 weeks has been adopted for the following reasons. Measurements of the biparietal diameter are easy at this time and, though less precise than early crown-rump length (by ±4 days), are considered clinically acceptable; the abdominal circumference measurement is a check on the biparietal diameter; early diagnosis of multiple pregnancy is facilitated; patients with low placental implantation are identified for further study; and gross abnormalities such as anencephaly are readily diagnosed. Though spina bifida cannot reliably be detected with realtime equipment, recent advances in imaging hold out much promise. Enlargement of the lateral ventricles may point to hydrocephalus, and exomphalos has been described at this time. At King's blood is also taken for estimation of alphafetoprotein at this visit. Nevertheless, even if the accuracy of scanning in the average district general hospital is generally as good as in the centres of excellence, there remains the problem of a systematic error of about a week either way in gestational estimation for most of the 750% of patients who present before 18 weeks, and at present this cannot be improved. To cope with this difficulty the Cardiff group8 has devised a "fail-safe" procedure. Women with serum alpha-fetoprotein concentrations above the 90th centile are carefully scanned for obvious abnormalities and multiple pregnancies and to reassess gestational age. Amniocentesis is offered where doubt remains or if the serum alphafetoprotein concentration is higher than the 95th-centile. Elsewhere early attendance remains disappointing. For instance, in Mid-Glamorgan in 1978 only 40%/ of women had attended by 16 weeks, and 3000 had not attended by 20 weeks. As in Aberdeen, many late attenders (42%) were primigravidae who had delayed reporting the pregnancy to the general practitioner. On the other hand, most women at particularly high risk during pregnancy and who have medical disorders such as diabetes refer themselves promptly for care, as do those patients with an unhappy obstetric history. These highly motivated mothers are those most in need of the benefits conferred by early attendance and in the main take full advantage of this facility. It might be argued that delaying most screening procedures to between 16 and 18 weeks, as is the practice at King's, might further encourage late booking. Furthermore, some patients, thinking themselves to be earlier in pregnancy than they really are, might miss out altogether. In practice, however, 16-18 weeks is currently the most popular time to book anyway, and errors in estimation of gestational age from the menstrual history are mostly overestimates.9 In a perfect world such considerations would be unnecessary, but until we can educate our prospective patients in what we consider to be prudent behaviour we must take the world as it is and compromise as best we can.