Two laboratories in the West Midlands have monitored 1000 `at risk' pregnancies. Of these 57% were referred for chromosomal indications and 43% for possible neural tube defects. The largest at risk groups (both 37%) were those mothers who had already had a pregnancy resulting in a baby with a neural tube defect (21% spina bifida and 16% anencephaly), and those who were referred because of the increased risk of Down's syndrome in pregnancies where the mother was over 35 years old. Six percent had already borne a child with Down's syndrome.
An estimate of the AFP level in the amniotic fluid was achieved in 985 (98·5%) of the pregnancies. Of these, 967 mothers could be reassured that the baby did not have an open neural tube defect and 18 abnormal fetuses were terminated or died spontaneously.
Chromosome studies were completed in 846 (85%) of the pregnancies with the consequence that 19 were terminated. Of these, 15 had an abnormal karyotype, including nine with Down's syndrome. Four male fetuses were terminated because of a high chance of carrying X linked disease.
The 1000 pregnancies monitored required 1080 amniocenteses. Cases were excluded from this study if the amniocentesis took place after the 20th week of gestation and if follow-up data concerning the outcome had not been received. This follow-up data was obtained through a questionnaire, but only ten cases were lost to the study at this point, mainly because the subject had moved away from the area before the birth of her child.
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