Trends in prenatal screening for and diagnosis of Down's syndrome: England and Wales, 1989-97.

All clinical cytogenetic laboratories in England and Wales notify the register anonymously of trisomy 21 or related karyotypes, together with the date, place of and indications for referral, parental age, and family history. Most send a copy of the notification to the referring physician for confirmation and completion. The outcome of the pregnancy is requested for all cases diagnosed prenatally (but may not be known for several months) and is known for 91% of registered prenatal diagnoses overall. In 1989 registrations numbered 1081; in 1993, 1144; and in 1997, 1336 (table). In contrast, births nationally dropped from 688 000 in 1989 to 642 000 in 1997. The proportion diagnosed prenatally at all maternal ages rose from 30% to 53% between 1989 and 1997. In 1989 only 9% of affected pregnancies were diagnosed prenatally in mothers under 35, rising to 45% in 1997. For mothers aged 35 and over prenatal diagnosis of affected pregnancies rose from 60% in 1989 to 72.2% in 1996 (table). However, the percentage diagnosed prenatally levelled out from 1994 onwards. Maternal age was the first indication for 78% of all prenatal diagnostic tests in 1989 but only 16% in 1997. Maternal serum screening results as an indication increased from 6% in 1989 to 37% in 1996 and fetal ultrasound findings from 13% to 43%. Around a fifth (8/43) of ultrasound referrals for diagnosis concerned the nuchal zone in 1989, but this was 55% (191/350) in 1997. Amniocentesis accounted for 78% (246/316) of prenatal diagnoses in 1989 and 61% (431/701) at a mean gestational age of 17.2 weeks in 1997. Between 1989 and 1992, 37% of all prenatal diagnoses followed amniocentesis before 16 weeks’ gestation; between 1993 and 1996 this fell to 31%. Prenatal diagnosis by chorionic villus sampling or placental biopsy decreased from 18% (58/316) (mean gestation 11.5 weeks) in 1989 to 10% in 1992 following reports of limb defects after biopsy before week 12, increasing to 37% in 1997 at a mean gestation of 13.7 weeks. From 1989 to 1997, 92% of cases diagnosed prenatally and with known outcome were terminated, 1.9% were miscarried, 1.5% were recorded as a stillbirth or neonatal death, and nearly 5% were live births surviving to discharge from hospital. In some of these an offer to terminate the pregnancy had been declined. When diagnosis occurred after 23 weeks, usually on the basis of ultrasound findings, only a quarter of affected pregnancies were terminated, while 23% were miscarried, stillborn, or died in the neonatal period.