Methods	Random allocation according to a table of random numbers. Randomisation code given out by a medical secretary at Rigshospitalet, Copenhagen (majority). Some women were randomised by envelopes (Fredriksborg county)
Participants	4606 women randomised between ages of 25 and 34. Exclusion criteria: women believed to be at risk of a child with a chromosomal abnormality, neural tube defect or increased risk of spontaneous abortion. Also women with known uterine abnormalities or intrauterine contraceptive devices were excluded along with multiple gestations
Interventions	Women in the study group were allocated to AC, all of which were carried out at the centre for prenatal diagnosis. The mean gestational age for AC was 16.4 +/−1.1 weeks. AC was carried out with a 20 G needle under direct ultrasound guidance. Women in the control group were allocated to the routine antenatal programme
Outcomes	Pregnancy outcome, abnormal karyotype and neonatal complications and congenital abnormalities
Notes
Risk of bias
Item	Authors’ judgement	Description
Allocation concealment?	Yes	A - Adequate