Methods	Consecutively-numbered sealed envelopes.
Participants	800 women in early pregnancy requesting prenatal diagnosis.
Interventions	4 operators performed all procedures - TC CVS with Portex cannula or AC at 16 weeks under ultrasound guidance
Outcomes	Pregnancy outcome, abnormal karyotype, antenatal complications and diagnostic accuracy
Notes	This study was part of the international MRC trial.
Risk of bias
Item	Authors’ judgement	Description
Allocation concealment?	Yes	A - Adequate