Methods	Random assignment.
Participants	3998 patients recruited in 8 US collaborating centres. Inclusion criteria: favourable placental position allowing both procedures to be performed, gestational age between 49 and 90 days. Exclusion criteria: active genital herpes, active vaginal bleeding or cervical polyps. 1190 randomised to TC CVS and 1163 to TA CVS
Interventions	TA or TC CVS. TC being performed with a plastic catheter and TA with an 18 to 22 G spinal needle
Outcomes	Sampling success, pregnancy outcome.
Notes	Initial cohort of 2353 women presented who delivered before July 1 1989
Risk of bias
Item	Authors’ judgement	Description
Allocation concealment?	Yes	A - Adequate

AC: amniocentesis

CRL: crown rump length

CVS: chorionic villus sampling

EA: early amniocentesis

G: gauge

TA: transabdominal

TC: transcervical

vs: versus