The Cochrane Database of Systematic Reviews 2006 Issue 3
Copyright 2006 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.

Plain language summary
Amniocentesis is safer after 16 weeks' gestation, and chorionic villus sampling is better done through the wall of the womb
Some parents want reassurance that their baby is all right genetically. This involves taking a sample either of the waters surrounding the baby (amniocentesis) or from the placenta (chorionic villus sampling (CVS) then testing it. The review of studies on ways of taking the sample found a small increase in the risk of miscarriage. Amniocentesis done at 16 to 18 weeks was the safest procedure. CVS is done earlier (about 10 to 13 weeks) and taking the sample through the wall of the womb was safer for the baby than through the vagina and cervix.

A major disadvantage of second trimester amniocentesis is that the result is usually available only after 18 weeks' gestation. Chorionic villus sampling (CVS) and early amniocentesis can be done between 9 and 14 weeks and offer an earlier alternative.

The objective was to assess comparative safety and accuracy of second trimester amniocentesis, early amniocentesis, transcervical and transabdominal CVS.

Search strategy
We searched the Cochrane Pregnancy and Childbirth Group trials register (March 2003) and the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 1, 2002).

Selection criteria
All randomised trials comparing amniocentesis and CVS.

Data collection and analysis
Two reviewers assessed eligibility and trial quality and performed data extraction. We analysed the data using RevMan software.

Main results
A total of 14 randomised studies have been included. In a low risk population with a background pregnancy loss of around 2%, a second trimester amniocentesis will increase this risk by another 1%. This difference did not reach statistical significance, but the increase in spontaneous miscarriages following second trimester amniocentesis compared with controls (no amniocentesis) did (2.1% versus 1.3%; relative risk (RR) 1.02 to 2.52). Early amniocentesis is not a safe early alternative to second trimester amniocentesis because of increased pregnancy loss (7.6% versus 5.9%; RR 1.29, 95% CI 1.03 to 1.61) and higher incidence of talipes compared to CVS (1.8% versus 0.2%; RR 6.43, 95% CI 1.68 to 24.64).

Compared with second trimester amniocentesis, transcervical CVS carries a significantly higher risk of pregnancy loss (14.5% versus 11%; RR 1.40, 95% CI 1.09 to 1.81) and spontaneous miscarriage (12.9% versus 9.4%; RR 1.50, 95% CI 1.07 to 2.11). One study compared transabdominal CVS with second trimester amniocentesis and found no significant difference in the total pregnancy loss between the two procedures (6.3% versus 7%). Transcervical CVS is more technically demanding than transabdominal CVS with more failures to obtain sample and more multiple insertions.

Authors' conclusions
Second trimester amniocentesis is safer than transcervical CVS and early amniocentesis. If earlier diagnosis is required, transabdominal CVS is preferable to early amniocentesis or transcervical CVS. In circumstances where transabdominal CVS may be technically difficult the preferred options are transcervical CVS in the first trimester or second trimester amniocentesis.