Amniocentesis and placental sampling for pre-birth diagnosis

Many women want to be reassured that their unborn baby is healthy. It is important that screening and diagnostic tests used are accurate and safe and can be done early enough in pregnancy to allow them the choice of terminating the pregnancy. Second trimester amniocentesis is most often used, at around 16 weeks' gestation. A needle is inserted through the abdominal wall into the uterus to remove amniotic fluid. Early amniocentesis or chorionic villus sampling (CVS) to withdraw placental tissue can be done  before 15 weeks. Either a transabdominal or vaginal (transcervical) approach is used for CVS.

We identified a total of 16 randomised controlled trials for the review. One study of 4606 women in a low-risk population found that a second trimester amniocentesis increased spontaneous miscarriages, 2.1% versus 1.3% with no intervention.

Early amniocentesis was not a safe early alternative to second trimester amniocentesis because of increased pregnancy loss and a higher incidence of deformed or club foot (talipes). It is also technically more demanding and involves a greater number of needle insertions, laboratory failures and false negative results.

Transcervical CVS also increased the risk of total pregnancy compared with a second trimester amniocentesis, mostly because of spontaneous miscarriages. Transabdominal CVS may be safer than the transcervical route, but the data are limited. Transcervical CVS is also more technically demanding than transabdominal CVS, with more failures to obtain sample and more multiple needle insertions required. It is more likely to cause vaginal bleeding immediately after the procedure, in approximately 10% of women.

Authors' conclusions: 

Second trimester amniocentesis is safer than early amniocentesis or transcervical CVS, and is the procedure of choice for second trimester testing. Transabdominal CVS should be regarded as the procedure of first choice when testing is done before 15 weeks' gestation. Diagnostic accuracy of different methods could not be assessed adequately because of incomplete karyotype data in most studies.

Read the full abstract...

A major disadvantage of second trimester amniocentesis is that the results are available relatively late in pregnancy (after 16 weeks' gestation). Chorionic villus sampling (CVS) and early amniocentesis can be done in the first trimester of pregnancy and offer an earlier alternative.


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's Trials Register (January 2008).

Selection criteria: 

All randomised trials comparing amniocentesis and CVS by either transabdominal or transcervical route.

Data collection and analysis: 

Two review authors independently assessed eligibility and trial quality and performed data extraction.

Main results: 

We included a total of 16 randomised studies.

One study in a low-risk population (N = 4606) with a background pregnancy loss of around 2% found that a second trimester amniocentesis will increase total pregnancy loss by another 1%. This difference did not reach statistical significance and the confidence intervals (CI) around this excess risk were relatively large (risk ratio (RR) 1.41; 95% CI 0.99 to 2.00). In the same study, compared with no intervention, the increase in spontaneous miscarriages following second trimester amniocentesis was statistically significant (2.1% versus 1.3%; RR 1.60; 95% CI 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 (RR 4.61; 95% CI 1.82 to 11.66).

Compared with a second trimester amniocentesis, transcervical CVS carries a higher risk of pregnancy loss, although the results are quite heterogeneous. One study compared transabdominal CVS with second trimester amniocentesis and found no significant difference in the total pregnancy loss between the two procedures.

Transcervical CVS is more technically demanding than transabdominal CVS, with more failures to obtain sample and more multiple insertions. However, the results related to comparative pregnancy loss between transabdominal and transcervical CVS are inconclusive, with significant heterogeneity between studies.