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.