Prenatal Diagnosis - Invasiveness

Invasiveness

Diagnostic prenatal testing can be by invasive or non-invasive methods. An invasive method involves probes or needles being inserted into the uterus, e.g. amniocentesis, which can be done from about 14 weeks gestation, and usually up to about 20 weeks, and chorionic villus sampling, which can be done earlier (between 9.5 and 12.5 weeks gestation) but which may be slightly more risky to the fetus. One study comparing transabdominal chorionic villus sampling with second trimester amniocentesis found no significant difference in the total pregnancy loss between the two procedures. However, transcervical chorionic villus sampling carries a significantly higher risk, compared with a second trimester amniocentesis, of total pregnancy loss (relative risk 1.40; 95% confidence interval 1.09 to 1.81) and spontaneous miscarriage (9.4% risk; relative risk 1.50; 95% confidence interval 1.07 to 2.11).

Non-invasive techniques include examinations of the woman's womb through ultrasonography and maternal serum screens (i.e. Alpha-fetoprotein). Blood tests for select trisomies (Down syndrome in the United States, Down and Edwards syndromes in China) based on detecting fetal DNA present in maternal blood have become available. If an elevated risk of chromosomal or genetic abnormality is indicated by a non-invasive screening test, a more invasive technique may be employed to gather more information. In the case of neural tube defects, a detailed ultrasound can non-invasively provide a definitive diagnosis.