At what gestational age should amniocentesis (amniotic fluid testing) be performed?

From the Guidelines

Amniocentesis should typically be performed between 15 and 20 weeks of gestation, with the optimal time being around 16-18 weeks. At this stage, there is sufficient amniotic fluid present to safely remove the small amount needed while minimizing risks to the pregnancy. Performing the procedure earlier than 15 weeks is associated with higher rates of procedure-related pregnancy loss and technical failure due to insufficient fluid volume. Waiting until after 20 weeks is possible but may delay important diagnostic information that could influence pregnancy management decisions. The timing also allows for adequate cell growth if genetic testing is being performed, as the amniotic fluid contains fetal cells that need to be cultured in a laboratory.

Some key points to consider when deciding on the timing of amniocentesis include:

• The risk of pregnancy loss from sonographically directed amniocentesis is approximately 1 in 300 – 600, as suggested by more recent literature 1.
• The American College of Obstetricians and Gynecologists recommends that all pregnant women, regardless of maternal age, should have the option of having an invasive diagnostic test to definitively identify fetal aneuploidies after appropriate counseling including risks and benefits 1.
• Patients should be counseled that results typically take 1-2 weeks for standard karyotyping, though rapid FISH testing can provide preliminary results for common chromosomal abnormalities within 24-48 hours.
• Prior to the procedure, patients should receive genetic counseling to understand the benefits, limitations, and risks, including a small risk of pregnancy loss.

It's worth noting that while there are multiple studies that support the optimal timing of amniocentesis, the most recent and highest quality studies are from 2008 1. However, the key findings and recommendations from these studies are consistent and support the optimal timing of amniocentesis between 15 and 20 weeks of gestation.

From the Research

Amniocentesis Timing

• Amniocentesis is typically performed between 15 and 20 weeks of gestation 2.
• This prenatal test is used to detect chromosome abnormalities, such as trisomy, and other genetic defects.
• The test involves withdrawing a sample of amniotic fluid from the uterus, which contains fetal cells that can be analyzed for genetic abnormalities.

Factors Influencing Amniocentesis

• Maternal age is a significant factor in determining the risk of chromosome abnormalities, with older mothers being at higher risk 2, 3.
• The use of Doppler ultrasound during amniocentesis can reduce the risk of sample contamination, particularly in multiparous patients with an anterior placenta 4.
• The thickness of the needle used for amniocentesis can also impact the risk of sample contamination, with thicker needles (20 G) resulting in less contamination than thinner needles (21 G) 4.

Alternative Prenatal Testing Options

• Non-invasive prenatal testing (NIPT) is a alternative option for detecting chromosome abnormalities, which involves analyzing cell-free DNA in the maternal blood 2, 3, 5, 6.
• NIPT can detect fetal sex chromosome abnormalities and copy number variation, but its positive predictive value is relatively low, and further prenatal testing and genetic counseling are still required 3.
• The detection rate of fetal sex chromosome abnormalities and total abnormalities is significantly higher in pregnant women over 35 years of age 3.