What might affect an amniocentesis?

Reasons you may not be able to have an amniocentesis or why the results may not be helpful include:

• If there is blood from your fetus in the amniotic fluid. This can falsely increase the level of the substances alpha-fetoprotein (AFP) and acetylcholinesterase (AChE) levels, which test for neural tube defects.
• If the amniotic fluid is exposed to light. This can falsely lower bilirubin levels.
• If there is blood or meconium in the fluid. This may cause an incorrect result for the test that checks to see whether your baby's (fetus's) lungs are mature.