What is amniocentesis?

Amniocentesis is a diagnostic test. A fine needle is passed through the abdomen, under continuous ultrasound guidance, to take a small sample of amniotic fluid containing cells shed by your baby. Those cells are examined directly. Diagnostic is the key word. Unlike NIPT, which estimates a chance, amniocentesis gives a definitive answer. Timing: RCOG guidance is that amniocentesis should not be performed before 15+0 weeks. If you need an answer earlier, CVS is the alternative, from 10+0 weeks and preferably from 11+0. What it can find: all chromosomes counted and examined, detecting both the trisomies NIPT screens for and the many abnormalities it misses. It can be extended with chromosomal microarray for smaller deletions and duplications, and with targeted testing for a specific condition known in your family. The risk: RCOG states the additional risk of miscarriage following amniocentesis by an appropriately trained operator is likely to be below 0.5%. For twins, around 1%. One technical point worth knowing: the first 1 to 2 mL of the sample is normally discarded, which reduces the chance of maternal cells contaminating the result. Chromosomal mosaicism is reported in approximately 0.25% of amniocentesis specimens. It is always your choice. Some women want certainty; others prefer to avoid any procedural risk. Both are reasonable, and the right answer depends partly on what you would do with the information.