A medical technique sometimes used during pregnancy (after the fourteenth to sixteenth weeks) whereby a hollow needle is inserted through the abdomen and into the mother's womb to pierce the amniotic sac (the so-called "bag of waters" that surrounds the fetus) in order to withdraw some of the liquid to examine it for chromosomal evidence of the sex of the unborn child as well as evidence of certain diseases or defects of the developing infant. The risk to the unborn child is statistically low, but the damage that may be induced is grave. Moreover, it is often used as a prenatal screening. When performed for such reasons, amniocentesis is forbidden on Catholic moral principles. But even for a good purpose it is morally questionable because of the risk to the life of the unborn child.