During pregnancy, a woman’s uterus houses and protects a developing fetus for about 9 months. Within the uterus, the fetus is surrounded by fluid, which in the 2nd and 3rd trimester is produced primarily by fetal urination. This fluid, called amniotic fluid, allows the fetus to float and move within the uterus. It also cushions and protects the fetus from injury and helps to maintain a constant temperature in the uterus. Pregnant women who are over 35 or have a family history of genetic disorders may undergo a procedure called an amniocentesis. In this procedure amniotic fluid is used to test for infections, genetic disorders, metabolic problems, and fetal lung maturity. During the procedure, the position of the fetus is first located using ultrasound. Then a long, slender needle is inserted through the skin of the abdomen into a safe location in the uterus and about 1 ounce of amniotic fluid is withdrawn. Following the procedure, the fetus will be monitored by ultrasound for a brief period. And, the amniotic fluid will be naturally replaced in about 3 to 4 hours.