Pregnancy 18 weeks amniotic fluid is less but the depth of amniotic fluid is about 3~8cm is normal, the depth of amniotic fluid is less than 3cm belongs to the amniotic fluid is too little, may be less water, premature rupture of fetal membranes, fetal deformity and other reasons. The depth of amniotic fluid in 18 weeks of pregnancy is about 3~8cm is normal phenomenon, after pregnancy, fetal development and growth of different, pregnant women living environment, lifestyle is different, resulting in pregnant women in the uterine cavity amniotic fluid amount is not the same, some people on the side of more, some people on the side of less, as long as in the normal range is normal. The depth of amniotic fluid in 18 weeks of pregnancy is less than 3cm belongs to the amniotic fluid is too little, may be the pregnant woman drink less water lead to the fetus blood supply is reduced, the fetus urine production is reduced will cause the amniotic fluid volume is low. Premature rupture of membranes at 18 weeks of pregnancy, amniotic fluid will continuously flow out of the amniotic cavity, which will also lead to low amniotic fluid volume. Fetal presence of urinary tract abnormalities amniotic fluid will also be reduced, is due to fetal urine is the main component of amniotic fluid, fetal urine is reduced after the amniotic fluid volume will also be reduced. Therefore, there are many reasons for the decrease in amniotic fluid volume in 18 weeks of pregnancy, and it is recommended to consult a doctor in time to clarify the exact cause of the problem.