Oligohydramnios means decreased amniotic fluid. There are several methods to evaluate the amniotic fluid in pregnancy.Amniotic fluid is made mostly by the baby's urine. The amount of amniotic fluid increases until 36 weeks, and then normally decreases.
The amount of amniotic fluid is measured either by checking the largest pocket of fluid by ultrasound or by measuring the largest amniotic fluid pocket in each of four quadrants and then adding those up. This is called the AFI or Amniotic Fluid Index.
Finding an amniotic fluid pocket that's at least 2 cm in vertical diameter is usually considered adequate
Alternatively, any AFI more than 25 cm gives you too much amniotic fluid, and any AFI less than 5 cm during the third trimester is too low.
Because the AFI has too many "false positive" results (too many people diagnosed with too low fluid when in fact the outcome was OK), the single pocket amniotic fluid evaluation has recently been found more reliable than the AFI. Any single pocket that's over 2 cm is considered adequate.
The following are cause for decreased amniotic fluid (oligohydramnios):
- Rupture of the fetal membranes.
- Placental insufficiency (high blood pressure, post-dates).
- Blockage or malformations of the bay's urinary tract: renal agenesis (no kidneys), polycystic kidneys, and obstruction of the urinary tract.