Amniotic Fluid

Amniotic Fluid

The rate of amniotic fluid turnover is 500 cc/hr. The volume of Amniotic fluid is maximum between 36-38 weeks (1l) and then decreases such that at term it is roughly 800-900 ml.

Amniotic fluid originates both from maternal and fetal sources:

• In early pregnancy – As an ultrafiltrate of maternal plasma. By the beginning of the second trimester – It consists of extracellular fluid which diffuses through the fetal skin.
• After 20 weeks – Cornification of skin prevents this diffusion and amniotic fluid is composed of fetal urine.

Function of Amniotic Fluid

During Pregnancy

1. It acts as a shock absorber, protecting the fetus from possible extraneous injury.
2. Maintains an even temperature.
3. The fluid distends the amniotic sac and thereby allows for growth and free movement of the fetus and prevents adhesion between the fetal parts and amniotic sac.
4. It has some nutritive value.

During Labor

1. The amnion and chorion are combined to form a hydrostatic wedge which helps in dilatation of the cervix.

Abnormality of Amniotic Fluid

OligohydramniosZ

Oligohydramnios is a condition where liquor amnii is deficient (< 200 ml at term).

Sonographically it is defined as:

• Absence of amniotic fluid pocket.
• Maximum vertical diameter of amniotic fluid pocket less than 2 cm.
• Amniotic fluid index less than 5 cm.

Causes of Oligohydramnios

• Drug (Prostaglandin Synthetase inhibitors and ACE inhibitors).
• IUGR.
• Leaking of fluid following amniocentesis or chorionic villus sampling.
• Maternal conditions like hypertension and preeclampsia.
• Post-term pregnancy.
• Premature rupture of membrane.
• Abruptiochronic.
• Chromosomal anomaly like triploidy.
• Renal anomalies of fetus.

Complications of Oligohydramnios

Fetal Complications

1. Abortion.
2. Deformity due to intra-amniotic adhesions or due to compression.
3. Fetal pulmonary hypoplasia (may be the cause or effect).
4. Cord compression.
5. Fetal growth restriction.

Maternal Complications

1. Prolonged labor.
2. Increased operative interference due to malpresentation.
3. The sum effect may lead to increased maternal morbidity.

Polyhydramnios

Defined when amniotic fluid index (AFI) is > 25 cm or finding of a pocket of fluid measuring 8 cm.

Grades of Polyhydramnios

1. Mild: defined as pockets measuring 8-11 cm in vertical dimension.Z
2. Moderate: defined as pocket measuring 12-15 cm in vertical dimension.
3. Severe: defined as free-floating fetus found in pockets of fluid of 16 cm or more.

Causes of Polyhydramnios

• Fetus produces more urine, e.g.:
  • Multifetal pregnancy (number of fetus is more: more urine).
  • Maternal hyperglycemia/diabetes: Maternal hyperglycemia → Fetal hyperglycemia → Fetal polyuria → increased amniotic fluid.
  • Twin to Twin transfusion syndrome.
• The amount of amniotic fluid will increase if fetal swallowing is impaired as in cases of:
  • Cleft lip and cleft palate.
  • Esophageal atresia or stenosis.
  • Duodenal atresia or stenosis.
  • Bowel obstruction.
  • Anencephaly.
• Other Causes:
  1. Placental Causes: Chorangioma of placenta and circumvallate placenta.
  2. Fetal Causes: Hydrops fetalis, Rubella, syphilis, Toxoplasma infection of fetus, Trisomy (note – Triploidy leads to oligohydramnios), Sacrococcygeal teratoma, Thalassemia of fetus.