The amniotic fluid

The Amniotic fluid

DR.Widad Mahmood

It is the fluid in the amniotic sac surrounding the fetus.

The amniotic fluid has a mixed origin and has heterogeneous composition. It's slightly turbid from the mixture of solid particles derived from fetal skin and amniotic epithelium.

May stained a green or brown if it contain meconium

Production of amniotic fluid :
It’s of both maternal and fetal origin, the relative importance of both mechanisms alters as pregnancy progresses.
In early pregnancy it is from the maternal plasma (ultra filtrate).

By the 2nd trimester diffusion through the fetal skin.

After 20 weeks, the fetal kidneys play an increasing role in the production of the amniotic fluid.( at term 500ml/day).

Pulmonary fluid contributes a small portion

Volume:
varies according to the gestational age:
12 weeks – 50 ml
20 weeks- 400 ml
36 weeks- 800ml - 1 liter
At term - it reduces to approximately 700ml

At term the specific gravity of the fluid is 1.010.

important contents near term are:
sodium 130mmol/l
urea 3-4mmol/l
protein 3 g/l
lecithin 30-100mg/l
alpha-fetoprotein 0.5-5 mg /l
It’s mildly bacteriostatic ,contain traces of steroid, non steroid hormones and enzymes.
PH = 7.

Colour

In early pregnancy it is colourless
At term becomes pale straw coloured due to presence of exfoliated lanugo and epidermal cells from fetal skin.

Abnormal appearance:

Greenish- due to presence of meconium
Golden yellow-due to presence of bilirubin resulting from fetal cell hemolysis due to Rh incompatibility
Greenish yellow- in post maturity
Dark brown/ blood stained – due to altered blood in accidental haemorrhage
Prune juice/dark brown- in presence of retained dead fetus.


Functions of amniotic fluid:
During pregnancy:
Act as a shock absorber to protect the fetus from external injury.
Maintains the fetal temperature.
Allows free movement and growth of fetus.
Prevents adhesion formation between the fetal parts and the amniotic sac.
Has some nutritive value because of small amount of protein and salt content.

During Labour:

It forms hydrostatic wedge to help dilatation of the cervix.
During uterine contractions , the amniotic fluid in the intact membranes prevents interference with placental circulation.
Provides pool for the fetus to excrete urine.
Protect the fetus from the ascending infections by its bactericidal action.

Clinical importance:

measurement of amniotic fluid index (AFI ), Study of amniotic fluid helps in knowing the well being and maturity of fetus
Intramniotic instillation of prostaglandins and hypertonic saline can be used for induction of abortion
Artificial rupture of membranes to drain liquor is a method of induction and augmentation of labour
Excess liquor (polyhydroamnios), less liquor known as (oligohydroamnios ) can be estimated by ultrasound .


Clinical assessment of amniotic fluid volume unreliable & objective definitions of abnormalities in amniotic fluid volume depends essentially on non-invasive method such as ultrasound. These include;

1-the deepest vertical pool (DVP)

2-amniotic fluid index (AFI)

Deepest vertical pocket (DVP):

it's performed by assessing a pocket of maximal depth of amniotic fluid on ultrasound which is free of umbilical cord & fetal parts.
DVP between 2-8 cm considered normal.
< 2 cm considered as oligohydromnios.
> 8 cm considered as polyhydromnios.

Amniotic fluid index (AFI):

is a score (expressed in cm) given the amount of amniotic fluid seen in ultrasound of a pregnant uterus. It's an index for fetal well-being & it's a part of biophysical profile. To determine AFI, we use 4 quadrant technique, when the deepest vertical length of each pocket of fluid is measured in each quadrant & summing them.
An AFI between 8-18 is considered normal
< 5 considered as oligohydromnios
> 20-24 considered as polyhydromnios