Amniotic fluid or Liquor amnii lies in the amniotic sac that protects the fetus, in the womb until the fetal membranes remain intact, this surrounds the fetus throughout pregnancy, lined by 2 fetal membranes inner amnion & outer chorion.
-It is an alkaline fluid.
– Its Osmolality is 260 mOsmo/L, which is lower than the maternal and fetal plasma (Osmolality 280 mOsmo/L).
– It is recycled every 3 hours.
– It is composed of 99% water & 1% dissolved substances that are a mixture of organic, inorganic substances, exfoliated fetal cells, Lanugo, vernix caseosa.
-In the early weeks of pregnancy (1-12 weeks), Maternal plasma (filtration of maternal plasma across the fetal membranes) contribute to it.
– Fetal skin (Transudation by fetal vessels) in next 12 – 20 weeks, until skin, becomes keratinized by 22 -25 weeks of gestation.
– Fetal urine (main contributor) after 18 weeks till term contributes to it. (Fetus starts urine production by 12 weeks, but after 18 weeks only it contributes significantly)
There are 4 main factor that helps in circulation of fluid –
- Fetal urine contributes about 650-1000 ml/day at term.
- Fetal lung secretion contributes about 350 ml/day.
- Fetal swallowing around 500-1000ml/day helps in the reabsorption of fluid.
- The intramembranous flow of fluid about 400 ml /day also reabsorbs fluid across the fetal membrane into the fetal circulation as the osmolality of fluid is lower than the fetal plasma.
12 weeks – 50 ml
16 weeks – 200 ml
40 weeks – 800 ml
42 weeks – 200 ml
Color of Fluid:
Normal Color at Term – Pale straw-colored & turbid
Colorless – Preterm
Dark red – concealed hemorrhage (APH)
Green -Meconium stained (can cause respiratory distress)
Saffron (greenish-yellow) -Post maturity
Tobacco juice (Brown & black) – IUD
Golden color– Rh incompatibility
- Protect the fetus by a cushioning effect, shock absorber.
- Allows free movement & growth of the fetus by providing a space for the fetus.
- Flushes birth canal when rupture.
- Maintains an even temperature in the sac.
- Hydrostatic pressure helps in cervical dilatation at the time of labor.
- Prevents infection to ascend in the uterine cavity.
Fluid is measured by 2 semi-quantitative methods by using USG.
- AFI (Amniotic fluid index) – In this uterus is divided into 4 equal quadrants & then placed a transducer in each quadrant and measure the fluid in a vertical pocket, and sum all the 4 quadrants. Normal value is 5-25 c.m.
- SDP (Single deepest pocket)- This will measure the single deep vertical pocket for the amount of fluid. The normal value is 2-8 c.m.