Oligohydramnios

Oligohydramnios means that there is an inadequate amount of amniotic fluid surrounding the baby in the womb. This watery compound supports the baby in a number of different ways, and it is important that there is the correct volume i.e. not too much and not too little, so as not to compromise to the baby’s well-being. 

What Does Amniotic Fluid Do?

  • It cushions and supports the baby in the womb
  • It helps the baby to move in the womb; particularly with turning
  • It helps in the development of the baby’s lungs, skin, limbs and muscles
  • It moves in and out of the baby’s lungs as it “breathes” and helps with lung maturation
  • The baby swallows amniotic fluid so there is a constant cycling and recycling through their body 

Where does Amniotic Fluid come from?

Early in the pregnancy, amniotic fluid is formed in the amniotic sac. Fluid comes directly from the mother’s circulating fluid volume and helps to support the baby before its kidneys take over this role. From around 11 weeks of gestation, the baby’s kidneys produce urine which is the source of the majority of amniotic fluid. 

Amniotic fluid or liquor is generally clear and has a distinctive, almost ammonia type smell. If a baby passes a bowel motion (meconium) whilst still in the womb, the amniotic fluid changes from being clear to speckled with black fragments or to a greenish colour, caused by the baby’s black meconium. 

How is it measured?

There are a few ways that the volume of amniotic fluid can be measured. An Amniotic Fluid Index (AFI) can be carried out under ultrasound. This is the sum of the deepest, most unobstructed, vertical fluid levels in each of the 4 quadrants of the womb.

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  • A measurement of between 7 and 18 cm is within a normal range
  • A measurement of less than 5 to 6 cm is classified as oligohydramnios
  • A measurement of greater than 18 cm is diagnosed as being polyhydramnios (too much amniotic fluid)

Oligohydramnios is suspected when there is less than 500 ml of amniotic fluid between weeks 32-36 of pregnancy. However, every pregnancy is unique and a smaller volume of amniotic fluid is not always a cause for concern. It is normal, for example, for the amniotic fluid to reduce in volume as the expected date of confinement (EDC) comes closer. 

According to The American Pregnancy Association, around 8% of pregnant women can have low levels of amniotic fluid, but only ½ (4%) of them end up being diagnosed with oligohydramnios. 

Mothers with oligohydramnios may not appear as large as they would normally be at their level of gestation. Appearing small and undersize can provide a clue as to whether there is sufficient fluid surrounding the baby. There may also be a change in the level of foetal movements, with a slowing down of activity, particularly of whole body movements. This is because amniotic fluid acts as a buffer to support the somersaults and tumbles babies do when they are still in the womb. 

What Causes Oligohydramnios?

  • Being overdue. Women who are two weeks or more past their due date are more at risk as amniotic fluid volume decreases. This can be by as much as 50% from 42 weeks of gestation.
  • It is more common in babies who have a congenital abnormality.
  • Babies who have abnormal kidneys or a problem with their urinary tract can produce less urine, which can result in lower amniotic fluid volume.
  • It is more common with babies who have particular chromosomal abnormalities.
  • If there is an interruption in the blood flow via the placenta, this affects the amount of blood flowing to the baby and consequently the amount of urine they produce.
  • When there is a leak or a rupture of the membranes. This may not always be obvious, particularly in cases where a mother has a hind water or slow leak of her amniotic fluid.
  • When a mother is unwell, has become dehydrated, has preeclampsia, problems with her own kidneys or gestational diabetes. 

What Can Happen with Oliogohydramnios?

  • Complications can arise particularly in women who are past their due date of pregnancy.
  • Pressure on the baby as well as their limbs and organs. This is because there is insufficient cushioning between the baby and the mother’s uterine wall.
  • An increased incidence of miscarriage or stillbirth.
  • Intrauterine growth retardation in the baby, prematurity and not being as robust and well at birth as they would otherwise be.
  • Lung immaturity in the baby with an increased likelihood of breathing difficulties after birth.
  • Pressure on the umbilical cord, which then affects the transfer of blood, oxygen and nutrients to the baby.
  • Oligohydramnios in early pregnancy can result in limb pressure deformities in the baby.
  • Oligohydramnios which continues for some time and is severe can lead to a range of problems in the baby. 

When is Oligohydramnios Likely to Happen?

It happens mostly in the third trimester of pregnancy. When it is diagnosed at the 20 week scan, the cause is usually due to renal (kidney) complications in the baby. 

What is the Treatment?

This depends on the stage of pregnancy. Monitoring, abdominal measurement, regular ultrasounds and recording of the baby’s movements are all fairly standard management options. If the volume is very low, then additional fluid in the form of saline, can be infused into the mother’s womb during her labour to alleviate any potential compression on the umbilical cord. If a mother has been unwell and becomes dehydrated, then infusing her with intravenous fluids helps to restore her circulating fluid volume, which in turn increases the amniotic fluid balance to a safe level.

 
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