Pregnancy Complications

Oligohydramnios (Low Amniotic Fluid)

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Oligohydramnios is better known as having low amniotic fluid. The opposite condition is called polyhydramnios. This is where there is too much amniotic fluid present in the womb. Measured by the amniotic fluid index (AFI), oligohydramnios is diagnosed as having less than the fifth percentile of fluid for your gestational age. The amount of amniotic fluid alters week by week and can be compared to the norms at any stage of pregnancy.

Amniotic fluid

During pregnancy, the uterus holds a fluid filled sac in which the baby develops. This is the amniotic sac and it consists of two membranes, the amnion and the chorion. These membranes ensure your baby is contained within the amniotic fluid throughout your pregnancy. When the baby is ready to be born, these membranes break open and out flows the fluid. This is also known as your waters breaking. The purpose of the amniotic fluid is to protect your baby against bumps and squashes. It also works to protect your baby against any infections and it helps with the maturation of your baby's digestive system and lungs.

Normal levels of amniotic fluid

The amount of fluid contained in the amniotic sac changes throughout the day. The levels rise and fall as the baby routinely swallows amniotic fluid and then passes it out of their body as urine. The volume of fluid increases as your pregnancy progresses. At 10 weeks gestation the volume is about 30ml. It peaks at 34-36 weeks of gestation with about 1 litre of fluid. The volume decreases towards term, with an average of about 800ml at 40 weeks. During the second half of pregnancy, the excretion of the baby's urine becomes a major source of the amniotic fluid production. Fluid secreted by the baby's respiratory tract is also a contributing factor. If there is not enough amniotic fluid surrounding the baby, it is known as oligohydramnios. If there is too much amniotic fluid it is called polyhydramnios.

Diagnosing oligohydramnios

Low amniotic levels may be suspected if your baby seems to be smaller than expected for its gestational age. Your midwife should pick this up when she either measures your bump or during an ultrasound. If you have already had a particularly small baby or one with a low birth weight, or you have blood pressure problems, lupus or a specific medical condition, your amniotic levels will be routinely checked. During an ultrasound the sonographer can check your fluid volume by measuring the depth of the pockets of amniotic fluid in four divided sections of the womb. By combining the four measurements they can score you on the amniotic fluid index (AFI). These are then compared to normal levels which have been determined for each gestational week. Measuring the deepest vertical pocket of fluid gives another measurement which can be compared to the norm. Less than two centimetres deep is considered low.

Causes of oligohydramnios

There are numerous clinical reasons as to why you may have developed low amniotic fluid levels, but the most common causes are:

  • Placenta problems.
  • Your waters have broken.
  • The baby has a problem.
  • Any medication you are on.
  • Maternal dehydration.

Placenta problems

There are many conditions that may cause your placenta to stop supplying enough nutrients and blood to your baby. Conditions include high blood pressure, lupus, diabetes and pre-eclampsia. If your placenta does not function efficiently, it can affect your baby's growth. You are most likely to be monitored regularly if you suffer from any of these conditions to ensure your baby's development is on track and that amniotic fluid levels are sufficient.

Your waters have broken

While sometimes there is a big gush of fluid when your waters break, you may find that all you have is a slow leak. This is sometimes wrongly thought to be an accidental urine leak. Your amniotic sac may break, allowing a large rush of fluid to escape at once, or it may just tear slightly resulting in a slow leakage of fluid. If you are unsure if your waters have broken or not call the doctor or midwife immediately. You are at risk of infection if your waters break early and you do not then proceed into labour. Antibiotics are sometimes prescribed to ward off infection. Depending on your gestational stage of pregnancy you may need to be induced for labour. If you are not induced you will be monitored until labour begins naturally.

The baby has a problem

Low amniotic fluid can be caused by your baby not passing enough urine. This can be a sign that there is a problem with the heart or kidneys or that there is some form of chromosomal abnormality with the baby. This is usually identified at your mid-pregnancy anomaly ultrasound scan. It is hard to hear that your baby may have a health problem. Referral to a specialist foetal medicine centre will provide you with experienced midwives and consultants who are skilled in looking after pregnant women and their unborn babies with health issues.

Mother's medication

It is known that some medication can cause oligohydramnios. ACE inhibitors are one such type of drugs which are used in the treatment of high blood pressure. Ibuprofen is a non-steroidal anti-inflammatory which can also cause low amniotic fluid. These medications are usually avoided and not prescribed during pregnancy. If you are unsure as to whether your regular medication is a cause for concern in pregnancy, check with your doctor as soon as possible.

Maternal dehydration

The amniotic fluid volume is largely affected by the mother's fluid balance. Increasing your uptake of fluid will increase the amniotic fluid levels in women with oligohydramnios. Ensure you are always well hydrated by drinking plenty of water. Make sure you eat well too and get lots of rest.

How the baby is affected

Factors such as what is causing the low amniotic fluid levels, how low the levels are and how far you are in your pregnancy will determine how much your baby is affected by oligohydramnios. If low fluid levels are detected during the first trimester or possibly even the early part of the second trimester, then you may well experience a miscarriage due to chromosomal or congenital factors. If you are diagnosed with oligohydramnios well into the third trimester, you will most likely just need to be monitored closely until the birth.

Prevention or cure of oligohydramnios

Unfortunately, there is not a cure for having low amniotic fluid volumes. The best thing you can do for yourself and the baby is to stay well hydrated, eat a healthy diet and get as much rest as possible. Worry can stop you sleeping, so try hard not to let the anxiety overtake your emotions and prevent you from getting a good night's sleep. You and your baby will be monitored closely and will be well looked after by your team of doctors and midwives during your pregnancy, giving you the best care possible.

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