Overview of Sonography Evaluation of Amniotic Fluid
During pregnancy, a fluid called amniotic fluid surrounds the developing baby, or fetus. This fluid is very important because it helps to protect and nourish the fetus. It creates a clean environment, controls the temperature, and provides a cushion against any bumps or jolts. The amniotic fluid also helps the baby to move around, which is important for the development of muscles and bones. It also helps organs to grow, especially the lungs.
The amount of amniotic fluid in the womb increases gradually until about the 34th week of pregnancy. After that, the amount of fluid lessens slightly until the 40th week (which is the normal time to give birth), and then it decreases more quickly after the 42nd week.
At the start of pregnancy, the amniotic fluid is similar to the mother’s blood serum. As the fetus grows, the composition of the fluid changes. For example, the levels of sodium decrease while those of urea, creatinine, and uric acid increase. The amniotic fluid also contains various hormones. At first, there are hardly any solid particles in the fluid. However, by the 16th week of pregnancy, a large number of cells from the baby’s skin and respiratory tree are present. Some of these cells are used in amniocentesis, a test to check the fetus’s health. Babies in the womb usually don’t pass stool, but if they do, it’s called meconium and can turn the amniotic fluid green. This might be a sign that the baby is under stress.
The amount of amniotic fluid is controlled by three main factors: the placenta, which transports water and other substances, the fetus, which produces urine and swallows fluid, and the mother, who affects the balance of fluid.
Especially before the 16th week, the fluid is maintained largely through a process called ‘intramembranous flow’. After this, the baby’s developing kidneys take over and produce urine which contributes to the fluid. Checking the amount of the amniotic fluid can give doctors information about how well the baby is doing.
If the mother or placenta isn’t working as it should, for example, if the mother is dehydrated, or if there’s a problem with the placenta, the amount of amniotic fluid can be affected.
Doctors assess the amount of amniotic fluid to check for problems such as oligohydramnios (too little fluid) or polyhydramnios (too much fluid). These conditions can indicate problems with the mother or the baby. Ultrasound scans are often used to do this. Some research suggests that ultrasound isn’t as good as other methods at measuring abnormal amounts of fluid. However, ultrasound is safe and gives real-time results – that is, the information is available immediately.
Doctors can use different techniques to check the amount of amniotic fluid. The main ones are the ‘deepest vertical pocket’ and the ‘amniotic fluid index’. Current guidelines suggest that these methods reduce the risk of getting the diagnosis wrong.
Understanding how amniotic fluid is produced, controlled, and assessed is very important for taking care of both the mother and the baby during pregnancy. Any problems with the amount of amniotic fluid can have a significant effect on the results at birth.
Anatomy and Physiology of Sonography Evaluation of Amniotic Fluid
At the start of pregnancy, the amniotic fluid, which is the liquid surrounding the unborn baby, largely comes from the mother and is similar to plasma because of various pressures. By the eighth week, the baby’s kidneys start working, producing urine, which becomes an important part of the amniotic fluid from around the 23rd to 25th week onwards. By this time, the baby’s urine accounts for the majority of the amniotic fluid, with up to 1500 mL being made everyday. In the last 20 weeks of the pregnancy, the baby also adds up to 350 mL per day to the amniotic fluid through its breathing and stomach secretions. This fluid then gets recycled, mainly by the baby swallowing it.
Other ways the amniotic fluid is recycled include pathways through the placenta, umbilical cord, and the baby’s skin, which all help to pass the fluid back and forth between the unborn baby and the amniotic fluid (about 400 mL per day). The volume of amniotic fluid increases steadily in the first half of the pregnancy, reaching its maximum at around the 36th or 38th week with an average of 800 mL. After that, it can decrease by 8% per week, reaching volumes as low as 400 mL by the 42nd week.
Typically, the mother’s serum, which is the liquid part of the blood, remains a bit less concentrated than the baby’s serum. This helps the flow of nutrients to the baby. After the unborn baby’s skin becomes fully formed, the amniotic fluid concentration is kept lower than the baby’s blood, allowing continuous flow of nutrients through the placenta. However, if the mother becomes dehydrated, this balance can get disrupted, impacting the volume of amniotic fluid. The fetus can also affect the flow due to hormonal changes which can alter the permeability of the membranes.
The term ‘Oligohydramnios’ is used when the volume of amniotic fluid is low. This can happen due to a number of reasons such as blockages in the unborn baby’s urinary tract or low urine production by the unborn baby’s kidneys. Inability to see any amniotic fluid at all is called “anhydramnios.” Blockages in the urinary tract can be particularly harmful for the baby. Low urine production usually signifies poor blood flow to the baby’s kidneys, often caused by the blood being diverted away from the kidneys to support the heart and the brain. This mechanism is also seen in critically ill adults. Another common reason for low amniotic fluid is the rupturing of the protective layer around the unborn baby, causing the fluid to leak out.
‘Polyhydramnios’ refers to a large volume of amniotic fluid. The increased volume is usually due to the baby swallowing less fluid than normal or producing more fluid. Having too much amniotic fluid can stretch the uterus, increasing the risk of premature contractions, delivery and rupture of the water bag before labor begins. It also increases the risk of heavy bleeding after delivery. If the baby is unable to swallow the typical amount of fluid due to issues with the GI tract, neurological disorders, or blockages in the esophagus, it may result in polyhydramnios. Increased fluid production can happen when the baby urinates excessively, often seen in cases where the mother’s blood sugar levels are uncontrolled due to diabetes.
Why do People Need Sonography Evaluation of Amniotic Fluid
Checking the amount of fluid in the womb, called an “amniotic fluid assessment,” is part of the regular check-up when a woman is pregnant. Sometimes, the amount of fluid present can indicate some health issues, and these findings can be unexpected.
Many experts say that all ultrasound exams during pregnancy should include a check on the amniotic fluid levels. There are certain risk factors that can change how much fluid is in the womb and might require more frequent checks. These risks can include conditions like severe high blood pressure requiring medication, complications from high blood pressure that are brought on by pregnancy, a baby not growing as expected, certain physical abnormalities in the baby, and pregnancies that are past due.
Also, if the size of the mother’s belly does not match up with how far along the pregnancy is, doctors will want to measure the amount of amniotic fluid.
Checking amniotic fluid is also a way to see if the membranes holding the baby’s fluid have broken. As well, it is part of a more comprehensive check-up called the “biophysical profile.” This check-up involves a five-part assessment of the baby’s overall health.
When a Person Should Avoid Sonography Evaluation of Amniotic Fluid
Even though ultrasound scans during pregnancy are usually safe, they should only be done when there is a medical need. This means they should only be done when they can help a doctor make a decision about your treatment or understand what’s happening in your pregnancy. Doctors use the principle of ‘as low as reasonably achievable’ to ensure that the fetus is exposed to the least amount of heat possible.
The Doppler technique, which uses sound waves to image blood flow, should not be used unless it’s absolutely necessary, because it uses much more energy than regular ultrasound scans. This could potentially expose the fetus to unnecessary risks.
Equipment used for Sonography Evaluation of Amniotic Fluid
Carrying out an ultrasound during pregnancy involves doing scans in real time. This means the doctor can see the baby’s movement and development as it happens, rather than just in static pictures.
In the early stage of pregnancy (the first trimester), instruments called intracavitary transducers might be used to measure the amount of fluid in the womb. These instruments are designed to be used inside body cavities.
However, most checks of amniotic fluid—the liquid surrounding and protecting the baby in the womb—are done in the second and third trimesters of pregnancy. During these checks, the doctor will use a different approach. They’ll scan the abdomen from the outside using a curvilinear transducer. This is a special kind of instrument that uses sound waves to create images, and it can provide a strength of more than 3 MHz. This means it’s very good at picking up fine details, making it easier for the doctor to assess the baby’s condition and development.
How is Sonography Evaluation of Amniotic Fluid performed
An ultrasound scan is used to measure the volume of the amniotic fluid which surrounds and protects the baby in the womb. This fluid level gives us important information about the health of the baby.
During this process, the mother lies flat on her back. The ultrasound technician will hold the ultrasound probe upright and move it in line with the mother’s body. The deepest pocket of amniotic fluid is then found and measured. The fluid will appear as a dark area on the ultrasound screen. To avoid any error, the measurements should not include parts like the baby’s body or umbilical cord. The pocket’s width has to be at least 1 cm, except in certain evaluations where it should be at least 2 cm.
For a different kind of measurement known as the Amniotic Fluid Index (AFI), the womb is divided into four equal sections with the belly button as the center point. The ultrasound technician will then find and measure the largest pocket of fluid in each quadrant. The four measurements are then added up to give the overall AFI. A method called Color Doppler is used to make sure that segments of the umbilical cord, which might not be seen with standard ultrasound imaging, are not included in the measurements.
The DVP measurement is used for pregnancies that are less than 24 weeks or those with multiple babies. Like the AFI, it involves measuring a single deepest pocket of fluid. A normal DVP measurement is between 2 to 8 cm. Measurements less than 2 cm could indicate a condition called oligohydramnios, where there is too little fluid, and readings above 8 cm could suggest a condition called polyhydramnios, where there is too much fluid.
These fluid measurements can vary due to several factors, including differences between different ultrasound operators, the mother’s position during the scan, and her hydration levels. Drinking lots of water before the procedure can help improve the readings, especially in diagnosing oligohydramnios.
The volume of amniotic fluid is a crucial part of the fetal biophysical profile, an ultrasound technique to assess the overall health and well-being of the baby. This method looks at four components within a 30-minute window:
- Baby’s breathing: The baby’s diaphragm should move continuously for at least 30 seconds.
- Baby’s movement: The baby should move its body or limbs at least 3 times.
- Baby’s muscle tone: The baby should actively extend a limb and return it to a bent position, or open and close its hand at least once.
- Amniotic fluid volume: The deepest pocket of fluid should be at least 2 cm.
Additionally, a non-stress test is conducted to monitor the baby’s heart rate. Generally, a full-term and healthy baby not under any stress will show all of these behaviors on an ultrasound. However, preterm babies may not necessarily exhibit all these behaviors. The next steps depend on a variety of factors including how far along the pregnancy is and which of these behaviors aren’t seen.
Possible Complications of Sonography Evaluation of Amniotic Fluid
Harms during an ultrasound to measure amniotic fluid index (the amount of fluid surrounding the fetus) or deepest vertical pool (the deepest pocket of this fluid) are almost never seen. During the ultrasound, some people might feel a bit uncomfortable due to the pressure from the ultrasound wand, or feel light-headed due to the pressure of the growing baby on a large vein in the body (vena cava) when lying flat on their back. Placing a rolled-up towel under a person’s side might help reduce these symptoms by relieving some of this pressure and helping blood flow better.
What Else Should I Know About Sonography Evaluation of Amniotic Fluid?
During pregnancy, doctors regularly measure the amount of fluid in the amniotic sac – the bag-like structure in the womb where your baby lives. This fluid, known as amniotic fluid, helps keep your baby safe and comfortable.
Typically, in a single-baby pregnancy, the normal amount of amniotic fluid – usually measured as Amniotic Fluid Index (AFI) ranges from 5 to 24 cm. When carrying twins or more babies, doctors use a measurement called the Maximum Vertical Pocket (MVP). This measurement identifies the amount of fluid for each baby.
Sometimes, there may not be enough amniotic fluid in the sac, a condition termed ‘Oligohydramnios’. A variety of issues, like problems with the placenta or issues with the baby’s urinary tract, can cause this. Oligohydramnios may lead to various complications, ranging from slowed growth in the baby to severe birth complications. Because of these risks, doctors might recommend inducing labor if Oligohydramnios is detected between 36-37.6 weeks of pregnancy.
On the other hand, there may be too much amniotic fluid, a condition called ‘Polyhydramnios’. This can be due to a few things — like high blood sugar, infections, or some facial abnormalities in the baby. It can also lead to complications like early labor and delivery difficulties. If Polyhydramnios becomes uncomfortable for the mother or threatens her breathing, doctors may need to remove some fluid.
In the case of twins, there could be a unique situation called ‘Twin-twin transfusion syndrome’ where one twin gets too much fluid while the other gets too little; This syndrome can lead to serious complications and requires regular screening to ensure doctors can act fast if needed.