The amniotic fluid index (AFI) measures the amount of amniotic fluid in the uterus during pregnancy. It is typically calculated during routine ultrasounds and can help determine fetal and maternal health. High or low levels of fluid can lead to complications during birth or development of the baby. Healthcare providers may intervene by adding or draining fluid or suggesting further testing.
The amniotic fluid index (AFI) is a measure and scale of the amount of amniotic fluid present in the uterus while a woman is pregnant. Typically, the calculation of the AFI is determined as part of routine ultrasound examinations, around the 20th week of pregnancy or shortly thereafter. Interest in fluid levels may show up sooner if the measurement is likely to be too high or too low, and conversely, some women never measure their fluid levels unless they have an ultrasound. The measurement and its comparison to the index can be important in helping determine fetal and maternal health and to ensure that the right amount of fluid is present to support fetal development.
In the test to determine the amount of amniotic fluid, the sonographer or ultrasound specialist divides the uterus into four quadrants and measures the volume of fluid in each quadrant with an ultrasound machine. The machine then calculates the total level of fluid volume that exists in the quadrants, arriving at a number in centimeters that can tell if the levels are problematic. “Problematic” is defined as having too much or too little fluid, which are also called polyhydramnios and oligohydramnios, respectively.
Polyhydramnios is diagnosed if a woman’s amniotic fluid index is 24 centimeters or higher. Oligohydramnios is diagnosed if the IFI is 5 centimeters or less. Women with a measurement that is close to one of these numbers, such as 6 or 23, might be closely observed and re-evaluated at a later date.
The importance of evaluating pregnant women with respect to the index is due to the problems associated with high and low levels of amniotic fluid which can complicate a birth or cause significant problems in the development of the baby. Low fluid levels can suggest maternal health issues, such as gestational diabetes or autoimmune conditions such as lupus. They are also associated with certain fetal birth defects, particularly of the kidneys, and with a higher incidence of problems during labor, including premature labor and an increased risk of a baby being deprived of oxygen during the birth process.
When a woman has an amniotic fluid index measurement that is too high, this is indicative of some forms of birth defects, especially those that can affect a baby’s ability to swallow fluids. Other causes occur more rarely but include an incompatibility between the Rh factors in the mother’s and baby’s blood or situations where twins have abnormal communications that threaten the health of one twin, which is called a twin-to-twin transfusion.
The AFI isn’t always an indication to do anything but watch carefully, and many women who have a low or high measurement have safe labor and healthy babies. When healthcare providers intervene, they may choose to add fluid if the AFI is low or drain it when it is too high. Other interventions might be suggested based on the ultrasound results. For example, suspicion of some birth defects might warrant more extensive ultrasounds, exams, or chromosomal testing to diagnose them prenatally.
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