eMedicine Specialties > Radiology > Obstetrics/Gynecology

Polyhydramnios: Imaging

Author: Prabhakar Rajiah, MD, MBBS, FRCR, Registrar, Department of Radiology, Central Manchester and Manchester Children's University Hospitals, UK
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Updated: Mar 26, 2009

Computed Tomography

Findings

CT scanning is generally avoided during pregnancy.

Magnetic Resonance Imaging

Findings

MRI is not essential in the imaging protocol for polyhydramnios, but if it is performed for fetal or maternal imaging, it can also be used for diagnosing polyhydramnios.

A customized medical image postprocessing software package can be used for segmentation and 3-dimensional (3D) modeling. Once the structures of interest in a 3D image volume are segmented, the postprocessing software creates a corresponding 3D surface model and automatically calculates the volume of each 3D reconstruction

Although allowances must be made for a smaller fluid volume (except in polyhydramnios) and for segmentation of the amniotic fluid in vivo being slightly more difficult owing to fetal motion, volumetric measurements are likely to represent the real values.

Degree of Confidence

MRI is good for assessing the volume of amniotic fluid and for diagnosing polyhydramnios. However, MRI is not mandatory for the diagnosis of polyhydramnios.

Kubik et al found that MRI is accurate in measuring the amniotic fluid volume, as it is in measuring placental volume and fetal weight.⁷

A good correlation was obtained between MR volumetry studies and the actual amniotic fluid volume. Although this would not be a cost effective method of diagnosing polyhydramnios, it would be of greater help in monitoring therapeutic response to polyhydramnios treatment. The common sequences used are T2-weighted single-shot fast spin-echo and high-spatial-resolution T1-weighted fast spin-echo images subsequent to a spoiled gradient-echo localizer.

False Positives/Negatives

Measuring the amniotic fluid volume is difficult when the quantity is low because no difference in signal intensity can be noted between a thin rim of fluid and the placenta and uterine wall; this similarity makes postprocessing and automatic segmentation difficult.

Although 3D reconstruction of a fetus is better with a large amount of amniotic fluid present, fetal motion adversely affects image processing and reconstruction.

Use of an automatic threshold for excluding amniotic fluid excludes other tissues containing the same signal intensity, such as fetal brain and fluid-filled fetal organs (eg, the urinary bladder).

Ultrasonography

Findings

Ultrasonography is the main modality for the diagnosis of polyhydramnios and evaluation of the fetus. The following features are assessed in polyhydramnios: amniotic fluid, possibly of multiple pregnancy, chorionicity in multiple pregnancy, fetal macrosomia, fetal thorax, fetal central nervous system, fetal gastrointestinal tract, cervical length, and posttreatment follow-up results.

Amniotic fluid

There are at least 3 methods for measuring amniotic fluid: (1) depth of the deepest vertical pool, (2) the 2-diameter pocket (depth X width of the longest pocket), and (3) the AFI.^8,9,10,11

With the AFI method, the uterus is divided into 4 quadrants. The depths of the deepest vertical pool in the 4 quadrants are measured and added to give the index. Occasionally, at less than 20 weeks, only the right and left lower quadrants are used. The normal index is 5-24. In polyhydramnios, it is more than 24. AFI of a normal population (ie, normative values) corresponding to the gestational age can be noted, and the percentile value of the particular patient can be calculated by using the mean and standard deviation.

The incidence of polyhydramnios can vary with the technique used. A single deep pocket more than 8 cm is diagnostic of polyhydramnios. With the single-pocket technique, the incidence is 0.7% (1.1% for oligohydramnios). With the 2-diameter pocket, the rate is 3% (30% for oligohydramnios), and with the AFI method, the rate is 0% (8% for oligohydramnios). Therefore, the single-deep-pocket method is the best technique because it classifies the least number of cases as being abnormal.

A simple rule of thumb is that in the first trimester, the fluid is more than the embryo/fetus; in the second trimester, the fluid is equal to the fetus; and in the third trimester, the fluid is less than the fetus.

Chorionicity in multiple pregnancy

Twins can be monochorionic or dichorionic. The difference can be assessed by careful observation. In dichorionic twins, the intermembrane septum is thick, with 3 or 4 membrane layers, and the membrane is more than 2 mm. The triangular sign is present and very specific. In dichorionic twins, the cause of polyhydramnios is the same as that in a singleton pregnancy.

In monochorionic twins, the intermembrane septum is thin, and the junction of membranes forms a T shape. In monochorionic twins, the most common cause of polyhydramnios is twin-to-twin transfusion syndrome

Fetal growth

Sonography may be useful in screening for growth retardation.

Fetal gastrointestinal tract

Ultrasonography may be helpful in evaluating the mouth, stomach, small bowel, and abdominal wall.

Fetal bladder dynamics

One technique involves the change in bladder dimension observed over 20-minute intervals. These changes can differentiate fetal polyuria from other causes of polyhydramnios. However, this technique has its limitations. It underestimates the degree of fetal urine production by at least 50%, and it is not useful in severe hydramnios because the bladder is already filled with urine, and any further increase in the bladder size is minimal.¹²

Cervical length

Cervical length is essential for assessing the risk of preterm labor. If the fetus is less than 24 weeks and if after amniotic drainage the cervical length is less than 25 mm, a cervical suture is required to prevent preterm labor.¹³

Posttreatment follow-up results

The AFI should be monitored twice a week when the patient is being treated with indomethacin. The treatment is stopped when the AFI is less than normal. The response is seen usually between 4 and 20 days.

Doppler imaging of the ductus arteriosus is also done within 24 hours of starting treatment and once weekly thereafter. Indomethacin is known to cause premature closure of ductus arteriosus, and if this happens, indomethacin is stopped.

Degree of Confidence

The values for amniotic fluid index, single deepest pocket, and 2 diameter pockets are not normally distributed throughout pregnancy. Therefore, a logarithmic transformation is required for gestational age–specific ranges. Normative values also vary within a population.

The incidence of detection of polyhydramnios varies with the technique used, as discussed inAmniotic fluidabove.

Sonographic assessment of amniotic fluid is a poor indicator of amniotic volume. The 95% confidence limit is wide compared with the dye-dilution technique for the measurement of amniotic fluid volume.

Magann et al showed that, while sonography and the single-deep-pocket method are good for measuring normal amniotic fluid volume (83-94%), they are not accurate in diagnosing polyhydramnios (33-46%) and oligohydramnios (11-27%).^9,10

If color Doppler imaging is used along with normal scanning, the AFI is less than that obtained without Doppler techniques. This difference increases the diagnosis of oligohydramnios.

Nuclear Imaging

Findings

Nuclear medicine studies have no role in the evaluation of polyhydramnios.

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