In the last couple of posts we have reviewed causes and complications related to excessive amniotic fluid, otherwise known as polyhydramnios or, simply hydramnios. Although 50-60% of cases of hydramnios are idiopathic (no identifiable cause) and about 90% of cases are mild to moderate, about 10% are severe and these latter are the more likely to be accompanied by considerable fetal and neonatal morbidity and mortality secondary to an underlying fetal cause of the hydramnios – chromosomal abnormality, congenital anomaly, fetal anemia, inborn error, or congenital infection. Evaluation of the pregnancy with hydramnios, therefore, focuses primarily on these concerns.
The first step in any evaluation of hydramnios is to take a detailed medical, obstetrical, and family history and to review the course, medical complications, and basic laboratory studies performed to date in the current pregnancy. It is not necessary to cover the extent of such a discussion herein but examples of pertinent information include: the current pregnancy being a multiple gestation; a previous pregnancy accompanied by hydramnios, fetal macrosomia, or diabetes (and the outcome of that pregnancy); known maternal diabetes; known Rh- or other isoimmunization; history of blood transfusion; hemorrhage or trauma during the current pregnancy; history of known/suspected exposure to parvovirus B19 (Fifth’s disease) or illness accompanied by fever and/or rash during the current pregnancy; exposure to young children at home or in the workplace; family history of inborn errors of metabolism or congenital birth defects – particularly, cardiac, gastrointestinal, and neural tube, and neuromuscular disorders; past or family history of aneuploidy or recurrent pregnancy loss; advanced maternal age; report of decreased fetal movement; maternal history of medications and nonprescription (licit and illicit) drug use during the pregnancy. It is also important to get some feel for the onset of the hydramnios related to timing in pregnancy (e.g., gestational age when noticed; slow onset vs. rapid onset) and maternal signs and symptoms of disease and cardiorespiratory compromise.
The next step is to perform a thorough, high resolution ultrasound examination. In this, the degree of hydramnios should be documented objectively by a four-quadrant amniotic fluid index (AFI). This will be valuable as a ‘baseline’ for comparison during subsequent ultrasound evaluations of the pregnancy. Fetal growth should be assessed to determine if the baby is abnormally large or growth-restricted for the gestational age of the pregnancy – either of which might help narrow down the differential diagnosis. A detailed anatomical survey of the baby should include: central nervous system and spine; face and facial midline structures; neck; thorax; heart and rhythm; diaphragm; gastrointestinal tract; genitourinary tract; and, extremities. In addition, it is important to document whether or not the baby appears to have normal movement – flexion and extension – of the extremities since, if this is not present, it might suggest an underlying neuromuscular disorder. Evidence of fetal hydrops (indicative of fetal anemia or high-output cardiac failure) should also be sought.
If there is a twin (or higher order multiple) gestation, it is important not only to assess fetal growth and anatomy, but to determine chorionicity of the twins (i.e., dichorionic-diamnionic; monochorionic-diamnionic; or, monochorionic-monoamnionic) and if there is any significant discordance for growth or amniotic fluid surrounding the babies. Twin pregnancies are at higher risk for fetal anomalies, chromosomal abnormalities, abnormalities of placentation, and in monochorionic twins, a condition called ‘twin-to-twin transfusion syndrome (TTTS)’ (a discussion of which will be reserved for another post at another time).
A critical step in the evaluation of the pregnancy complicated by hydramnios (as it was in that complicated by oligohydramnios) is performing Doppler flow velocimetry (DFV) studies. These should be done at least on the fetal umbilical and middle cerebral arteries (MCA) and should be considered for the fetal ductus venosus and umbilical vein and the maternal uterine arteries. The goals of DFV under these circumstances are to ascertain if there is any difficulty perfusing the placenta (increased resistance indices) from either the fetal or maternal side; assess whether there is any evidence of fetal blood flow redistribution (“cranial sparing”) related to relative ‘placental insufficiency’ (decreased resistance to blood flow in the MCA); if there is increased peak systolic velocity (PSV) of blood flow in the MCA which would be suggestive of significant fetal anemia; or if there is evidence of fetal cardiac decompensation (abnormal wave forms – increased resistance or pulsatility - in the fetal ductus venosus or umbilical vein). DFV is a critical evaluation in the monochorionic twin pregnancy, especially if there is discordance for growth and/or amniotic fluid, that might help differentiate simple intrauterine growth restriction, or hydramnios related to aneuploidy or fetal anomalies, from TTTS.
Once a comprehensive ultrasound has been completed, a discussion should be held with the patient about what else can be done at this time, diagnostically and therapeutically, if indicated. Again, a detailed discussion of this is beyond the purpose of our post today, but some examples are as follows depending on the findings: 1) If the baby is growth-restricted and/or has visible abnormalities (major structural or subtle), an amniocentesis should be offered for fetal chromosomal studies and congenital infection, particularly, for cytomegalovirus (CMV). 2) Growth restriction with hydramnios and abnormal resistance to fetal placental-perfusion by umbilical DFV carries about a 50% chance of aneuploidy, even in the absence of visible abnormalities, so fetal karyotype should be encouraged with this combination of findings as well; 3) If there is increased PSV (> 1.5 MoM) in the fetal MCA, even in the absence of hydrops fetalis, then the baby may need to be evaluated for significant anemia – best done by percutaneous umbilical blood sampling (PUBS) with preparations made for coincident transfusion. This becomes even more critical if the baby already has hydrops; 4) If a fetal arrhythmia has been identified, medical therapy should be attempted to correct this condition; 5) If a twin gestation is present and there appears to be TTTS, then the patient should be counseled and offered a referral to one of the few sites in the country with the expertise to handle this condition.
As a routine part of maternal evaluation, especially if no readily apparent cause of the hydramnios is identified by ultrasound, I will frequently recommend the following: blood type and antibody screen; thyroid studies; a full 3-hour glucose tolerance test (unless the patient has already been diagnosed with diabetes); serologic testing for evidence of recent CMV or Parvovirus B19 infection and consider screening for toxoplasmosis and syphilis. If a woman is a known diabetic, I will include a hemoglobin A1C level and make efforts to optimize her diabetic control.
If a correctible cause for the hydramnios, such as fetal anemia, has not been identified and/or there are significant risks to the pregnancy because of the hydramnios itself, especially, if the pregnancy is less than 30 weeks and there is premature labor to contend with, or the mother has developed cardiorespiratory compromise secondary to massive hydramnios, there are limited options for management. Acute management of maternal cardiorespiratory decompensation may require amnioreduction. This is an amniocentesis procedure in which a large bore needle/catheter is inserted into the uterus and the fluid slowly drained until the AFI is in a ‘normal’ range of 10-20 cm. The most common risks to this procedure are rupture of membranes, premature labor, and placental abruption if the fluid is decompressed too rapidly. Unfortunately, since under normal circumstances, amniotic fluid volume is replaced daily, the fluid will often reaccumulate within 48-72 hours, necessitating repetitive procedures. Under these circumstances, the risk of the previously noted complications, as well as of infection, increase further.
As an adjunct to amnioreduction, or if the situation is not so acute, another option is to use potent prostaglandin synthetase inhibitors that have the effect of decreasing fetal urine production (and, hence, amniotic fluid) and may also decrease uterine contractions that usually accompany hydramnios, thereby, decreasing the risk of premature labor. Indomethacin has had the widest experience in this regard and is relatively safe for both mother and baby. After an initial loading dose of 100 mg, I will frequently place the patient on 25-50 mg of indomethacin every 6 hours. It usually takes at least 4 days (sometimes much longer) to get any response to this regimen. Once indomethacin has been started, it is important to monitor both amniotic fluid and the fetal ductus arteriosus which can constrict in response to the drug and is a primary means of maintaining the “fetal circulation” (bypassing the lungs and allowing proper distribution of well-oxygenated blood throughout the body) while the baby is in utero. One must be especially careful about using indomethacin in women who have underlying kidney problems, cardiac disease, long-standing diabetes, hypertensive disorders, pregestational and pregnancy-related preeclampsia, or evidence of infection because if their renal output also drops significantly, they can be pushed into congestive heart failure.
Another prostaglandin inhibitor that has also been tried, and with which I must admit limited experience, is sulindac (usually dosed at 200 mg every 12 hours). Sulindac has greater selectivity for the cyclooxygenase 2 (COX-2) enzyme and appears to be capable of reducing fetal urine output with less of an effect on the ductus arteriosus, although its effect on the fetal kidneys is also less than that of indomethacin. It may be safer to use later in gestation than indomethacin which I will usually stop at 32 weeks (and no later than 34 weeks) gestation. The risk of premature delivery is so high with severe hydramnios requiring amnioreduction and/or prostaglandin inhibitor therapy that I often couple their use with a course of corticosteroids to accelerate fetal lung maturation in the event that delivery occurs or becomes necessary.
In closing, I would like to mention only one other caution about hydramnios that is often over-looked with regard to my last statement in the paragraph above. If hydramnios is present and associated with diabetes and/or fetal macrosomia, fetal lung maturation may be delayed as much as 2-3 weeks as the result of hyperinsulinemia in the baby. Hyperinsulinemia suppresses the development of lung surfactants and one last study that should be considered, and is highly recommended, prior to the elective delivery of baby because of hydramnios, or macrosomia, is an amniocentesis to assess fetal lung maturity, especially if the planned delivery is by cesarean section.
Well, this concludes our series on amniotic fluid. As I said at the outset, evaluation of amniotic fluid is an important part of every pregnancy and understanding the causes, complications, and management of the pregnancy with abnormalities of amniotic fluid is a daily part of my routine. I have tried to make our discussions digestible for the nonclinician as well as a valuable overview for the primary care professional involved in the care of women during pregnancy and hope that we have accomplished that here! Thanks for reading! Dr T