This time I am tackling the myth of "low amniotic fluid." It seems that the more silly excuses OBs give for induction get shot down, the more they like to come up with. What baffles me is how they can justify doing so, knowing all the literature out there that does NOT support active management in most cases...but they do anyway. And since the average pregnant woman in America has neither the time nor the inclination to go read a bunch of research articles, I have dedicated my precious time to doing it for you. Let's look at the issue:
Disclaimer: This post is discussing the merits, or lack thereof, of inducing labor for low AFI levels, at term, and in the absence of any other risk factors. I do not address preterm labor, preterm premature rupture of membranes, or congenital defects. If any of these things apply to you, then this post does not.
This website describes the basics of low amniotic fluid (oligohydramnios), or low AFI. The level of amniotic fluid is determined by measuring, by ultrasound, an estimate of the various pockets of fluid around the baby. This is important because too much or too much fluid might indicate some sort of problem. A fluid level of between 5 and 18 cm. is considered normal. The level of fluid cycles, and so can fluctuate daily, or even hourly. You may begin to see for yourself the margins for error with this diagnostic tool.
First of all, what's the big deal with low AFI anyway? Extremely low, and/or persistent low AFI can be associated with various congenital defects, such as uteroplacental insufficiency, congenital anomalies, viral diseases, idiopathic fetal growth restriction (FGR), premature rupture of the fetal membranes, fetal hypoxia, meconium-stained fluid, and/ or postmaturity syndrome. It can also contribute to malpresentations, umbilical cord compression, and difficult or failed external cephalic version. (ref) For these reasons, doctors have typically associated low AFIs with poor fetal outcomes. However, as Leeman and Almond discuss, "A number of studies over the past 15 years have shown an association between oligohydramnios and poor fetal outcomes. These were predominantly retrospective studies, which failed to control for the presence of factors known to be associated with oligohydramnios such as intrauterine growth restriction (IUGR) and urogenital malformations."
In other words, though low AFI might be associated with some of these conditions, it doesn't necessarily mean the baby is in any immediate danger with a low AFI, with no other indicators. That is, low AFI by itself doesn't really mean much. In addition, there have been several studies which also show no correlation between low AFI and poor outcomes. (here, here, here, here, among others)
Knowing this, it would make sense if doctors chose not to actively manage low AFI in the absence of any other factors, as it doesn't usually mean negative outcomes, but this is not the case. According to this survey, submitted to perinatologists across the nation, ninety-two percent of respondents consider isolated low AFI (IO) to be a risk factor for various adverse outcomes. With a favourable cervix, 82% would consider inducing labour without documented lung maturity prior to 39 weeks. When asked whether induction of labour in cases of IO reduces perinatal morbidity, 45% were unsure and 21.4% thought it would not. Only 33% believe induction could decrease adverse outcomes. So most of the respondents considered low AFI to be a risk factor of some sort (it can be), and a full 78% either didn't know or thought induction would result in improved outcomes (it doesn't). This is pretty consistent with my own (anecdotal) experience with different mothers I have come in contact with in my own personal life. In every case of mothers that I have personally known being induced for low AFI, there was not a single other supporting factor. In both this and this blog post, I have mentioned before that I believe that low AFI is more likely to be used as a convenient "medical" reason to induce a woman who is postdates, especially if she refuses an induction for convenience.
Questionable motivations aside, let's assume for a moment that low AFI by itself can mean serious problems for the baby. How does one determine a dangerously low AFI? It is assessed by using ultrasound to measure the levels of fluid around the baby. Naturally, this is not an exact science. Gloria LeMay, in her article, "Low Amniotic Fluid....I don't think so," she describes the process like this:
What the ultrasound technician is doing could be compared to viewing an adult in a see-through plexiglass bathtub from below the tub. In such a scenario, it would be difficult to assess how much water is in the tub above the body that is resting on the bottom of the tub. You might be able to get an idea of the water volume by measuring how much water was showing below the elbows and around the knees, but if the elbows were down at the bottom of the tub, too, you might think there was very little water. This is what the technician is trying to do in late pregnancy—find pockets of amniotic fluid in little spaces around the relatively large body of an 8 lb. baby who is stuffed tightly into an organ that is about the size of a watermelon (the uterus). If most of the amniotic fluid is near the side of the uterus closest to the woman’s spine, it can not be seen or measured. This diagnosis of low amniotic fluid frightens the parents-to-be into acquiescing to an induction of labour.That is to say, it can easily be inaccurate. In the above referenced article from gynob.com, it is even admitted that ultrasound measurements can be wrong:
This article quotes a 1998 study, which concludes, "that indexing amniotic fluid by measuring the pockets of amniotic fluid...is 'a poor screening test' to identify infants at risk." In this article, several studies and factors can explain the inaccuracies inherent in this method of measurement:
If there were to turn out to be a normal amount of fluid with rupture of membranes during an induction, then the low AFI that prompted the induction was either temporary or wrong. It happens, but ultrasound's the best thing we have to go by, even with its inherent error. (emph. added)
Although the AFI is widely accepted as the standard to diagnose oligohydramnios in the United States, many studies have found it to be an inaccurate method for assessing the actual amount of amniotic fluid, especially in the lower or higher ranges.[3,4,6-9] Rutherford et al. point that poor intra- and interobserver reliability may account for some of the low positive predictive value. Serial measurements have shown mean differences of 1 cm of amniotic fluid volume when conducted by the same ultrasound operator, and 2 cm variance in measures of volume when conducted by multiple operators. Variation can exist because of subjectivity of the ultrasonographer, the amount of pressure applied to the abdomen, and fetal position or movement. Additional variables that may alter AFI summation of the 4 uterine quadrants of amniotic fluid is the influence of the environmental temperature, altitude, maternal glucose control in diabetes, maternal hydration, and the status of the amniotic membranes. (original linked references in article)
Many factors can influence the measurement of the AFI, not the least of which involve subjectivity on the part of the sonographer(s). Observing and approximating measurements through a solid surface, around a moving object, under constantly changing conditions, is a guess, at best.
So if induction for low AFI does nothing to improve fetal outcomes, what does it accomplish? In short, a higher risk of induction complications, including c-section.
"Although small and insufficiently powered, these studies suggest that isolated oligohydramnios does not appear to be associated with adverse outcomes, but it may cause fetal intolerance of labor, which does result in higher cesarean rates." (ref)
"In a case-control study by Conway, 183 low-risk, term parturients with oligohydramnios were matched to 183 women of similar gestational age and parity who presented in spontaneous labor. The patients with isolated oligohydramnios were induced and showed an increased cesarean delivery rate. The increased rate of cesarean delivery was not due to nonreassuring fetal surveillance and was attributed to the induction process (LOE: 2).25" (ref)
"CONCLUSION: Active induction of labor in term low risk gestations with isolated oligohydramnios translated into higher labor induction, operative vaginal delivery and cesarean section rates. This led to increased maternal risk and an increase in costs with no differences in neonatal outcome." (ref)
"CONCLUSION: Isolated oligohydramnios is not associated with adverse perinatal outcomes. However, it increases the risk for labour induction and Caesarean section." (ref)
One other interesting note. In this article, the question is asked, " Is the increase in cesarean section secondary to fetal intolerance of labor from low AFI or the induction process itself?" I would submit that the question is moot. If the fetus is experiencing stress related to low AFI, would it make sense to add more stress by inducing? In either case, it is clear that induction is a strong contributor to another completely unnecessary c-section, and therefore, that induction for a diagnosis of low AFI is yet another unsubstantiated OB myth.