Oligohydramnios and Post-Term Pregnancy

Abstract & Commentary

Synopsis: Renal artery Doppler was more predictive of oligohydramnios than the umbilical RI. The reduced renal artery end-diastolic velocity suggests that increased arterial impedance is an important factor in the development of oligohydramnios in prolonged pregnancies.

Source: Oz AU, et al. Obstet Gynecol. 2002;100(4): 715-718.

Oz and colleagues recently re-explored the possible etiology of the oligohydramnios sometimes seen in post-term pregnancy. They evaluated 147 patients whose pregnancies had exceeded 287 days with Doppler waveform analysis of the middle cerebral arteries (MCA), umbilical arteries (UA), and renal arteries (RA).

Twenty-one fetuses had oligohydramnios. There were no significant differences in resistance indices of the MCA and UA between those with and without oligohydramnios. However, RA resistance was significantly higher in those post-term pregnancies with oligohydramnios. Interestingly, the oligohydramnios groups had a higher rate of adverse outcome (more than 24 hours in the nursery, 5 minute Apgar scores of < 7, and fetal distress in labor). They also delivered infants that were smaller (mean weight, 3414 g vs 3665 g).

Comment by John C. Hobbins, MD

Most often oligohydramnios is a symptom of possible trouble rather than a cause of it. Oligohydramnios accompanying IUGR is a function of fetal "brain sparing" in which the fetus shunts blood to his or her brain (as demonstrated by an increase in end diastolic flow in the MCA), at the expense of renal plasma flow. Since some post-term pregnancies with oligohydramnios are productive of infants displaying signs of dismaturity (akin to IUGR), it has been assumed that the same mechanism behind the oligohydramnios is being put into play. This group did not find this. In fact, the only fetal area showing alterations in vascular resistance were the renal arteries, suggesting a different, and perhaps primary, renal reason for the oligohydramnios.

So there appears to be 2 different mechanisms for oligohydramnios, and in each condition it is a sign of potential compromise. However, in IUGR there are far better ways to assess fetal condition than by the presence or absence of oligohydramnios. Umbilical artery waveform analysis is the most sensitive indicator of fetal condition in IUGR, irrespective of oligohydramnios. However, since oligohydramnios did occur more frequently in this study in the post-term pregnancies with adverse outcome (43.7% vs 18.8%), and MCA and UA waveforms did not correlate with oligohydramnios, one might assume that Doppler, in general, is not the way to follow post-term pregnancies.

Randomized trials have indicated that if one uses fetal heart rate monitoring to manage post-term pregnancies, perinatal mortality is not different than if induction were undertaken at 42 weeks. However, there is a suggestion that the cesarean section rate is higher in the induction group. Yet, once oligohydramnios occurs in post-term patients all bets should be off.

Actually, in many areas of the United States, the pendulum has swung so far away from conservative management of post-term pregnancy that 40 weeks and 1 day is considered grounds for induction. In fact, even in pregnancies between 36 and 40 weeks, the slightest hint of oligohydramnios seems to represent an indication for induction. A recent audit of our delivery log has shown that the most common indication for induction was oligohydramnios at term.

In the United States, the induction rates have risen from 9.5% in 1990 to an alarming 19.4% in 1998. Induction is accompanied by increased costs and a clear increase in cesarean sections. Older and recent investigations have shown that elective induction in term primigravidas results in almost a doubling of cesarean births (20% vs 14%).

Conway recently assessed outcomes of 183 patients induced between 37 and 42 weeks for isolated oligohydramnios, compared with a control group entering in spontaneous labor at the same gestational ages with a normal amount of amniotic fluid. A significant increase in cesarean sections was noted (15.8% vs 6.6%; P < 0.01), but there was no difference in any outcome variable, including fetal distress in labor.

Lastly, the assessment of amniotic fluid is very subjective, even when using a quantitative index such as the AFI, and the definition also can vary appreciably. I recently heard someone define oligohydramnios as being > 2 standard deviations below the mean for gestation. This happened to be an AFI of less than 8, a finding in our experience that is quite common at term.

For an excellent review of labor induction, the reader is referred to an article by Rayburn and Zhang.

Dr. Hobbins is Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver.

Suggested Reading

1. Rayburn WF, Zhang J. Obstet Gynecol. 2002;100(1): 164-167.

2. Yeast JD, et al. Am J Obstet Gynecol. 1999;180: 628-633.

3. Smith LP, et al. Am J Obstet Gynecol. 1984;148: 579-585.

4. Conway DL, et al. J Matern Fetal Med. 1998;7: 197-200.