Objective
To analyze pregnancy outcome in cases of isolated oligohydramnios at preterm.
Methods
A retrospective cohort study of singleton pregnancies diagnosed with isolated oligohydramnios at preterm (n = 108). Pregnancy outcome was compared with a matched control group of low-risk preterm pregnancies with normal levels of amniotic fluid in a 3:1 ratio (n = 324).
Results
Pregnancies complicated by isolated oligohydramnios were characterized by a higher rate of preterm deliveries (26.9% vs 12.3%, P < .001), most of which were iatrogenic, and a higher rate of labor induction and cesarean delivery. Neonates with isolated oligohydramnios were characterized by a lower birthweight and a higher rate of neonatal morbidity. These differences were eliminated when the analysis was limited to the subgroup of pregnancies with isolated oligohydramnios that were managed expectantly and delivered spontaneously at term.
Conclusion
Adverse pregnancy outcome in cases of isolated oligohydramnios diagnosed at <37 weeks appears to be related to a considerable degree to iatrogenic prematurity.
The optimal management in cases of isolated oligohydramnios is a matter of controversy. Following reports on increased risk of adverse outcome in pregnancies complicated by oligohydramnios, labor induction became the standard management. However, it was later suggested that the previous observations regarding adverse pregnancy outcome in cases of oligohydramnios are the result of other pregnancy-related complications that are associated with oligohydramnios (ie, fetal growth restriction and hypertensive complications) and to the higher rate of interventions in these pregnancies (ie, labor induction and cesarean delivery) rather than the result of oligohydramnios itself. Indeed, recent studies have shown that the outcome of low-risk pregnancies complicated solely by oligohydramnios (isolated oligohydramnios) is comparable to that of low-risk pregnancies with normal levels of amniotic fluid.
There is only a small number of studies that address the management of isolated oligohydramnios at preterm (<37 weeks of gestation), and these studies are further limited by a small sample size, variation in the definition of oligohydramnios and in the gestational age at diagnosis of oligohydramnios, lack of appropriate control groups, and lack of information regarding the severity of oligohydramnios, and timing and indications for interventions.
Considering the fact that induction of labor at preterm (compared with induction at term) is complicated by an increased risk for prematurity-related complications and by a higher rate of induction failure, there is an urgent need for studies investigating the optimal management in these cases.
In the current study, we aimed to compare pregnancy outcome in cases of isolated oligohydramnios at preterm with that of a control group of low-risk preterm pregnancies with normal levels of amniotic fluid, as well as to compare the outcome of expectant vs active management in these cases.
Materials and Methods
Study population
We conducted a retrospective cohort study of all singleton pregnancies diagnosed with isolated oligohydramnios (amniotic fluid index [AFI] <5 cm) at 24 + 0 to 36 + 6 weeks of gestation after sonographic evaluation in the ultrasound unit of a university-affiliated tertiary hospital, between 1996 and 2007. Data were compared with a control group consisting of low-risk preterm pregnancies with normal levels of amniotic fluid (AFI 5-25 cm) based on sonographic evaluation in the same ultrasound unit, matched to the study group by gestational age at the time of sonographic evaluation in the ultrasound unit, maternal age, and parity in a 3:1 ratio.
Pregnancies complicated by any of the following conditions at the time of initial evaluation in the ultrasound unit were excluded from both the study and control groups: chronic or gestational hypertension, preeclampsia, pregestational or gestational diabetes, a history of prior preterm delivery, major congenital anomalies, uncertain pregnancy dating, suspected fetal growth restriction (defined as sonographically estimated fetal weight below the 10th percentile according to local reference curves ), abnormal umbilical artery Doppler studies, suspected placental abruption, placenta previa, suspected chorioamnionitis, and premature rupture of membranes (PROM).
To determine whether adverse pregnancy outcome in the isolated oligohydramnios group is related to the presence of oligohydramnios or whether it is merely the result of the higher rate of interventions in this group, we compared the characteristics and outcome of those women with isolated oligohydramnios who were actively delivered (ie, underwent labor induction or cesarean delivery) before 39 + 0 weeks of gestation because of the presence of oligohydramnios (active management subgroup) with those who were managed expectantly and either delivered spontaneously, underwent indicated delivery before 39 + 0 weeks because of indications other than oligohydramnios, or underwent indicated delivery at ≥39 + 0 weeks for any indication, including oligohydramnios (expectant management subgroup). Cases in the latter group were followed with nonstress testing (NST, twice weekly) and sonography (1-2 times weekly) until either delivery or resolution of oligohydramnios.
Data collection
Potential cases for the study and control groups were initially identified using our comprehensive database of sonographic examinations. Subsequently, the medical charts of these patients were reviewed in detail for the following information: maternal demographics, medical and obstetric history, gestational age at delivery based on last menstrual period (LMP), and, whenever available, confirmation by first-trimester ultrasound, maternal or fetal pregnancy related complications, onset and mode of delivery, intrapartum complications, and short-term neonatal outcome.
Definitions
Respiratory morbidity was defined as any of the following: the presence of either respiratory distress syndrome (RDS), transient tachypnea of the newborn (TTN), pulmonary hypertension, or need for ventilatory support. Infectious morbidity was defined as the presence of culture-proven sepsis, meningitis, or pneumonia. Central nervous system (CNS) morbidity included seizures or intraventricular hemorrhage (IVH, any grade). Composite neonatal outcome was defined as the presence any of the following: respiratory, infectious or CNS morbidity (as defined above), neonatal death, necrotizing enterocolitis (NEC), need for phototherapy, hypoglycemia, or hypothermia.
Statistical analysis
Data analysis was performed with the SPSS v15.0 software (SPSS, Inc, Chicago, IL). Student t test was used to compare continuous variables between the groups, and χ 2 test was used for categorical variables. For the purpose of subgroup-analysis, the gestational-week group was determined as the number of completed weeks of gestation (eg, an infant born at gestational age of 35 6/7 was included in the 35-weeks group). Multivariate logistic regression analysis was used adjust the risk associated with isolated oligohydramnios for potential confounders. Differences were considered significant when P value was less than .05.
Results
Characteristics of the study and control groups
Of the total 21,718 women at gestational age of 24 + 0 to 36 + 6 weeks who underwent sonographic evaluation in our ultrasound unit during the study period, 988 (4.5%) women had oligohydramnios diagnosed. Of these, 108 (10.9% of all cases of oligohydramnios before 37 weeks, 0.5% of all sonographic examinations before 37 weeks) were defined as isolated oligohydramnios, and to whom a control group of 324 (3:1 ratio) low-risk preterm pregnancies with normal levels of amniotic fluid was matched by gestational age at the time of initial sonographic evaluation in the ultrasound unit, maternal age, and parity.
The women in the study and control groups were similar with regard to the demographic and obstetric characteristics ( Table 1 ). The mean gestational age at diagnosis of isolated oligohydramnios was 33.9 ± 2.0 weeks (range, 27–36 weeks) ( Table 1 ).
| Characteristic | Isolated oligohydramnios n = 108 | Normal amniotic fluid n = 324 | P value |
|---|---|---|---|
| Maternal age, y | 28.2 ± 5.3 | 29.2 ± 5.2 | .09 |
| Age >35 y | 18 (16.7) | 46 (14.2) | .5 |
| Nulliparity | 62 (57.4) | 186 (57.4) | N/A |
| Previous cesarean delivery | 11 (10.2) | 37 (11.4) | .7 |
| Gestational age at the time on initial sonographic evaluation | 33.9 ± 2.0 | 33.9 ± 2.0 | N/A |
| Estimated weight at the time on initial sonographic evaluation, g | 2227 ± 548 | 2278 ± 549 | .4 |
| Percentile | 44 ± 22 | 48 ± 26 | .1 |
| Male fetus | 55 (50.9) | 180 (55.6) | .4 |
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