A comparison between amniotic fluid index and the single deepest vertical pocket technique in predicting adverse outcome in prolonged pregnancy

Abstract

Objective

to compare perinatal outcome in induced postterm pregnancies with normal amniotic volume and in patients with prolonged pregnancy undergone induction for oligohydramnios, evaluated by two different ultrasonographic methods.

Methods

amniotic fluid volume was measured, using Single Deepest Vertical Pocket (SDVP) and Amniotic Fluid Index (AFI), in 961 singleton uncomplicated prolonged pregnancies. In 109 of these patients, hospitalization was planned for induction of labor, during or after 42 weeks of gestation, for oligohydramnios, postterm pregnancy and other indications in 47, 51 and 11 cases, respectively. Perinatal outcome included: rate of caesarean section, fetal distress, non reassuring fetal heart tracing, presence of meconium, umbilical artery pH < 7.1, Apgar score at 5 minutes < 7, admission to neonatal intensive care unit (NICU).

Results

oligohydramnios was diagnosed in 4.89% of cases, when at least one of the two methods was used. A reduced AFI and SDVP value identified 4.47% and 3.75% of cases, respectively, even if without statistical difference. No statistical differences were reported in perinatal outcomes in postterm versus prolonged pregnancies with oligohydramnios, also in relation to the two different ultrasonographic methods.

Conclusions

oligohydramnios is more frequently diagnosed using AFI than SDVP, consequently determining a higher rate of induction of labor. Moreover, perinatal outcome in prolonged induced pregnancies is not affected by oligohydramnios.

Introduction

Terms of postterm and prolonged pregnancy are poorly defined. Prolonged pregnancy is considered as menstrual age ranged from 41 to 41 6/7 weeks of gestation, while a pregnancy beyond 42 0/7 weeks is defined postterm (1). Frequently, postterm pregnancy is associated with oligohydramnios (2). It is probably related to a decrease in placental function and/or in fetal renal perfusion with reduction of urine production (3–5). In this period of pregnancy, amniotic fluid volume is considered an important predicting factor of fetal wellbeing. In fact, oligohydramnios is usually associated with an increased risk of fetal heart rate tracing abnormalities, meconium-stained amniotic fluid and caesarean sections for fetal distress (6–8). At present, there is no consent about the efficacy in predicting adverse perinatal outcome with the ultrasonographic methods currently used to determine amniotic fluid volume (9–13, 8).

Aims of the present study concern: a) the comparison between the perinatal outcomes in prolonged pregnancies, complicated with oligohydramnios, and the outcomes in postterm pregnancies without oligohydramnios; b) the evaluation of perinatal outcome in relation to different ultrasonographic methods for the examination of amniotic fluid volume in those cases complicated with oligohydramnios.

Materials and methods

In the period between May 2011 and April 2013, we examined 961 singleton prolonged pregnancies at our Department. The mean maternal age was 33.1 ± 3.4 years (years ± SD; range 24–45 years) and the gestational age at the time of the last ultrasound examination was 290.35 ± 1.42 days (days ± SD; range 287–295 days). Gestational age was calculated from the date of the last menstrual period and confirmed at the first trimester scan. All pregnancies were uncomplicated, without any maternal and/or fetal disease; patients with fetal anomalies and/or abnormal fetal karyotype, detected during pregnancy or at delivery, were excluded. Amniotic fluid volume was determined for each patient at least 2 days before delivery, measuring both SDVP and AFI. SDVP is a semi-quantitative method that measures the largest vertical pocket of amniotic fluid, free from fetal parts or loops of umbilical cord (14). Oligohydramnios is defined as a single pocket with a depth < 2.0 cm (10, 14, 15). AFI is another semi-quantitative measurement of amniotic fluid volume; it is determined by the sum of maximum vertical pockets of amniotic fluid, in each of the four quadrants of the uterus with no fetal parts or loops of umbilical cord (16, 17). In prolonged and postterm pregnancies, oligohydramnios is defined as an AFI < 5.0 cm (15–17). An ultrasonographic device with a 3.5-MHz volume transducer (model Voluson 730, GE Healthcare, Zipf, Austria) was used.

Hospitalization was planned for induction of labor at or after 42 weeks. Labor was induced in 109 patients (11.34%). The indications for induction of labor were: reduction of amniotic fluid volume (n=47), postterm pregnancy (n=51), non-reassuring fetal cardiotocography (n=9) and other indications (n=2). Non-reassuring fetal cardiotocography was diagnosed when variability was reduced in presence of persistent severe variable and/or late decelerations (18).

Perinatal outcome was assessed considering: rate of caesarean section, caesarean delivery for fetal distress, non reassuring fetal heart tracing, presence of meconium, umbelical artery pH < 7.1, Apgar score at 5 minutes < 7, admission to neonatal intensive care unit.

Statistical analysis was performed by T-student and Chi-Square test (p<0.01).

Results

Oligohydramnios was diagnosed in 47 patients (4.89%), when at least one of the two methods used to evaluate amniotic fluid was below the cut-off considered. Considering an AFI value < 5.0 cm, oligohydramnios was diagnosed in 43 patients (43/961; 4.47%), while in 36 cases using SDVP < 2.0 cm (36/961; 3.75%) (p = n.s.). Labor was induced in all pregnancies with ultrasound diagnosis of oligohydramnios. Consequently, rate of induction of labor was higher when oligohydramnios was diagnosed by the AFI value (43/47 vs 36/47 cases), even with no statistical differences between the results of the two different methods. Furthermore, 32 out of 43 patients with an AFI < 5.0 cm, revealed a SDVP < 2.0 cm (74.42%). Similarly, in 32 out of 36 patients (88.89%) with a SDVP < 2.0 cm, AFI appeared < 5.0 cm. Moreover, 4 cases reported a SDVP < 2.0 cm and an AFI > 5. 0 cm and 11 cases showed a SDVP > 2.0 cm and an AFI < 5.0 cm. In Table 1 perinatal outcomes in deliveries induced for prolonged pregnancies with oligohydramnios and for postterm pregnancy are described. No statistical differences were reported in relation to the different two methods used to diagnose oligohydramnios.

Table 1.

Perinatal outcomes in induced patients in relation to the different results of oligohydramnios and in postterm pregnancies.

Perinatal outcomes	Oligohydramnios				Postterm pregnancy
	Total cases	AFI < 5.0 cm and SDVP < 2.0 cm	AFI < 5.0 cm and SDVP > 2.0 cm	AFI > 5.0 cm and SDVP < 2.0 cm
	n=47	n = 32	n = 11	n = 4	n = 51
	n (%)	n (%)	n (%)	n (%)	n (%)
Non-reassuring fetal heart rate tracing	18 (38.30)	13 (40.62)	4 (36.36)	1 (25.00)	14 (27.45)
Cesarean section	21 (44.68)	15 (46.87)	4 (36.36)	2 (50.00)	18 (35.29)
Cesarean delivery for fetal distress	16 (30.09)	11 (34.37)	4 (36.36)	1 (25.00)	11 (21.57)
Presence of meconium	19 (40.42)	13 (40.62)	5 (45.45)	1 (25.00)	16 (31.37)
pH umbilical artery <7.1	1 (2.13)	1 (3.12)	0 (0.00)	0 (0.00)	1 (1.96)
Apgar score<7 at 5 min	4 (8.51)	3 (9.37)	1 (9.09)	0 (0.00)	3 (5.88)
NICU	1 (2.13)	1 (3.12)	0 (0.00)	0 (0.00)	1 (1.96)

Discussion

The aim of the antepartum surveillance tests is the prevention of adverse pregnancy outcomes, particularly in prolonged pregnancy. Multiple factors contribute to pregnancy outcomes in women undergoing antepartum fetal surveillance. Amniotic fluid volume is one of the parameters proposed for the assessment of fetal wellbeing. Different tests have been proposed to determine amniotic fluid volume as mean of evaluation of fetal wellbeing in prolonged and postterm pregnancies. The most common methods used to evaluate the amniotic fluid volume are AFI (16, 17) and SDVP measurements (14). The accuracy of amniotic fluid assessment is related, not only to the different methods and diagnostic criteria used, but even to other factors. Experience of operators, fetal position, presence of abdominal mass or scar could interfere with an adequate measurement (19, 8).

In different studies AFI and SDVP were compared as a screening for preventing adverse pregnancy outcomes (20, 21, 13). In our experience, AFI measurement detects more frequently oligohydramnios than SDVP, resulting in a higher rate of induction of labor, even with no significance at the statistical analysis, according to the reports of Morris et al. (22). Magann et al. (23) reported that 72% of women with an AFI ≤ 5.0 cm, still had a SDVP measurement greater than 2.0 cm. Randomized controlled trials and a recent meta-analysis (24, 20, 21, 25, 13) confirm these results, even if with different percentages and some confounders factors such as high risk pregnancies and evaluation at different gestational periods (close to / at term or in the postterm period).

In our case series, oligohydramnios was diagnosed with both AFI and SDVP measurements in 68.1% of the patients. Furthermore, an AFI < 5.0 cm and a SDVP > 2.0 cm was detected in 23.4% of cases and the other 8.5% showed an AFI > 5.0 cm and a SDVP < 2.0 cm.

Considering perinatal outcomes, our results showed no significant differences when labor was induced in pregnancies with normal and low amniotic fluid volume, despite the different diagnostic methods used. Therefore, the amniotic fluid volume, even when reduced, could not influence perinatal outcomes in induced prolonged pregnancy. Similar results are reported by Hofmeyr (26) and Verrotti et al. (27). Consequently, further studies need to be performed to determine the clinical validity and utility of amniotic fluid volume measurement in prolonged pregnancy, thus considering the great number of variables analyzed in the restricted group of our study. A prerequisite should be the reproducibility of the methods used for amniotic volume measurement, particularly in case of oligohydramnios. In fact, although these methods assess better a normal volume, they are characterized by a low sensitivity and specificity when detecting an abnormal volume (28).