Key Points
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Amniotic fluid (AF) volume is usually well regulated in pregnancy.
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Subjective or semiquantitative ultrasound measurement systems are used to identify and categorise disorders of AF volume.
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Low (oligohydramnios) or high (polyhydramnios) AF volumes are associated with increased maternal and perinatal complications.
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Obstetric management is based on the underlying cause of the abnormal AF volume.
Introduction
The fetus exists in a fluid-filled environment which assists in pulmonary maturation and musculoskeletal development, provides some protection from infection and trauma, protects the umbilical cord from compression and provides some nutrition. Regulation of the amniotic fluid (AF) volume is not well understood, but aberrations from normal are associated with increased perinatal morbidity and mortality.
Amniotic Fluid Physiology
Amniotic fluid is 98% to 99% water, with variance in chemical composition with advancing gestation. In early pregnancy, AF is a dialysate of maternal and fetal plasma, with water and solutes traversing the fetal skin bidirectionally. By the second trimester, when the fetal skin keratinises and becomes impermeable to water, the AF becomes increasingly hypotonic and is derived predominantly from fetal urine and lung fluid. AF is removed mainly by fetal swallowing and absorption into fetal blood in the chorionic plate vessels (intramembranous pathway). This dynamic process of AF regulation results in a fairly stable volume of 800 mL from 24 weeks’ gestation until near term, when there is a decline. The precise mechanisms of AF regulation remain uncertain, although the intramembranous pathway is currently believed to be the principal regulator of AF volume.
There are several studies assessing the change in AF volume with increasing gestation. The earlier studies of Queenan and colleagues (1972) and Brace and Wolf (1989) using dye-dilution techniques demonstrated an increase in AF volume from 15 to 20 weeks’ gestation with a maximum volume at 33 to 34 weeks’ gestation. Both these authors demonstrated a progressive decrease in AF volume from the peak until term. A decade later, Magann and colleagues conducted another study in 144 singleton pregnancies using dye-dilution techniques to assess AF volume over gestation in an attempt to overcome some of the methodological problems with the earlier data. Using a growth curve model, Magann and colleagues demonstrated a continued increase in AF volume across gestation, peaking at 40 weeks’ gestation ( Fig. 43.1 ).
Amniotic fluid provides several important functions for fetuses:
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The trophic factors found in AF (e.g., insulinlike growth factor, granulocyte colony-stimulating factor), have been postulated to play a role in fetal growth and development.
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Protective support permitting fetal growth and movement
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Antimicrobial function (e.g., human β-defensins 1–4)
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Fetal lung growth
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Fetal musculoskeletal development
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Maturation of the gastrointestinal system
Methods for the Clinical Assessment of Amniotic Fluid Volume
Research studies of absolute AF volume using dye dilution techniques, radioactive isotopes or direct measurement at hysterotomy, although important for determination of actual fluid volumes, are not applicable for use in the clinical practice setting. Recognising the importance of AF in obstetric outcomes—normal, increased or decreased—the assessment of AF volume for daily clinical practice has centred around ultrasound using either subjective or semiquantitative methodologies. Of the semiquantitative methodologies, only amniotic fluid index (AFI) and maximum vertical pocket (MVP) are in routine clinical practice.
Subjective Assessment of Amniotic Fluid Volume
The subjective assessment of AF volume is a visual interpretation of AF using ultrasound examination without an objective measurement. It is unclear how frequently this is used in clinical practice. There are limited data comparing the subjective evaluation of AF volume with direct measures; however, the few publications available demonstrate a satisfactory correlation. For experienced sonographers, the subjective impression of normal compared with abnormal AF volume may be used but is difficult to translate across users and time.
Semiquantitative Ultrasound Assessment of Amniotic Fluid Volume
There are two main semiquantitative measurement systems of AF volume in clinical use: the AFI and the MVP (also known as the single deepest vertical pocket). Since first suggested in 1998 by Moore and Brace, there have been several studies assessing the relationship between AF volume (both AFI and/or MVP) and dye-determined or directly measured AF volume. Disappointingly, the sonographic estimates of normal AF volume do not correlate consistently well with the direct or dye-determined techniques (sensitivities, 71%–98%). For oligohydramnios, detection with ultrasound techniques compared with dye-determination or directly measured volumes the sensitivities are poor, ranging from 6.7% to 27%.
The AFI is determined by summing four vertical quadrants with the transducer positioned in a sagittal place perpendicular to the floor ( Fig. 43.2 ) and was first introduced by Phelan and colleagues in 1987 for term pregnancies. This measurement system was subsequently expanded to include second and third trimester pregnancies (16–42 weeks’ gestation), and gestation-specific normal ranges for AFI were developed. The MVP technique ( Fig. 43.3 ) is also widely used with a 2-cm cutoff being most widely clinically accepted as discriminating normal from low AF. The ultrasound threshold for low AF volume is generally accepted as an AFI of 5 cm or less or a MVP of 2 cm or less because as these values have been associated with an increased risk for adverse perinatal outcomes. Interestingly, the upper limit of AFI to define polyhydramnios is less well-defined but is typically greater than 24 cm (or MVP >8 cm).
Amniotic Fluid Volume and Perinatal Outcome
For clinicians, the absolute relationship of true AF volume with sonographic techniques is of lesser importance than the association of sonographic estimation of AF and adverse pregnancy outcomes. Measurements of AF volume are routinely performed as a component of fetal surveillance protocols throughout pregnancy. There is a well-recognised association with fetal abnormality and extremes of AF volume (see later discussion).
In a systematic review, Nabhan and Abdelmoula assessed four randomised controlled trials in singleton pregnancies comparing AFI and MVP as components of antepartum fetal surveillance to prevent adverse pregnancy outcome. Use of AFI to determine oligohydramnios compared with MVP was associated with an increase in the diagnosis of oligohydramnios (relative risk [RR], 2.33; 95% confidence interval [CI], 1.67–3.24), an increase in labour induction (RR, 2.10; 95% CI, 1.60–2.76) and an increase in caesarean delivery for fetal compromise (RR, 1.45; 95% CI, 1.07–1.97). Additionally, there was no difference in adverse perinatal outcomes using either measurement technique (admission to neonatal intensive care unit, cord arterial pH <7.1, Apgar score <7 at 5 minutes or meconium). These authors concluded that MVP was the preferred measurement method for AF volume, with AFI overestimating the presence of oligohydramnios and leading to increased obstetric intervention with no improvement in pregnancy outcomes.
The association of AF perturbations and pregnancy outcomes in structurally normal fetuses with intact membranes has been recently reviewed by Morris and colleagues. In their systematic review of 43 studies comparing AF volume assessments with adverse pregnancy outcomes in 244,493 fetuses, the authors observed:
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Strong association between oligohydramnios (variously defined) and
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Birth weight below the 10th percentile in high-risk populations (odds ratio [OR], 6.31; 95% CI, 4.15–9.58)
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Birth weight below the 10th percentile in low-risk or unselected populations (OR, 2.34; 95% CI, 1.76–3.09)
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Neonatal death (OR, 8.72; 95% CI, 2.43–31.26)
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Perinatal mortality in high-risk populations (OR, 11.54; 95% CI, 4.05–32.9)
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Strong association between polyhydramnios (MVP >8 cm or AFI >25 cm) and birthweight greater than the 90th percentile (OR, 11.41; 95% CI, 7.09–18.36)
Associations of oligohydramnios and assessments of neonatal morbidity were only moderate with substantial heterogeneity between the studies. There was no association between polyhydramnios and low birth weight. For all outcomes assessed, however, the prediction models for abnormalities of AF volume and adverse outcomes were poor. In particular, although specificity was generally good, sensitivity was low, indicating an increased risk for an adverse outcome with an abnormal result but a normal result did not alter outcomes. Assessment of AF volume should not be used in isolation but rather in combination with other prognostic features (e.g., umbilical artery Doppler) to more accurately predict outcomes.
Amniotic Fluid Assessment in Multiple Pregnancies
Multiple pregnancies have a recognised baseline increase in adverse perinatal outcomes compared with singletons. The presence of a twin gestation adds another layer of complexity to AF volume assessment in terms of techniques of assessment, cause of AF variance and management. The dividing membrane in diamniotic multiple pregnancies complicates the conduct and reproducibility of ultrasound-based AF assessments. A recent systematic review on the assessment of AF volume in twin pregnancies demonstrated that AFI and/or MVP was typically used for ultrasound volume studies. The AFI and MVP performed equivalently when compared with dye-dilution techniques, but concerns have been raised about the reliability of AFI in twin pregnancies. It has been reported that there is no significant relationship between gestation and MVP in diamniotic twins, although a recent study of uncomplicated monochorionic diamniotic twins has demonstrated this does not hold true in this subset. This may well be secondary to the predominance of dichorionic pregnancies in the initial paper and the focus solely on monochorionic pregnancies in the second. Most centres have moved to using MVP for multiple pregnancies because of its ease of use, reliability and more robust performance characteristics for oligohydramnios ( Fig. 43.4 ). In general, an MVP less than 2 cm is used to define oligohydramnios, and an MVP greater than 8 cm to define polyhydramnios. For monochorionic twins, an MVP greater than 10 cm is used to define polyhydramnios after 20 weeks’ gestation.
Oligohydramnios
Low AF, or oligohydramnios, is usually defined as an MVP less than 2 cm or an AFI les s than 5 cm. It remains of concern that a consistent universal definition is not used, creating difficulty in comparing studies and outcomes. The pregnancy outcome principally depends on the underlying cause of the low AF volume and the gestation at recognition. Oligohydramnios in early midpregnancy typically has a different cause to that diagnosed in the late second or the third trimester, reflecting the different perinatal outcomes. The three main underlying causes of oligohydramnios are:
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Premature rupture of the membranes (PPROM)
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Congenital abnormalities
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Intrauterine growth restriction
The cause of periviable PPROM is poorly understood, but there are several consistent risk factors associated with its occurrence, including antepartum haemorrhage, history of preterm labour, cervical cerclage, multiple pregnancy, cigarette smoking and previous PPROM. PPROM occurring in early pregnancy is typically associated with poor perinatal outcomes, particularly if before 22 weeks’ gestation. Early studies provided an extremely dismal prognosis with survival rates of 10% for fetuses with PPROM at 13 to 21 weeks’ gestation. A recent observational study of 143 cases of periviable PPROM (16–24 weeks’ gestation) reported a 17% survival rate to discharge rate for cases at 16 to 20 weeks’ gestation ( n = 24) and 39% for those at 20 to 24 weeks’ gestation ( n = 82) ( P = .042), demonstrating some improvement in later gestations with contemporary neonatal management. Factors associated with increased survival in periviable PPROM include gestation at membrane rupture, latency (time period from PPROM until delivery) and mode of delivery. The ultrasound assessment of AFI was a poor predictor of survival. For babies born alive after periviable PPROM, respiratory morbidity (pulmonary hypoplasia, respiratory distress syndrome and bronchopulmonary dysplasia) is common. Bronchopulmonary dysplasia was reported in 29% of survivors in a recent review. The occurrence of periviable PPROM requires careful multidisciplinary counselling with realistic outcome data provided and the management options of expectant management or pregnancy interruption clearly explained.
Congenital malformations of the fetal renal tract with absence of functioning renal tissue or lower urinary tract obstruction are recognised causes of second trimester oligohydramnios. These severe fetal conditions are readily recognised by ultrasound with absence of renal tissue bilaterally ( Fig. 43.5A ), abnormal renal appearances ( Fig. 43.5B ) or megacystis ( Fig. 43.5C ) (See Chapter 33 on fetal renal abnormalities). The outcome for these fetuses tends to be universally dismal.
