Universal late pregnancy ultrasound screening to predict adverse outcomes in nulliparous women: a systematic review and cost-effectiveness analysis

BACKGROUND

Currently, pregnant women are screened using ultrasound to perform gestational aging, typically at around 12 weeks' gestation, and around the middle of pregnancy. Ultrasound scans thereafter are performed for clinical indications only.

OBJECTIVES

We sought to assess the case for offering universal late pregnancy ultrasound to all nulliparous women in the UK. The main questions addressed were the diagnostic effectiveness of universal late pregnancy ultrasound to predict adverse outcomes and the cost-effectiveness of either implementing universal ultrasound or conducting further research in this area.

DESIGN

We performed diagnostic test accuracy reviews of five ultrasonic measurements in late pregnancy. We conducted cost-effectiveness and value-of-information analyses of screening for fetal presentation, screening for small for gestational age fetuses and screening for large for gestational age fetuses. Finally, we conducted a survey and a focus group to determine the willingness of women to participate in a future randomised controlled trial.

DATA SOURCES

We searched MEDLINE, EMBASE and the Cochrane Library from inception to June 2019.

REVIEW METHODS

The protocol for the review was designed a priori and registered. Eligible studies were identified using keywords, with no restrictions for language or location. The risk of bias in studies was assessed using the Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) tool. Health economic modelling employed a decision tree analysed via Monte Carlo simulation. Health outcomes were from the fetal perspective and presented as quality-adjusted life-years. Costs were from the perspective of the public sector, defined as NHS England, and the costs of special educational needs. All costs and quality-adjusted life-years were discounted by 3.5% per annum and the reference case time horizon was 20 years.

RESULTS

Umbilical artery Doppler flow velocimetry, cerebroplacental ratio, severe oligohydramnios and borderline oligohydramnios were all either non-predictive or weakly predictive of the risk of neonatal morbidity (summary positive likelihood ratios between 1 and 2) and were all weakly predictive of the risk of delivering a small for gestational age infant (summary positive likelihood ratios between 2 and 4). Suspicion of fetal macrosomia is strongly predictive of the risk of delivering a large infant, but it is only weakly, albeit statistically significantly, predictive of the risk of shoulder dystocia. Very few studies blinded the result of the ultrasound scan and most studies were rated as being at a high risk of bias as a result of treatment paradox, ascertainment bias or iatrogenic harm. Health economic analysis indicated that universal ultrasound for fetal presentation only may be both clinically and economically justified on the basis of existing evidence. Universal ultrasound including fetal biometry was of borderline cost-effectiveness and was sensitive to assumptions. Value-of-information analysis indicated that the parameter that had the largest impact on decision uncertainty was the net difference in cost between an induced delivery and expectant management.

LIMITATIONS

The primary literature on the diagnostic effectiveness of ultrasound in late pregnancy is weak. Value-of-information analysis may have underestimated the uncertainty in the literature as it was focused on the internal validity of parameters, which is quantified, whereas the greatest uncertainty may be in the external validity to the research question, which is unquantified.

CONCLUSIONS

Universal screening for presentation at term may be justified on the basis of current knowledge. The current literature does not support universal ultrasonic screening for fetal growth disorders.

FUTURE WORK

We describe proof-of-principle randomised controlled trials that could better inform the case for screening using ultrasound in late pregnancy.

STUDY REGISTRATION

This study is registered as PROSPERO CRD42017064093.

FUNDING

This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 25, No. 15. See the NIHR Journals Library website for further project information.

THE EXTRATHYROIDAL DISTRIBUTION OF 131-I THYROXINE

The elimination of radioactivity from the blood stream has been examined in 6 subjects after intravenous administration of 131I-thyroxine. About 50 per cent of the radioactivity left the blood within 60–90 minutes and 75 per cent within 6 hours. In both in vivo and in vitro experiments there was a rapid linking of the labelled thyroxine to the serum proteins.

The elimination of radioactivity from the blood and the excretion in urine and faeces was followed in 12 subjects for 12 days. To calculate the distribution and exchange of the labelled thyroxine between the various extrathyroidal pools, a model comprising 4 pools was constructed. The working of the model was analysed by means of an analogue computer. There was a close agreement between the curves thus obtained and those yielded by empirical methods. According to the 4-pool model 6 – 7 times the intravascular thyroxine pool was exchanged with the extravascular pool in 24 hours.

A ONE-WAVELENGTH, TWO-AREA METHOD IN MICROSPECTROPHOTOMETRY FOR PURE AMPLITUDE OBJECTS

A one-wavelength method for nuclei in smears is tested and discussed. It eliminates need for visual estimates of areas and measurements to check for possible slight variations in absorption spectrum which would vitiate applicability of the two-wavelength method. Extinction ( E1) is obtained from an intranuclear plug measurement; area of absorbing material is computed as a function of ratios between "one minus transmittances", one intranuclear, the second including in addition "free" or "non-absorbing" areas around it. If T1 represents intranuclear transmittance, T2 total transmittance (nucleus plus free area), A1 the area of the nucleus and A2 the area from which T2 was obtained (absorbing plus nonabsorbing media), total extinction times area is computed as

[See equation in the PDF file]

MODELS AND THEORY FOR UREA METABOLISM

1. Theories have been developed to try to interpret the effects of finite time of equilibration between plasma and body water, and the mechanism of renal function, on the variations of activity and specific activity of urea in plasma and urine after initial injection. 2. The magnitudes of errors likely to arise through approximations made in estimating the pool of body urea etc. have been derived. 3. Experimental results do not fit exactly with extreme models postulated, but are usually intermediate.

THE DEPOSITION OF AEROSOLS IN THE RESPIRATORY TRACT. I. MATHEMATICAL ANALYSIS AND COMPARISON WITH EXPERIMENTAL DATA

The deposition of aerosol in the human respiratory tract was calculated as a function of the particle size and particle density of the aerosol, the manner of breathing, and various parameters which control the degree of mixing of inspired air with the dead space air and with the lung air. The computations were performed on an electronic computer, which allowed a full analysis of the effects of the various parameters. Agreement between the computed total deposition curves, and published experimental data was very satisfactory. The computed curves exhibited the minimum in total deposition previously established experimentally, in the region of 0.3 μ. The maximum in the alveolar deposition curve was about 1 μ for particles of unit density, confirming the results of Brown, Cook, Ney, and Hatch for nose breathing. Its position was a function of the density of the aerosol particles, but was relatively insensitive to the tidal volume and other breathing parameters. The opposite held in regard to the value of the maximum, which was totally uninfluenced by the particle density, but was affected by the breathing parameters.

EQUATIONS FOR MEASURING BLOOD FLOW BY EXTERNAL MONITORING OF RADIOISOTOPES

It is possible by external monitoring alone to measure blood flow per unit volume through any vascular bed accessible to external monitoring of the radioactive tracer if the tracer input function or if the quantity of tracer entering the system is known. The methods proposed exploit the fact that the mean transit time through the system is the flow per unit volume of distribution of tracer. The equations are free of assumptions that require solutions as exponentials or any other specified frequency function of transit times.

If the input function is known, measurement of the concentration of tracer in blood leaving the part sensed by the external detector, combined with external detection of radioactivity, can lead to measurement of absolute blood flow.