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The influence of maternal body mass index on fetal anomaly screening

European Journal of Obstetrics & Gynecology and Reproductive Biology, pages 181 - 184

Abstract

Objectives

Maternal obesity is increasing and affects the quality of ultrasound images. The aim of this study was to examine the relationship between maternal body mass index (BMI) and requirement for repeat fetal anomaly scans and to determine whether gestational age at the initial scan affects the completion rate.

Study design

This was a retrospective cohort study of BMI and ultrasound scan data from 1000 women with singleton pregnancies delivering over a two month-period in a tertiary obstetric unit. Statistical analysis was used to determine the correlation between BMI and number of scans for completion and between gestational age and number of scans for completion. Women were classified into four groups according to BMI for comparison of the need for repeat scans using t test and relative risk.

Results

The number of scans required to complete the anomaly scan was directly related to increasing BMI (r = 0.243,p < 0.001). 52.1% of obese women required more than one scan to complete the anomaly scan compared to 27.4% of non-obese women (RR 1.90, 95% CI 1.60–2.27,p < 0.001). 12.5% of obese women require more than two scans to complete the anomaly scan compared to 2.35% of non-obese women (RR 5.32, 95% CI 2.97–9.50,p < 0.001). Completion of the initial anomaly scan was not related to gestational age at the time of the scan.

Conclusions

Obese women required more attempts to complete the anomaly scan and the number of scans required to complete was directly related to booking BMI. There was no relationship between gestation and completion of the scan, suggesting that delaying the anomaly scan to 20 + 6 weeks (or even beyond) would not provide a solution to reducing the requirement for repeat scans. The results provide important information for pre-screening counselling of obese women. Further work is needed to optimize the methods and timing of ultrasound screening to reduce the burden on sonography provision and improve screening outcomes.

Keywords: Maternal obesity, Body mass index, Fetal anomaly, Screening, Ultrasound scan.

Introduction

Body mass index (BMI) is commonly classified as follows: BMI <30 Non-obese, BMI 30.0–34.9 class 1 obesity, BMI 35.0–39.9 class 2 obesity and BMI 40 and over class 3 or morbid obesity ( Table 1 ) [1] . Prevalence of maternal obesity is increasing and has become one of the most commonly occurring risk factors in obstetric practice[2] and [3]. Obstetric complication rates in obese women, including incidence of congenital anomalies, are higher than in non-obese women and these rates of complication increase with increasing BMI[4], [5], and [6].

Table 1 The BMI characteristics of the study population (n = 1000).

BMI (kg/m2) WHO classification Number of women (%)
≤29.9 Non-obese 808 (80.8)
30.0–34.9 Obesity class 1 115 (11.5)
35.0–39.9 Obesity class 2 51 (5.1)
≥40 Obesity class 3 26 (2.6)

Routine antenatal care in the UK includes a fetal anomaly ultrasound scan performed between 18 + 0 weeks (126 days) to 20 + 6 weeks (146 days) of gestation [7] . Ultrasound examination in obese pregnant women can be challenging due to attenuation of ultrasound in adipose tissue and sub-optimal views of fetal structures and a relationship between obesity and requirement for repeat anomaly scans has been demonstrated[8] and [9].

As maternal obesity has increased over recent years, the increasing requirement for repeat anomaly scans and impact on sonography service provision has been noted within our unit. Initial and first repeat anomaly scans are allocated 30 min appointment time in accordance with current guidelines [7] . If the anatomical survey is still incomplete after the second scan, local guidelines direct women to the fetomaternal unit for further imaging. With many obese women requiring repeat imaging for completion of this scan there is a clear implication for the service in terms of sonographer hours and cost. There is evidence that adequate ultrasound visualization of specific fetal structures is improved at later gestations within the anomaly scan ‘window’ period [10] but whether this equates to a significant effect on completion of the anatomical survey in practice is not known.

The aim of this study is to examine the relationship between maternal BMI at the booking appointment, requirement for repeat anomaly scans and gestational age and to examine the impact that this has on our fetal anomaly screening service.

Materials and methods

We performed a retrospective analysis of routinely collected data from the electronic maternity database (PROTOS) and imaging results reporting (ICE) systems at the Jessop Wing of the Royal Hallamshire Hospital in Sheffield, a tertiary maternity unit in a large teaching hospital with around 7800 deliveries per year.

All women delivering over a two-month period from 1st June to 31st July 2011 were identified (1200). Two hundred cases were excluded (incomplete data 184, multiple pregnancies 8, fetal anomalies identified at first anomaly scan 7 and anomaly scan performed prior to 126 days gestation 1), leaving 1000 for the final analysis ( Fig. 1 ).

gr1

Fig. 1 Case selection flow chart.

Statistical analysis was performed with Excel and SPSS Statistics v21. Spearman rank correlation coefficient was calculated to determine whether a direct relationship exists between BMI and the number of scans required to complete the anomaly scan and also between gestational age at the initial anomaly scan and completion of this scan. Completion of the anomaly scan was defined according to the standards set out in the NHS Fetal anomaly screening program [7] . In some cases the scan could not be completed due to poor image quality and/or fetal position.

Women were divided into four groups according to the WHO BMI classification in order to compare the number of scans required to complete the anomaly scan for each group using Studentttest and calculate relative risk for requirement of repeat scans. Ninety-five percent confidence intervals were calculated where appropriate and statistical significance was assigned wherep-value was less than 0.05.

Results

Of the 1000 women included in the analysis, mean (SD) BMI at booking was 25.8 (5.55) kg/m2(95% CI 25.48–26.17, range 15.2–50.8). This included 808 (80.8%) non-obese and 192 (19.2%) obese women. Of the obese group, 115 (11.5%) had Class 1, 51 (5.1%) Class 2 and 26 (2.6%) Class 3 or morbid obesity ( Table 1 ).

The mean number of scans required to complete the anatomical survey for all women was 1.36 (95% CI 1.33–1.40). Overall, 321 (32.1%) women required two or more scans to complete the anomaly scan. 278 (27.8%) required two scans to complete and the remaining 43 (4.3%) required a third ultrasound scan. The mean (SD) number of scans to complete increased for each class of obesity from 1.43 (0.61) for Class 1 to 1.88 (0.65) for Class 2 to 2.15 (0.73) scan for Class 3. With increasing BMI there was a significant increase in the mean number of scans required up to BMI 35 (two samplet-test,p < 0.02) ( Table 2 Fig. 2 ).

Table 2 Number of repeat scans required to complete the fetal anomaly scan by BMI class.

BMI class Number of scans to complete anomaly USS Mean (SD) number of scans to complete anomaly scan Total scans to complete per 1000 women
1 2 ≥3
Non obese (n = 808) 587 (72.6%) 202 (25.0%) 19 (2.35%) 1.30 (0.51) 1297
Class 1 (n = 115) 73 (63.5%) 35 (30.4%) 7 (6.1%) 1.43 (0.61) p < 0.02 a 1426
Class 2 (n = 51) 14 (27.5%) 29 (56.9%) 8 (15.6%) 1.88 (0.65) p < 0.001 a 1882
Class 3 (n = 26) 5 (19.2%) 12 (46.2%) 9 (34.6) 2.15 (0.73) p < 0.001 a 2154

a Unpaired two sample t test p value comparing mean number of scans of group to non-obese mean.

gr2

Fig. 2 Number of scans required to complete the anomaly scan by BMI.

The number of scans required to complete the anatomical survey was directly related to BMI (Spearmanr = 0.243, 95% CI 0.18–0.30,p < 0.001). One hundred (52.1%) of the 192 obese (BMI ≥ 30) women required more than one scan compared to 221 (27.4%) of the 808 non-obese women (RR 1.90, 95% CI 1.60–2.27,p < 0.001). Twenty-four (12.5%) of the 168 obese women still did not have the anomaly scan completed after two attempts compared to 19 (2.4%) of the non-obese group (RR 5.32, 95% CI 3.0–9.5,p < 0.001). Twenty-one (80.7%) of the 26 women with Class 3 obesity required more than one scan (RR 2.95, 95% CI 2.37–3.67,p < 0.001). Nine (34.6%) of the 26 women with Class 3 obesity required more than two scans to complete (RR 14.72, 95% CI 7.4–29.4,p < 0.001). The mean (SD) gestation at the first anomaly scan was 137.2 (5.02) days (95% CI 136.9–137.5, range 126–179) with 59 (5.9%) women having the first anomaly scan later than the recommended in the guideline. Gestation at first anomaly scan was not related to completion of the initial scan. Gestation at first scan was directly related to BMI (Spearmanr = 0.205, 95% CI 0.14–0.27,p < 0.001). Mean gestation at first scan increased significantly for each class of obesity from 136.5 days for non-obese women days to 145.0 days for Class 3 obesity ( Table 3 ).

Table 3 Comparison of mean gestational age at first anomaly scan by BMI.

BMI group Mean gestational age at first scan (SD) in days Difference from sample mean (days) Significance level
All BMI 137.2 (5.02) 0.00  
Non obese 136.5 (4.61) −0.74 p < 0.001 a
Class 1 138.6 (4.51) 1.40 p < 0.001 a
Class 2 141.9 (5.62) 4.68 p < 0.001 a
Class 3 145.0 (5.45) 7.94 p < 0.001 a

a One sample t test comparing group mean to population mean.

Comments

Obesity rates in our study population reflected national figures, with approximately 20% of women classified as obese at the time of booking [4] . Our results demonstrate the negative effect of increasing maternal BMI on the fetal anomaly screening program in our unit. Increasing BMI resulted in increasing numbers of scan appointments required to complete the anatomical survey with the effect being most pronounced for women in the Classes 2 and 3 obesity groups.

The rate of repeat scans determined in this study can be used to demonstrate the impact of maternal obesity on scanning resources. Based on our results for all women requiring a second scan, we estimate the annual number of repeat scans in our unit to be around 2500 or around 24 additional hours per week of sonographer time. For all classes of obesity, obese women require a minimum of 248 extra scans per 1000 women to complete the anomaly scan compared to non-obese women. In a unit such as ours with a current rate of obesity of around 20%, this equates to an additional 387 scans per year or around 4 h per week ( Table 4 ).

Table 4 Number of repeat scans required to complete the fetal anomaly scan.

BMI Number requiring at least one repeat scan to complete anomaly scan n (%) Repeat scans per 1000 women Repeat scans per year in our unit a
All women n = 1000 321 (32.1) 321 2504
Non-obese n = 808 221 (27.4) 273 1703
Obese n = 192 100 (52.1) 521 813

a Based on 7800 deliveries.

It is notable that a similar smaller study by Phatak and Ramsay [9] found very different results, which raises further questions about the anomaly screening program at our center. Despite a similar rate of obesity (21.4% vs. our 19.2%), the number of women requiring a repeat scan appointment was notably less for both obese (7.1% vs. 52%) and non-obese (1.2% vs. 27.4%) women in this prospective study. This was the case despite allocation of a shorter appointment time (15 min vs. our 30 min). This may reflect differences in policy on timing, technique and sonographer experience but still demonstrates a worrying trend in our unit, which will increase the demands on the sonography service. These results are important in tailoring the pre-screening counseling provided to obese women in our unit.

One of the overarching aims of this study was to propose methods of reducing the burden of repeat anomaly scans on our sonography services and to increase the completion rates and therefore effectiveness of fetal anomaly screening in obese women. There is evidence that delaying the anomaly scan to a later gestational age, when the fetal structures are larger, decreases the impact of suboptimal images in maternal obesity [10] . This had led to the suggestion that delaying the anomaly scan to later in the ‘window’ period or even beyond 20 + 6 weeks in obese women could reduce the need for repeat scans in our unit. This study failed to demonstrate a significant correlation between gestational age at the first anomaly scan and completion of the scan at this initial appointment therefore it cannot be recommended to delay the anomaly scan for obese women to increase completion rates. It is likely that the failure to demonstrate this relationship was due to the unexpected finding that obese women appeared to have their initial anomaly scan performed at a later gestational age compared to non-obese women ( Table 3 ). Although this was not an established policy, it appears that a tendency to delay the anomaly scan booking for some obese women existed where there was difficulty in obtaining adequate images at the dating scan on the presumed basis that the fetus would be larger and therefore easier to assess. Despite this bias and failure to correlate gestational age to scan completion, it has been useful to demonstrate that this strategy of delaying the scan has not solved the problem of need for repeat scans. It is likely that the effect we have found of increasing BMI on completion rate would have been even larger if the gestational age for each BMI group had been the same but a prospective study would be required to determine the true impact of delaying the anomaly scan on completion rate. However, delaying the scan within or beyond the guideline upper limit of 22 + 0 weeks has implications in terms of women's choices if an anomaly is detected. There is growing evidence to suggest a role for first trimester transvaginal ultrasound scan in detection of fetal anomalies [11] . It is possible that a combination of first trimester transvaginal scan and late second trimester transabdominal scan could increase completion rates for the anatomical survey in obese women.

In conclusion, this study demonstrates that maternal booking BMI is directly related to number of scans to complete the fetal anomaly scan. Obese women required more attempts to complete the anomaly scan and number of scans required increases with body mass index. There was no relationship demonstrated between gestation and completion of the scan, suggesting that delaying the anomaly scan to 20 + 6 weeks (or even beyond) would not provide a satisfactory solution to reducing the requirement for repeat scans. Increasing levels of maternal obesity and the associated complications in pregnancy have wide-ranging implications for obstetric service provision. The impact of obesity on fetal anomaly ultrasound screening service provision is significant and likely to increase with rising levels of maternal obesity. Further work is needed to optimize ultrasound screening for obese women and reduce the burden of sonography service provision, particularly concerning the timing and method of ultrasound scans.

Conflict of interest statement

No conflicts of interest declared.

Condensation

The effect of maternal obesity on the need for repeat fetal anomaly scans has significant implications for both sonography service provision and effectiveness of screening.

Acknowledgements

No funding was required for this study.

References

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Footnotes

Jessop Wing, Royal Hallamshire Hospital, Sheffield Teaching Hospitals NHS Trust, Tree Root Walk, Sheffield, South Yorkshire S10 2SF, United Kingdom

lowast Corresponding author. Tel.: +44 114 2711900; fax: +44 114 2711901.