Challenges of third trimester scanning in obese women

32.1

Introduction

• Obesity has been recognised as an important public health problem, and worldwide the prevalence of obese pregnant women continues to rise.

• In the United States, approximately 61%–64.5% of the total population is classified as either overweight or obese; in the United Kingdom, 33% of women are overweight and 23% are obese, giving a total of 56% of women over the recommended BMI.

• Recent reports of the UK Confidential Enquiry into Maternal Deaths have reported obesity to be a factor associated with direct maternal deaths. Over a quarter (29%) of the women who died in this triennium were obese and a further 26% were overweight.

32.2

Why do obese pregnant women need a third trimester scan?

• Obese pregnant women have increased chances of comorbidities like type 2 diabetes mellitus (DM), chronic hypertension, and chronic kidney disease, compared to pregnant women with normal BMI. All these diseases are known to be associated with placental insufficiency, and hence have implications for impaired foetal growth. Another spectrum of disordered growth is occurrence of macrosomia/large for gestational age foetuses in diabetic mothers. The prevalence of macrosomia is reported to be higher in diabetic mothers who are also obese.

• Obesity itself is associated with development of various antenatal complications, such as gestational diabetes mellitus (GDM), preeclampsia, and venous thromboembolism, which necessitate third trimester scans for foetal growth and well-being. Meta-analysis has shown that the estimated risk of stillbirth may be twice as high in obese pregnant women as that in normal weight pregnant women.

• In the maternity units in the UK, it is not a routine practice to offer a third trimester scan to low-risk pregnant women, rather the need for a third trimester scan is guided by clinical assessment of foetal growth by symphysio fundal height (SFH) assessment. Obesity precludes correct assessment of SFH, which is difficult to perform in such women due to excessive maternal adipose tissues. The RCOG recommends that women with BMI >35 kg/m ² should be referred for serial ultrasound assessment of foetal growth as obesity contributes to the limited predictive accuracy of SFH.

• Knowledge of foetal lie and presentation is essential to help pregnant women formulate an appropriate birth plan. Unfortunately, maternal obesity often results in inconclusive clinical assessment of foetal presentation, hence the need for an ultrasound scan.

With advances in technological advances, ultrasound now is considered as a safe modality for foetal assessment, especially after the embryonic stages of development, provided the ALARA principle is adhered to.

32.3