Ultrasound in the management of obesity in pregnancy

Abstract

T he prevalence of obesity in women is increasing on a world-wide basis, not only in developed countries but now also in developing countries. Here in Australia, about 40-50% of the population are either overweight or obese. 1,2 This has been linked to an increased risk of pregnancy complications including fetal growth disorders, pre-eclampsia and gestational diabetes. [3][4][5] In the post-natal period, maternal haemorrhage and infections are more common. The neonate is also at risk of low Apgar scores, jaundice and hypoglycaemia. [6][7][8][9] The underlying pathophysiology in obesity is a chronic inflammatory state resulting from cytokines released by excess adipose tissue. In pregnancy, there is a natural increase in inflammation, and in combination with obesity, deranged metabolic homeostasis may occur with subsequent pathology. [3][4][5]10 Traditionally, obesity has been quantified in terms of the body mass index (BMI), calculated as the weight in kg divided by the square of height in metres. This was originally invented by Adolphe Quetelet, a Belgian astronomer and social scientist, around 1840. There are several limitations to this index. It does not reflect the actual distribution of body fat, 11,12 and this is a major issue since it is well known that central (abdominal) fat distribution is more closely correlated with adverse outcomes such as hypertension, diabetes and cardiovascular disease, than peripheral fat. 13 It does not differentiate between muscle mass and fat mass, and hence there may be ethnic differences in reference standards. 14 This measurement is somewhat biased in that tall people tend to have BMI that are unduly high for the amount of fat that they carry.Accurate measurements of central adiposity can be obtained by computed tomography, magnetic resonance imaging or body densitometry. Apart from cost, these methods may not be suitable for pregnant women in routine clinical practice. An alternative index of central adiposity is the subcutaneous fat thickness (SFT). Abdominal SFT has no correlation with age, number of pregnancies, or history of laparotomy, 15 but it becomes thinner as pregnancy progresses due to the increase in abdominal volume and pressure. The techniques for the sonographic measurement of the SFT and other measures of intra-abdominal fat were first described by Armellini et al. in non-pregnant women in 1991. 16 They found these measurements reliable and superior to skinfold measurement with callipers.The first clinical correlations of ultrasonic assessment of adiposity were reported in a small study by Martin et al. 17 Subcutaneous and visceral fat was measured in pregnant women at around 12 weeks' gestation. A glucose challenge test (GCT) was carried out at around 26 weeks' gestation. They found that visceral adipose tissue depth above the upper quartile was associated with a positive GCT, whereas subcutaneous fat was not correlated. Similar results were reported by Gur et al., 18 who also noted that sonographic visceral fat was superior to waist circumference and BMI ...