Maternal–Fetal Medicine


Table 1: Body Mass Index Classification Classification


Underweight Normal weight Overweight Obese class I Obese class II Obese class III


18.5–24.9 25–29.9 30–34.9 35–39.9 ≥40


BMI = body mass index. Adapted from WHO, 1997.1


on multiple gestations suggest that low birth weight, prematurity and growth discordance are similar as in normal-weight women.25


BMI Category (kg/m2) <18.5


However,


one study has suggested that the accuracy of estimated fetal weight (EFW) in the second twin is decreased in obese women.26


This finding


does have potential clinical impact, as significant growth discrepancy in a non-vertex second twin may influence counselling and patient choice in mode of delivery, especially if the second twin is found to be significantly larger.


In a study examining pre-pregnancy risk factors for stillbirth, obesity ranked as the most prevalent.27


Many hypotheses have been proposed


completion of first-trimester and early second-trimester anatomical scanning requires further study.


The importance of detailed and accurate fetal anatomical scanning in the obese gravida is critical given the considerable evidence of increased rates of congenital anomalies. These risks are increased even when controlled for comorbidities such as diabetes.10–12 regions most often affected include the neural tube13–15


Anatomical and the heart.16


In contrast to abdominal wall defects such as gastroschisis (which is less common in obese women), anomalies more likely to occur in obese women include hydrocephaly, cleft lip, cleft palate, anorectal atresia, and limb reduction anomalies.17


Our patients should be counselled regarding these fetal risks as well as the significant limitations in our ability to complete their routine anatomical scan. The sonographer tasked with imaging the fetus of the obese gravida also needs to be aware of the increased anomaly risks and the understandable concern the patient may have when presenting for her scan. The North American First- and Second-Trimester Evaluation of Risk (FASTER) Research Consortium studied over 8,000 women and showed that the detection rate for fetal cardiac anomalies was significantly decreased for obese women as compared with their normal-weight counterparts.18


Smaller studies have repeatedly


shown that anatomy is suboptimally visualized to an alarming degree (20–50 %) in obese women, and in some cases can never be completed to satisfaction.19–22


Apart from shifting the timing of the anatomical scan to either earlier or later in the gestation, there are several strategies that may be used to improve the quality of the given scan. As the maternal adipose layer increases the distance between the ultrasound transducer and the fetus, some investigators have used the maternal umbilicus as a window to the fetus, where the maternal abdominal wall is thinnest.23


The advent


of lower-frequency transducers (e.g., 1 MHz), harmonic imaging, and post-processing software may further enhance our ability to image the obese gravida.24


Modalities such as 3D ultrasound scanning and magnetic resonance imaging (MRI) may prove useful in some instances for scanning in pregnancies affected by obesity, but their role has yet to be defined in this population.


As obese women may present with comorbidities such as polycystic ovary syndrome (PCOS) and infertility, multiple gestations may present additional challenges in imaging. Limited data on the impact of obesity


94


to explain this, including underlying placental vasculopathy secondary to inflammation, hyperlipidemia, and free radical damage. While there are no definite guidelines to aid in prevention of stillbirth in obese women, as this population is frequently affected by comorbidities such as hypertension, diabetes, and obstructive sleep apnea, assessment of the fetal wellbeing in the third trimester is often performed. The effectiveness, frequency, and timing of these assessments in the obese gravida—i.e., external foetal monitoring versus biophysical scoring—require further study.


Diabetes and Estimation of Fetal Weight Multiple studies and consensus documents confirm that obesity is a significant risk factor for pre-gestational as well as gestational diabetes.28–32


While there is agreement that early gestational diabetes screening may be advisable in obese women, the timing has yet to be determined. In our centre, we have elected to start gestational diabetes screening at the earliest prenatal visit, given the likelihood of undiagnosed diabetes and insulin resistance. The effectiveness of this strategy and best methods of treatment (e.g., insulin and/or oral hypoglycemics) need further study.


Prediction of gestational diabetes may be facilitated by first-trimester measurement of maternal visceral adipose through transabdominal ultrasound, as this has been shown to correlate well with third-trimester gestational diabetes screening.33


Maternal obesity is an independent risk factor for foetal macrosomia.28–32 Fetal macrosomia is an important counselling issue in pregnancies affected by obesity, especially if maternal diabetes is present. As the risks include fetal shoulder dystocia, birth trauma, and asphyxia, macrosomia should be considered and assessed for severity.34,35 Unfortunately, the estimation of fetal weight is fraught with difficulty, with the fetal weights in the highest percentiles associated with the poorest accuracy when compared with birth weight—as high as 10 % error.36,37 This implies that some patients may elect for or be counselled toward Caesarean delivery in anticipation of suspected macrosomia. As the risks of Caesarean delivery in the class III obese woman are substantial, a careful discussion regarding labor and delivery risks in view of maternal health status and fetal assessment is essential, with disclosure of the limitations in our ultrasound measurements. More investigation is needed to determine whether there are formulae to estimate fetal weight that might be more appropriate in the obese population.


While detection of macrosomia is important in assessing pregnancies affected by obesity, the diagnosis of small for gestational age (SGA) and


US OBSTETRICS & GYNECOLOGY


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68