The Impact of Pre-Pregnancy Maternal Obesity on Nuchal Translucency Measurement at Time of First- Trimester Screening: A Single Center Retrospective Analysis

Research Article

Austin J Obstet Gynecol. 2016; 3(3): 1060.

The Impact of Pre-Pregnancy Maternal Obesity on Nuchal Translucency Measurement at Time of First- Trimester Screening: A Single Center Retrospective Analysis

Young OM¹* and Canavan TP²

1Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Washington University in St. Louis School of Medicine, USA

2Division of Ultrasound, Magee-Women’s Hospital/ University of Pittsburgh Medical Center, USA

*Corresponding author: Young OM, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Washington University in St. Louis School of Medicine, 660 South Euclid Avenue, Campus Box 8064, St. Louis, Missouri 63110, USA

Received: September 23, 2016; Accepted: October 13, 2016; Published: October 17, 2016


Objective: To evaluate the impact of pre-pregnancy maternal obesity on Nuchal Translucency (NT) measurement during a first trimester screen ultrasonographic examination.

Methods: Using the Magee-Women’s Hospital Ultrasound Database, a retrospective cohort study of women with singleton pregnancies undergoing successful NT screening from January 2008 to October 2013 was performed. Body Mass Index (BMI) was calculated from available pre-pregnancy height and weight data. The relationship among obesity, gravidity, parity, diabetes status, maternal age, ethnicity, pre-pregnancy weight and NT measurement was analyzed via multivariable linear regression. BMI categories and NT thresholds for consideration of invasive fetal testing for aneuploidy were examined via χ2 test and t-test, when appropriate. A P-value <0.05 was considered statistically significant.

Results: Among 24, 862 women with singleton pregnancies, 7,408 (29.8%) cases were identified. After adjustment for confounders, increasing pre-pregnancy maternal BMI was significantly associated with a larger NT measurement (P<0.0001). Those with a BMI ≥40kg/² had a significant association with a NT measurement of 3.5mm or greater (P=0.028). Mean NT measurements between BMI <30kg/² and BMI ≥ 30kg/² were not statistically significant but trended toward an increased NT in the BMI ≥ 30kg/² cohort (1.62mm vs. 1.67mm: p=0.31).

Conclusion: Pre-pregnancy BMI greater than or equal to 40kg/² is significantly associated with a NT measurement of 3.5mm or greater at time of first trimester screen.

Keywords: Maternal Obesity; Body Mass Index; First-Trimester Screen; Nuchal Translucency


NT: Nuchal Translucency; BMI: Body Mass Index; CDC: Centers For Disease Control; FMF: Fetal Medicine Foundation; WHO: World Health Organization


Obesity has become an increasingly prevalent public health problem in the United States of America, reaching epidemic proportions. According to 2009 Centers for Disease Control and Prevention (CDC) epidemiologic data on obesity in the United States, 35.7% of the American population is considered overweight or obese [1]. Currently, on the review of the literature, over 20% of American pregnancies are complicated by maternal obesity [1]. Obesity has been well correlated with numerous adverse maternal pregnancy outcomes such as hypertensive disorders of pregnancy, gestational diabetes, and increased rates of operative delivery [2]. Moreover, a recent metaanalysis demonstrated that, for obese mothers, the odds of having a fetus with a neural tube defect is 1.87 times higher compared to a mother of normal weight [3]. Thus, obesity can portend the possibility of detrimental consequences for both mother and fetus.

First trimester screening is employed for the detection of fetal aneuploidy with use of Nuchal Translucency (NT) measurement and assessment of maternal serum markers. Its use has been validated in the First-Trimester or Second-Trimester, or Both for Down syndrome (FASTER) Trial in 2005 [4]. However, this study did not include any data relating Body Mass Index (BMI) to the components of first trimester screening [4]. An increased NT has also been associated with adverse outcomes including spina bifida, skeletal dysplasia, congenital heart disease, congenital diaphragmatic hernia or intrauterine fetal demise [3,5,6]. With nearly 29.7% of adults in the state of Pennsylvania being classified as obese, the implications of obesity on adverse pregnancy outcomes in our population become even more essential to recognize and define [7]. There are insufficient data to suggest whether or not pre-pregnancy maternal obesity plays any role on the value of the NT measurement obtained at the time of first trimester screening and what is both the extent and clinical implications of such an increase [8,9]. Therefore, given the ever-increasing rate of pre-pregnancy maternal obesity, we aim to determine if pre-pregnancy maternal obesity, as defined by BMI, increases the value of the NT measurement obtained on a first trimester screening ultrasonographic examination among a diverse obstetric population in western Pennsylvania. Our hypothesis is that women with pre-pregnancy obesity have a higher incidence of having an abnormal nuchal translucency on first trimester screening.

Materials and Methods

We reviewed a retrospective cohort study utilizing the Magee- Women’s Hospital Obstetric Ultrasound Database for all women with singleton pregnancies between January 2008 and October 2013 who presented for first trimester screening. Magee-Women’s Hospital is a tertiary care center in western Pennsylvania whose obstetric service delivers over 11,000 babies per year and performs over 100,000 ultrasound examinations per year. Pregnancy dating was determined by the crown-rump length, as measured on the day of the NT measurement. All women with singleton gestations were included in our analysis if they had a successful NT measurement obtained between 11 weeks 0 days and 13 weeks 6 days and had a crown rump length between 41mm and 84mm. The following cases were excluded from analysis: those with a cystic hygroma, a fetal anomaly diagnosed at the time of NT measurement, fetal demise or those who declined testing. All NT measurements were performed at Magee-Women’s Hospital, an obstetric tertiary care center performing over 100,000 ultrasounds per year.

In our ultrasound unit, both sonographers and maternalfetal medicine physicians are able to obtain NT measurements in accordance with previously published criteria established by the Fetal Medicine Foundation (FMF) only if they are certified to do so [10]. All NT measurements were taken with the fetus in a neutral, sagittal position with the widest part of the nuchal translucency being measured. Calipers were placed on the inner border of the horizontal lines defining the nuchal measurement. Each fetus had 3 measurements of the NT taken, with the image fulfilling FMF criteria being accepted as the NT measurement for that respective fetus. All NT measurements were obtained using a Philips IU22 ultrasound system using a C5-1 MHz trans-abdominal probe (Philips Healthcare, Andover, Massachusetts). All images were automatically stored into the picture archiving and communications systems software for any future review. We hypothesize that pre-pregnancy maternal obesity does influence the fetal NT measurement, with a larger BMI leading to a larger NT. We used the previously accepted World Health Organization (WHO) criteria to define obesity as having a BMI greater than or equal to 30kg/² [11,12]. BMI was calculated using available pre-pregnancy height and weight data, which were obtained from both the electronic medical record and, if not immediately available, patient self-report at presentation for nuchal translucency measurement.

The primary outcome of our study was an abnormal NT measurement. We examined the association between the NT measurement and maternal BMI, as calculated from the recorded height and weight at the time of the examination, via multivariable linear regression. Measurements of both NT and of maternal obesity were transformed into categorical variables in order to observe the association of increased NT measurements with maternal obesity, defining 3.0mm and 3.5mm as an increased NT measurement and using WHO definition of obesity as previously described, via a chisquare test. Pearson’s correlation coefficient was used to examine the correlation between pre-pregnancy weight and NT measurement.

Furthermore, we planned subgroup analyses a priori using a twostep approach. Firstly, we investigated the relationship of maternal obesity class (i.e. BMI ≥ 30kg/² and BMI ≥ 40kg/²) on having an increased NT measurement, as compared to non-obese controls (BMI < 30kg/²), via an analysis of variance with Bonferroni correction for multiple comparisons. Secondly, we analyzed the association between maternal obesity and an increased NT measurement by ethnicity classification, while controlling for maternal age. Other covariates controlled for in the analysis included gravidity, parity, ethnicity, gestational age at NT measurement, smoking history and maternal diabetes status. Definitions of a NT measurement threshold at which to consider further testing for fetal aneuploidy (i.e. chorionic villus sampling, amniocentesis or non-invasive prenatal testing) vary by gestational age and institution; however, current literature does support using either 95th percentile or 99th percentile as cut-offs for an abnormal measurement [13]. Nicolaides and associates provided two different recommendations for a NT threshold for invasive testing: one study suggesting 3.0mm, given its correlation with aneuploidy, while a second study suggested a threshold of 3.5mm since it represents the 99th percentile irrespective of gestational age or crown rump length [13,14]. Therefore, a priori, we considered an abnormal nuchal translucency measurement as both 3.0mm and 3.5mm in separate analyses. The statistical significance of the associations was analyzed by the means of a two-sample Student’s t test.

We defined statistical significance as a p-value of less than 0.05. STATA 13.1 software was used for the conduct of all analyses (StataCorp, College Station, TX). This study was approved by the University of Pittsburgh Institutional Review Board #PRO13100433.


A total of 24, 862 women were included in our analysis with mean pre-pregnancy weight and BMI of 70.0kg and 25.9kg/², respectively. 7,408 women were classified as obese, representing approximately 29.8% of our total cohort. NT measurements for all women were obtained at a mean of 12.24 weeks gestation. Gravidity data was available for 23, 317 women (93.8%), with a mean gravidity of 2.39 (SD: 1.604). Parity data was only available for 12,451 women (50.1%). Of these patients, 6336 (50.9%) were considered multiparous, having given birth to one or more living children.

Our data demonstrate that the average NT measurement does increase with increasing BMI category, as noted in (Table 1) [1]. In evaluating our data for non-obese women and the previously delineated classes of obesity, there was significant difference in NT measurements between non-obese women and those with a BMI ≥ 30kg/² (p=0.046). When adjusted for maternal age, ethnicity, gravidity, parity, maternal diabetes and maternal smoking status, BMI had a significant association with NT measurement (p<0.001) in our regression model. For every 1kg/² increase in maternal BMI, there is, however, only a 0.00442 increase in fetal NT measurement (r2=0.0107). The regression model also showed a significant association of NT measurement with African-American ethnicity (p<0.001), Asian ethnicity (p=0.002), parity (p=0.007) and smoking (p=0.039). There was a slight positive correlation between patient’s pre-pregnancy weight and NT measurements obtained (r=0.0223).

Citation: Young OM and Canavan TP. The Impact of Pre-Pregnancy Maternal Obesity on Nuchal Translucency Measurement at Time of First-Trimester Screening: A Single Center Retrospective Analysis. Austin J Obstet Gynecol. 2016; 3(3): 1060.