The primary goal of this study was to establish a standard for normal fetal growth (velocity) and size for gestational age in the U.S. population. Additional goals were to create an individualized standard for fetal growth potential and to improve accuracy of fetal weight estimation. The primary NICHD Fetal Growth Studies Singletons found significant differences in fetal growth and individual fetal dimensions by self-reported maternal race/ethnicity with some differences occurring earlier than others but remaining throughout gestation (Buck Louis et al. American Journal of Obstetrics and Gynecology 2015). These findings suggest that assessment of fetal growth by ultrasound needs to be evaluated clinically using racial/ethnic-specific standards for early identification of potential abnormalities and to minimize misdiagnosis of intrauterine growth restriction and unnecessary clinical interventions. In 2018, researchers developed a model to compute fetal growth velocity percentiles for any given set of gestational week intervals, and determine the association between fetal growth velocity and birthweight. Findings suggest that growth velocity adds additional information over a single measurement of fetal size alone which has potential to change clinical practice (Grantz AJOG 2018). Researchers also investigated the relationship between first trimester vaginal bleeding and fetal growth patterns. First trimester vaginal bleeding was common, affecting 18% of pregnancies. More than 1 day of bleeding was associated with asymmetric, decreased fetal growth, with statistically significant differences beginning in the third trimester and an increased risk of SGA at birth. The magnitude of decrease in birthweight was small and it remains unknown whether early pregnancy bleeding is associated with short-term or long-term morbidity and if additional intervention would be of benefit (Beaver Obstetrics and Gynecology 2018). Obese Cohort Obesity is common among women of reproductive age and is known to increase the risk for maternal and fetal pregnancy complications. The NICHD Fetal Growth Studies enrolled 468 obese women with singleton pregnancies with the goal of comparing fetal growth patterns between women with obesity and non-obese women. Furthermore, because pregnancy complications such as GDM and preeclampsia are more common in women with obesity, this additional cohort offers the opportunity to examine how fetal growth is impacted by such complications. The researchers found that as early as 32 weeks' gestation, fetuses of obese women had higher weights than fetuses of nonobese women (Zhang et al. JAMA Pediatrics 2018). Dichorionic Twin Cohort Twin gestations represented 3.4% of U.S. births in 2013, yet there is limited contemporary data on the estimation of fetal growth trajectories in twins. The NICHD Fetal Growth Studies enrolled 171 dichorionic twin pregnancies. The primary objective was to empirically define the trajectory of fetal growth in dichorionic twins using longitudinal two-dimensional ultrasonography and to compare the fetal growth trajectories for dichorionic twins with those based on a growth standard developed by our group for singletons. Additional research is focusing on the influences of maternal and pregnancy characteristics on fetal growth. Twin pregnancies put additional demands on maternal nutritional status due to the increased maternal and fetal tissue mass. However, the current gestational weight gain recommendations for twin pregnancies are provisional due to limited research specific to twins. In 2017, we studied the associations between maternal weight gain across pregnancy and the growth of the twins as determined by ultrasonography (Hinkle et al. American Journal of Clinical Nutrition 2017). We found that maternal weight gain in the second trimester was associated with fetal growth of the twins. Specifically, the findings were driven by an association with the abdominal circumference earlier in second trimester and the long bones (femur and humerus length) later in the second trimester. The larger intrauterine size persisted to delivery, which was demonstrated by the significant association between maternal weight gain in the second trimester and birth weight. These findings should help to direct intervention studies to determine whether modification of a womans weight gain trajectory can enhance fetal growth and pregnancy outcomes in women with dichorionic twin pregnancies. Biomedical Markers in Relation to Gestational Diabetes and Fetal Growth The NICHD Fetal Growth Studies is the basis for studying the pathogenesis of gestational diabetes (GDM) and fetal growth and to identify factors that can improve early prediction of GDM. This work is grounded within an evolving body of research suggestive of important roles of maternal metabolism and nutrition in the development of GDM and in fetal growth. Pathway specific biomedical markers, and non-targeted metabolomics and lipidomics were measured longitudinally in 107 GDM cases and 214 non-GDM controls in the NICHD Fetal Growth Studies-Singleton Cohort (c.f. Gestational Diabetes Mellitus: Epidemiology, Etiology, and Health Consequences). Notably, it was discovered that thyroid function in pregnant women may be involved in the pathophysiology of gestational diabetes (Rawal et al. Journal of Clinical Endocrinology and Metabolism 2018). These findings, in conjunction with previous evidence of thyroid-related adverse pregnancy outcomes, support the potential benefits of thyroid screening among pregnant women. In addition, a significantly increased risk of gestational diabetes was observed in association with saturated fatty acid levels as early as the first trimester of pregnancy (Zhu et al. American Journal of Clinical Nutrition 2018) providing impetus for future investigations that target circulating saturated fatty acids in pregnant women to improve our understanding of their distinct nutritional, metabolic, and physiologic roles in cardiometabolic outcomes. In 2018, new research also discovered that HbA1C levels can potentially help identify women at risk for gestational diabetes early in pregnancy, when lifestyle changes may be more effective in reducing their risk (Hinkle et al. Scientific Reports 2018). Furthermore, new research found that womens adipokine levels (Hinkle et al. International Journal of Obesity 2018) and vitamin D status (Francis et al. Nutrients 2018) in pregnancy appear to be involved in regulating fetal growth. While more work is eeded, the data offer insight into how maternal body fat composition may influence different aspects of fetal growth.
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