ABSTRACT Fetal brain abnormalities (FBA) are one of the most common prenatal sonographic abnormality detected and account for ~20% of birth defects posing a substantial burden on the health care system. FBA can be isolated or syndromic and have vast phenotypic heterogeneity. The paired approach of prenatal diagnosis using ultrasound to characterize aberrant phenotypes with genetic analysis to determine causal lesions has improved the ability to accurately counsel families about diagnosis, prognosis, and recurrence risk. Recently, prenatal exome sequencing (ES) has been applied in cases of lethal or multiple fetal abnormalities to determine a molecular diagnosis that otherwise could not be identified with traditional testing. Our group and others using ES have shown a diagnostic rate of 23.6% in cases of multiple fetal abnormalities, but only 2.6% in isolated FBA abnormalities, indicating a need to improve diagnostic capabilities for FBA. We posit that the overabundance of unresolved fetal cases is due to a gap in our understanding of the repertoire of genotypes underlying prenatal FBA and limitations of population genetics to establish causality of rare variants in novel candidate genes. Our team who is at the forefront of prenatal genetic diagnostics and in vivo zebrafish modeling of human disease will overcome the current challenges of diagnosing prenatal FBA. We will intersect exome- and genome-wide variation with a relevant model system (zebrafish). We hypothesize that we will 1) generate initial discoveries directly relevant to human brain development by modeling novel candidate FBA genes in zebrafish; and 2) improve prenatal diagnosis for FBA using whole genome sequencing (WGS) and deep phenotyping. We will: 1. Perform bioinformatic analysis of 200+ clinically ascertained fetuses with FBA and their parents using a tiered filtering strategy on already available parent-fetus trio exome data 2. Perform WGS on 114 prospectively enrolled fetuses and their parents paired with comprehensive prenatal and postnatal phenotypic data to further characterize genotype/phenotype of FBA; 3. Establish relevance of candidate genes to FBA development and determine variant pathogenicity using genome-editing and phenotyping tools in zebrafish. Our work will expand the understanding of molecular processes governing human brain development, establish a clinical-research hybrid platform readily applicable to FBA and other anatomical defects detectable by fetal imaging, build an animal model of aberrant FBA development with potential for future use in therapeutic target identification. Our immediate results will improve counseling/management of prenatally diagnosed FBA and lead to future work to develop novel therapeutic and preventative strategies for FBA.