A method for preparing an enriched sequencing library, the method comprising: (a) non-surgically obtaining a test sample comprising a plurality of double-stranded deoxyribonucleic acid (dsDNA) fragments; (b) preparing an enriched sequencing library from the test sample, wherein preparing the sequencing library comprises: (i) ligating double-strand DNA adapters to both ends of the dsDNA fragments to generate a plurality of adapter-fragment constructs; (ii) adding a first set of primers to the adapter-fragment constructs, wherein the first set of primers comprise single-stranded oligonucleotide less than about 50 nucleotides in length; (iii) hybridizing the first set of primers to the adapter-fragment constructs and extending the first set of primers in a first nucleic acid extension reaction using a polymerase to generate a plurality of amplified adapter-fragment constructs; (iv) enriching the amplified adapter-fragment constructs for adapter-fragment constructs derived from dsDNA fragments less than about 150 bp in length to generate an enriched sample comprising enriched adapter-fragment constructs; (v) adding a second set of primers to the enriched sample, wherein the second set of primers comprise single-stranded oligonucleotide greater than about 50 nucleotides in length; and (vi) hybridizing the second set of primers to the enriched adapter-fragment constructs and extending the second set of primers in a second nucleic acid extension reaction using a polymerase to generate a sequencing library. (a) non-surgically obtaining a test sample comprising a plurality of double-stranded deoxyribonucleic acid (dsDNA) fragments; (b) preparing an enriched sequencing library from the test sample, wherein preparing the sequencing library comprises: (i) ligating double-strand DNA adapters to both ends of the dsDNA fragments to generate a plurality of adapter-fragment constructs; (ii) adding a first set of primers to the adapter-fragment constructs, wherein the first set of primers comprise single-stranded oligonucleotide less than about 50 nucleotides in length; (iii) hybridizing the first set of primers to the adapter-fragment constructs and extending the first set of primers in a first nucleic acid extension reaction using a polymerase to generate a plurality of amplified adapter-fragment constructs; (iv) enriching the amplified adapter-fragment constructs for adapter-fragment constructs derived from dsDNA fragments less than about 150 bp in length to generate an enriched sample comprising enriched adapter-fragment constructs; (v) adding a second set of primers to the enriched sample, wherein the second set of primers comprise single-stranded oligonucleotide greater than about 50 nucleotides in length; and (vi) hybridizing the second set of primers to the enriched adapter-fragment constructs and extending the second set of primers in a second nucleic acid extension reaction using a polymerase to generate a sequencing library. (i) ligating double-strand DNA adapters to both ends of the dsDNA fragments to generate a plurality of adapter-fragment constructs; (ii) adding a first set of primers to the adapter-fragment constructs, wherein the first set of primers comprise single-stranded oligonucleotide less than about 50 nucleotides in length; (iii) hybridizing the first set of primers to the adapter-fragment constructs and extending the first set of primers in a first nucleic acid extension reaction using a polymerase to generate a plurality of amplified adapter-fragment constructs; (iv) enriching the amplified adapter-fragment constructs for adapter-fragment constructs derived from dsDNA fragments less than about 150 bp in length to generate an enriched sample comprising enriched adapter-fragment constructs; (v) adding a second set of primers to the enriched sample, wherein the second set of primers comprise single-stranded oligonucleotide greater than about 50 nucleotides in length; and (vi) hybridizing the second set of primers to the enriched adapter-fragment constructs and extending the second set of primers in a second nucleic acid extension reaction using a polymerase to generate a sequencing library.