It does not, because (1) many birth defects are the result of de novo mutations, and (2) most of the people afflicted by serious birth defects cannot reproduce, so their aberrant genetic profile is not preserved in the gene pool.
On the other hand, those who are pro-abortion are very likely changing the allelic distribution of the gene pool. A woman whose maternal instinct is so weak that she can kill her own children as a form of birth control is less likely than other women to have living children. This is as much a result of psychological factors as it is of physical factors. Abortion damages the reproductive organs, making post-abortive women more likely to have future pregnancy loss or premature birth.
In short, pro-aborts are culling the very characteristics that make them pro-abort out of the human gene pool.
My comment after the election was that all the feminists had to do to get Hillary elected was to turn out their base. But the base couldn’t turn out - it was ABORTED!