That is no longer the case. With current reproductive technology there are at least two biological meanings of "mother" and will soon be a third. A gestational mother is the woman in whose womb a fetus was incubated. An egg mother is the woman whose fertilized egg became the fetus. Once human cloning becomes an established technology, a mitochondrial mother will be the woman whose egg, with its nucleus replaced by the nucleus of the clone donor but with its own extra-nuclear mitochondrial DNA, developed into the fetus. And once genetic engineering becomes a mature technology, permitting us to produce offspring whose DNA is a patchwork from multiple donors, the concept of "a" biological mother (or father) will be very nearly meaningless.