See link for significant background information. We again want
specially crafted virus, similar to what was described there.
A few days after conception, and inside the womb, what was
originally a fertilized ovum has become something known as a
"blastocyst", which attempts to implant into
the womb. Every
in a blastocyst is a special type of stem cell, a "totipotent" stem
cell. So far as we know, this is the only stage of human
development where lots of totipotent stem cells exist. Very
likely, during the first couple weeks after implantation, all of
them will become less-capable "pluripotent" stem cells.
Suppose our specially crafted virus targeted only totipotent
cells? If sprayed into the womb through the cervix, no part of
woman's body would be affected by that virus. Only the
totipotent stem cells in the blastocyst would be affected.
And what do we want this virus to do? Simple! We want them
tell each totipotent stem cell to start processing a different
of DNA code, perhaps muscle-cell DNA code. The entire
blastocyst turns into a tiny blob of muscle cells (or other chosen
cell-type), and after that, does absolutely nothing. The blob
be flushed out of the womb with the next menstrual cycle,
with any virus particles that did not find a target. (If, the very
next month, the woman decides she wants to get pregnant,
there will be no virus there, to interfere.)
This is a "morning after" type of birth-control system, but likely
needs to be applied for several days, because of uncertainty
regarding just when a blastocyst will be THERE, to be
to this virus.