This is a view that may be acceptable in the "Modern Theravada" groups but the ancients saw it differently.
30. Herein, firstly the span has no sign because there is no definition such as:
Just so much must be lived, no more than that. For beings [die in the various
stages of the embryo, namely], at the time of the kalala, of the abbuda, of the pesi, of
the ghana, at one month gone, two months gone, three months gone, four months
gone, five months gone … ten months gone, and on the occasion of coming out
of the womb. And after that they die this side or the other of the century.
The kalala is that time shortly after the meeting of the male seed with the female egg when the fetus is just beginning to develop. Once this occurs the matter is ripe for teh arising of a new consciousness.
52. But when that minimal amount arises in the two kinds of generation
termed egg-born and womb-born, it amounts to no more than a drop of cream of
ghee on a single fibre of new-born [kid’s] wool, and it is known as the “embryo
in the first stage” (S I 206).
At this stage the size of the fetus is post microscopic where someone with very keen eyesight under good light might just be able to perceive the rupa.