For rebirth to occur in the human and animal realms there must be present suitable rupa i.e, the mix of egg from mother and matter from father. Once these are present the rebirth consciousness can arise there.
Of course at this first stage of life the embryo is rather small:
Vis. CHAPTER XVII
151. The least decads the first has got respectively are three or two: together with the
rebirth-linking consciousness that is mixed with materiality and comes first in
the pair “mixed and unmixed,” there arise, at the least, the two decads (see
18.5f.) of physical basis and body, or else the three decads of physical basis, body,
and sex. There is no reducing the materiality below that.
152. 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).
Now about the materiality which include the sense bases.
As ven. Dhammanando and volo have said/implied these are very tiny and subtle at anytime . And the eyebase, for example, at this stage does not have the surrounding 'eye' , eye socket and eyelids etc.
Could there be any actual seeing consciousness at this early stage with only this - I couldn't dig up a reference to say yes or no, but I only had a quick look.
In any event - according to the texts - it seems that the pasada rupas are indeed present from birth, but only later , after the embryo grows do eyes, ears etc, which are easily recognisable appear.
vis. CHAPTER XX22. Herein, when materiality is being generated in any
being, it is first generated from kamma. For at the actual moment of rebirthlinking
of a child in the womb, first thirty instances of materiality are generated
in the triple continuity, in other words, the decads of physical [heart-]basis, body,
and sex. And those are generated at the actual instant of the rebirth-linking
consciousness’s arising. And as at the instant of its arising, so too at the instant
of its presence and at the instant of its dissolution.14