And since an aura is an amalgam between your body's actual state - which for pregnancy is not entirely binary, but it can be made to believe it is - and your emotions, there'll likely be a very subtle change when you actually become pregnant, another one when you find out you're pregnant, still others (multiple others) when your body shifts to supplying different specific nutrients for the infant's development, and yet another when your body starts to shift in order to actually birth the child.
IMO, at no time before birth is the infant's aura going to be perceptible, for the simple fact that it's entirely encompassed by that of the mother. There may be differences in the aura of the mother, ones that are subtly different for one thing or another, that enable a very talented and knowledgeable aura reader to tell one way or the other if the child is a boy or a girl, or maybe even the race of the infant - but I think those differences would be noticeable relatively late in the game, far later than a simple amniotic fluid sample would give you the same information.
Spirits, depending on their sensitivity and interest, could have widely differing reactions. Anima and shadow spirits might be very interested indeed, but for totally different reasons ...
... though it's notable that the insect spirits in 'Missing Blood' did not know that a host was pregnant at the time of their investiture with the insect spirit, and so complications ... arose.