Osamu Tezuka's 1971 entry into the transforming magical girl genre. And by entry, I of course mean introducing the very concept of it. Plus it's another example of the entertainment value of orphans! At the end of this list I should work out exactly what percentage of shows feature orphans.
Melmo and her brother Totoo are left as orphans when their mother dies in a traffic accident, so her dead mother convinces God to give Melmo some magic candy that allows her to shapechange. Blue candy allows her to turn into an adult, Red to turn into a baby. Both at once turn her into a fetus and from that form turn into any animal she likes.
According to Tezuka Osamu @World the show was intended to be something of a sex education film. Something that is borne out by that first sequence in the opening credits featuring the pollen in the breeze. Unlike the transformation of the heroine into an adult, this sex education angle hasn't been one that has been directly imitated by further magical girl shows.