I think in this relationship the one who carried wouldn't matter much, even if it was the other way around (Reimu carrying, instead of Marisa). I just never thought that it would be Marisa carrying the baby, but ok, it still works.
And for this picture a tag that would've said Pregnant might've warned people a bit better. Because if it wasn't for Science Babies tag I would've just thought that Reimu was simply resting on Marisa's lap because her belly isn't all that obvious (could be because of the clothes and her sitting position, also Reimu hugging Marisa's belly doesn't help either).