Interesting and quite original topic ( although not completely original as you found in your search). I think the main characteristic of a parasite is that it ' lives off its host'. A human foetus definitely qualifies. you could also argue a foetus is harmful, since many woman die in childbirth.
Of course, in the end it's not, as you showed with your points above. This topic is gonna freak some woman out though, it would be a scary way to think if you were pregnant!