In his Complete Works, V4, lecture "The Ramayana", Swami Vivekananda says (available here - http://www.advaitaashrama.org/cw/content.php):
Now, there was another king, called Janaka, and this king had a beautiful daughter named Sitâ. Sita was found in a field; she was a daughter of the Earth, and was born without parents. The word "Sita" in ancient Sanskrit means the furrow made by a plough. In the ancient mythology of India you will find persons born of one parent only, or persons born without parents, born of sacrificial fire, born in the field, and so on — dropped from the clouds as it were. All those sorts of miraculous birth were common in the mythological lore of India.