It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Jews said: A child is born cross-eyed when a man approaches his wife from behind. So Allah the Exalted revealed this verse: "Your wives are a place of sowing of seed for you, so come to your place of cultivation however you wish" (__QUR_N__). That is, approach the woman from the front or from behind, but intercourse should only be in the lawful (legal) way.