The [etymology of *love child*](http://www.etymonline.com/index.php?search=love%20child) says it derived as a polite form of "love brat" which was used around the 18th century.

My question is when two people are in love and they have a child, could you not call him/her a "love child"?

Edit: Why does it have to have the (rather negative) connotation that it is outside of marriage, when love itself is a positive emotion?

Funny enough, "love child" makes sense in Indian English, where there is a concept of "love marriage" and "arranged marriage" - [as has been earlied posted](https://english.stackexchange.com/questions/19541/are-there-synonyms-for-love-marriage).