Most results show a low to moderate negative correlation between IQ and fertility. This isn't exactly my area of expertise, but I can give you the references I found on this topic: Lynn, R. & Harvey, J. (2008). The decline of the world's IQ. read more
A negative correlation between fertility and intelligence (as measured by IQ) has been argued to have existed in many parts of the world at various times; however, critical review of these arguments shows these conclusions to be "superficial and illusory" and not supported by empirical data. read more