"Anaemia, prenatal iron use, and risk of adverse pregnancy outcomes: systematic review and meta-analysis" 
BMJ Jun 2013
http://www.bmj.com/content/346/bmj.f3443
"Conclusions Daily prenatal use of iron substantially improved birth weight in a linear dose-response fashion, probably leading to a reduction in risk of low birth weight. An improvement in prenatal mean haemoglobin concentration linearly increased birth weight."