The H19 large intergenic non-coding RNA (lincRNA) is one of the most highly 
abundant and conserved transcripts in mammalian development, being expressed in 
both embryonic and extra-embryonic cell lineages, yet its physiological function 
is unknown. Here we show that miR-675, a microRNA (miRNA) embedded in H19's 
first exon, is expressed exclusively in the placenta from the gestational time 
point when placental growth normally ceases, and placentas that lack H19 
continue to grow. Overexpression of miR-675 in a range of embryonic and 
extra-embryonic cell lines results in their reduced proliferation; targets of 
the miRNA are upregulated in the H19 null placenta, including the 
growth-promoting insulin-like growth factor 1 receptor (Igf1r) gene. Moreover, 
the excision of miR-675 from H19 is dynamically regulated by the stress-response 
RNA-binding protein HuR. These results suggest that H19's main physiological 
role is in limiting growth of the placenta before birth, by regulated processing 
of miR-675. The controlled release of miR-675 from H19 may also allow rapid 
inhibition of cell proliferation in response to cellular stress or oncogenic 
signals.