Published: May 04, 2012 9:26 PM EDT

PITTSBURGH (AP) - Mitt Romney is a grandfather again, and he has a surrogate to thank.
    
Tagg Romney, the presumptive Republican presidential nominee's oldest son, announced Friday that that he has newborn twin sons. Tagg Romney tweeted "big thanks" to the surrogate who gave birth to the twins, named David Mitt and William Ryder.
    
The two additions mean Mitt Romney and his wife, Ann, now have 18 grandchildren.
    
Tagg and his wife, Jen, have appeared on the campaign trail with the former Massachusetts governor, sometimes with their four other children. This is the second time the couple has used a surrogate. Their son Jonathan was born in 2010 via a surrogate.

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