Nos. 17 and 18 have arrived.

Mitt Romney’s eldest son, Tagg, announced Friday the births of twin boys, David Mitt and William Ryder, bringing the presumptive Republican presidential nominee’s number of grandchildren to 18.

In a statement posted to Facebook, Tagg Romney said that David Mitt and William Ryder are “healthy and happy.” The twins were born to a surrogate, he wrote, adding that “life truly is a miracle.”

David and William are Tagg and Jen Romney’s fifth and sixth children. Mitt and Ann Romney’s four other sons and their wives have 12 children among them.

In his full statement, Tagg Romney wrote:

Happy 2 announce birth of twin boys David Mitt and William Ryder. Big thanks to our surrogate. Life is a miracle.…

— Tagg Romney (@tromney) May 4, 2012

“Jen and I are happy to announce the birth of twin boys, David Mitt and William Ryder. Everyone is healthy and happy. They weighed in at 5 lbs 13 oz and 19-3/4 inches and 6 lbs. 13 oz and 19 inches. A special thanks to our gestational surrogate who made this possible for us. Life truly is a miracle, and we feel so blessed to be able to celebrate the arrival of these precious boys into our family. For those keeping score at home, these are grandchildren numbers 17 and 18 for my parents.”

Ann Romney also celebrated the births on Twitter on Friday, writing: “Grandchildren 17 and 18 are here — congratulations @TRomney and Jen! We can’t wait to meet David and William.”

The grandparents might have that chance this weekend. Mitt Romney is expected to be off the campaign trail on Saturday and Sunday, spending time in the Boston suburb of Belmont, where both the elder Romneys and Tagg’s family live.

Mitt Romney often talks about his 16 grandchildren on the campaign trail, but at one stop last fall he let slip that more were on the way, saying he had 17 grandkids before correcting himself.

“I’m counting ones that aren’t hatched yet,” Romney said.