Credit: Stefanie Keenan/

It's a boy -- and a girl!

When first-time parents Neil Patrick Harris and his longtime partner David Burtka welcome twins via surrogate this fall, the newborns will be a son and a daughter, an insider confirms to

While mum on the twins' gender, the How I Met Your Mother star, 37, Tweeted over the weekend that he and chef/actor were expecting. "We're super excited/nervous./thrilled," Harris wrote.

Also first-time parents, Harris' HIMYM costars Alyson Hannigan and Cobie Smulders both welcomed daughters in spring 2009.