The Hereafter actress will give birth to the baby - whose sex is still unknown - later this year, reports US Weekly.
The couple are already parents to son Theo, who was born in 2007. They met as students at New York University and married in June 2006.
Howard previously confessed that she suffered from post-natal depression following Theo's birth and felt "totally overwhelmed in every area" of her life.
The actress, who did not seek any treatment for depression, added: "I was crying in the shower all the time, not feeling like myself, not feeling connected to my son or to my husband - certainly not to myself - not being able to make decisions."
She can next be seen in 50/50, a cancer comedy starring Joseph Gordon-Levitt and Seth Rogen which opens in US cinemas on September 30.
A UK release date has not yet been confirmed.
Howard recently described the film as "truthful" and insisted that it would uplift movie-goers despite its dark subject matter.