The actress talked about her fertility struggle on the June 13 episode of Dr. Berlin's "Informed Pregnancy Podcast," revealing how she took a pregnancy test after dreaming she was pregnant.
She was driving with Bell when she decided to break the news.
"I turned to him and I was like, 'Is now a bad time to show you this?'" Mara recalled. "I showed him the stick. He was, like, at a stop light, and he just burst out laughing and was like, 'Oh, my God. How is that possible?'"
But the two, who met on the set of 2015's "Fantastic Four" and married two years later, soon faced awful news when their doctor couldn't find the embryo.
Mara then took blood tests to confirm what she suspected was happening.
"I knew that there was no way that was possible, but you've got that tiny hint of hope," she said.
Mara learned she had a blighted ovum, when a fertilized egg attaches to the uterine wall but the embryo doesn't grow.
"I didn't know what that was at the time. I've learned what that is. So she said, 'Basically, you've miscarried, but it just hasn't left your body yet.'"
It took three difficult months for her body to fully miscarry, Mara said. Medication she was prescribed didn't work.
"Everything just took so much time, by the time it was all over," she said. "It just dragged out forever," Mara said.
When she later found out she was pregnant again, she didn't get her hopes up.
"We were both so nervous," she said. "I was nervous that it was going to happen again, you know? So it just ruins the excitement. I shouldn't be too excited."
It was a healthy embryo, and the couple announced their happy news in January. They welcomed their daughter in May.