Backstreet Boys star Kevin Richardson has opened up about fertility struggles that have prevented him and his wife from expanding their family.

The singer has revealed he and his other half Kristin really suffered before their second child, son Maxwell, was born in 2013, because for the longest time it "just wasn't happening".

"Me and Kristin had our first-born, Mason, with no issues at all," he tells Entertainment Tonight, but then they realised making babies wasn't a given for a loving couple.

"We both got checked out and Kristin had some issues. We couldn’t believe it. It was like, 'Wow. OK, so this isn’t going to be easy. Maybe it’s not in the cards for us to have another child'. That was a realisation that smacked us in the face."

Kristin previously shared her pregnancy struggle in the documentary Pushing Motherhood.

After a year of trying to get pregnant naturally, Kevin and Kristin looked at alternatives and underwent two rounds of in vitro fertilisation.

"We tried IVF, we tried artificial insemination, and none of that worked," Kevin explains to ET. "She was injecting her body with all kinds of hormones to make her ovulate, then on top of that was all the stress. She also couldn’t exercise and she’s a dancer, so it was totally unnatural. I just tried to be as supportive as I could, let her lead and be there for her."

The Backstreet Boys star is reliving the couple's fertility issues in a hard-hitting new film, titled If I Could Tell You, which chronicles a woman's search for a black market sperm donor in a desperate bid to get pregnant.

He admits he and his wife even started considering adoption, adding, "It just didn’t feel right, so we were like, 'Let’s stop this. Let’s reset your body, reduce stress, make you feel happy and healthy, then go back to the roots of how it’s supposed to be done. If it’s meant to be, it’s meant to be, and if not, we’ll adopt'."