Claire Kittle made the announcement in an emotional post for Valentine's Day -- saying she and George found out about the pregnancy on Christmas ... just three weeks before the Niners began their run in the playoffs.

Claire showed the couple -- as well as all of their immediate family members -- were stoked for the news ... but less than a month later, four days before George and S.F. played the Seahawks in the Wild Card round, she said a routine checkup at the doctor led to a terrifying discovery.

"The nurse giving my ultra sound went silent for awhile and I asked 'I’m still early there's probably nothing to see yet right?'" Claire said. "She responded with 'I see a pregnancy here it's just not in the correct place, it’s ectopic."

"I felt my soul leave my body," she added.

Claire went into surgery hours later ... and lost the pregnancy.

"The point of sharing this is not for pity but to acknowledge something extremely personal, hard & emotional that has taken place in our life," she wrote in her social media post, which included multiple loving pictures of her and George.

"To simply talk about it because I felt alone when I got the news, I never knew anyone else who had an ectopic pregnancy before. So here's my direct line to you, i've had one, you're not alone. It sucks, It's hard, be strong."

The All-Pro tight end's wife said she leaned on George and family members in the coming days and weeks to get through the difficult time.

George, meanwhile, played in all three of the Niners' playoff games while dealing with the heavy heart -- catching 10 passes ... as S.F. fell just short of making the Super Bowl.