The couple, who married in October, already have a four-year-old son, Joshua, and are due to become parents again in June.
Mrs Muamba wrote on Twitter: “Thanks for all the lovely well wishes @fmuamba and I are so happy. We’ve been so blessed and can’t wait to meet our new baby.”
Muamba was playing for Bolton in an FA Cup quarter-final against Tottenham Hotspur in March when he suffered a cardiac arrest and collapsed.
His heart stopped beating for 78 minutes, but he defied the odds to make an incredible recovery. He had hoped to resume his football career, but was advised against it by doctors and retired from the sport in August.
In October, he launched the Hearts & Goals campaign to increase the number of defibrillators in public places and provide CPR training to reduce the 100,000 people in the UK who die of sudden cardiac arrest every year.