The former Blossom star and his wife are expecting a baby girl they've already named Liberty Grace in April (10).
But with weeks to go before her due date, Yawn-Nelson has been diagnosed with Rh disease, which could cause Hemolytic disease of the newborn (HDN). In severe cases, the condition develops in the foetus and can cause the baby to suffer heart failure and be stillborn.
The couple already has a healthy three-year-old daughter named Charli and the Lawrences are praying for the health of their new addition.
Lawrence tells America's Life & Style magazine, "There are so many unknowns about it because it is so rare. All you can do is pray hard and be positive."