It seems the child’s development is linked to the flow of time within the Blood Wheel Coffin.
"Your Chaotic Black Lotus and my Eternal Undying Lotus have fused to give birth to a dual-lotus entity. My constitution is quite unique, so it cannot be measured by human standards."
"This child has used the dual-lotus connection to manipulate the time flow within the Blood Wheel Coffin. Though it has only been sixty-six days for us, he has actually been gestating for sixty-six thousand six hundred and sixty-six days."
Chapter 604