I had a female cousin who was severly retarded and crippled, basically an invalid. She started her period around age 13 and lived to age 25. Her mother and grandmother had to deal with that along with feeding and changing her. They went through a LOT. Thankfully, she was put into a nursing home around age 18. The mother worked at the nursing home and the grandmother visited several times a day. THe situation was SO very sad.
I once saw a news show with a mother and her Down Syndrome daughter. The daughter was in her 20's, and could hold a job and ride the bus, but still needed the support of living in her family home. The woman liked sex and found plenty of "men" to have sex with her.
She was in no position to parent, she couldn't fully understand what pregnancy might entail. Her mother had no power to keep her from leaving the house. The mother wanted to have her sterilized, but the daughter needed to consent. After explaining the operation to the daughter, the daughter refused. Not because she wanted to have a baby, or for any thoughtful reason, she was simply afraid of having an operation (as most young children are).
The mother was the one who would have to bear the brunt of the responsibility for a pregnancy, and yet she was completely powerless.