I don't think this is necessarily a JW thing, but a selfish thing on the part of many parents. I am 39 and I have a child that is one. People ask when I am going to have #2 and I say, "Never."
They seem shocked. When I explain I have a perfect child, just wonderful, there couldn't be another baby more beautiful, so why should I mess with perfection. I have enough money for her college education, a 2 bedroom house, day care expenses, etc. It would not be economical to have another, especially if my financial situation took a downturn. And then, after 40 the chances of a DS kid go way up.
Now, if I had a DS child, what would the effect be on my 1 year old to grow up always with a mentally retarded sibling. Facing facts means I'd have to teach her to tolerate other kids making fun of her and her sib. It would mean there is a good chance that in my daughters young adult-hood, she'd be left to care for her DS sibling, since people at 60 get cancer, heart attacks, etc.....even if I was 70 and I died, that means my daughter would be 30 and caring for another adult for the REST OF THEIR LIVES. I am willing to make decisions that effect my life. I am not willing to make those kinds of decisions for my baby.
JW's do say "Jehovah willing" an awful lot, but worldly people say, "the Lord provides" or "God works in mysterious ways." People don't stop and think, they see movie stars having babies at 40 and don't realize they have gotten other eggs or in-vitro .... People think, "Oh an only child" or they think "I better have a baby, who cares if I am 45!"
Ya gotta think past yourself and see the whole picture.