I don’t think they were “going for another kid”. They were having sex and she got pregnant. That can happen. When they first got married she used birth control and became pregnant anyway. That pregnancy resulted in a miscarriage. After that they committed to God the number of children they would have. God has blessed them with children and with the finances to provide for them. So now that they have 18 children do you want them to now decide it is in their hands and not God’s?
Maybe they don’t know how to use birth control.
I don’t know, hope their baby is healthy.