Have to wonder if Jill's 'Yay, Derrick is graduating law school!' was really 'Yay, we can start trying for another baby bc Dreck said no more kids until he's done with law school and now he's done!'. Also, I can absolutely see Dreck under estimating the difficulty of the bar and just assuming he would pass.
As far as how far along Jill was, IIRC she said she miscarried a couple of days after she tested positive. Knowing the Duggars, Jill probably tested early & often, meaning she was most likely barely 4 weeks pregnant, if that. Certainly, people are entitled to grieve however they ch