I agree with this. I'm taking a statistics class in grad school now, and I was imagining what my professor would say about these studies. I think she would say, this is interesting and probably warrants further study, but this is not enough data to make a real conclusion. The problem is that it's really hard to design a study about the effects of almost anything on fetal development, because it's unethical to purposely have pregnant patients exposed to something that we think could be harmful. So there are a million things that might be safe but we just don't know, and this is one of them. No judgement to anyone who used MJ while pregnant, I know that it can be a huge help for nausea and other symptoms, so you gotta do what you gotta do!
My academic background involves assessing epidemiological studies like these. I currently don't have full access to read the full studies, but just from glancing at the abstracts, I think the evidence from these studies is insufficient to make a conclusion on the long term effects of marijuana on development. The sample sizes seem to small (they may be too small to detect a difference). It might not be appropriate to generalize the results to the entire population of pregnant women.
This doesn't mean that smoking marijuana is harmful. I'm just saying that I personally will take such papers with a pinch of salt until I see several long-term studies with consistent results. Or studies with very large sample sizes.