You would expect that. Typically the first trimester is when the fetus is most susceptible. It happens with other viral infections, exposure to chemicals, exposure to alcohol, or exposure to any sort of thing. That's what you expect, though there is also evidence to suggest it can have an effect in the last trimester as well. We'll see where that one goes.
On your comment about malnutrition, this may be multifactorial, which means you need several things to come together. What's still interesting is that this is from one area of Brazil only and there tends to be a lower socio-economic status in that area, so it may be that the combination is what causes this, and that's what we'll find out.