Again, not based on any knowledge of the justice system, but..
The police know Helen stabbed Rob (I think?) so isn't the immediate next step to look for a motive. WHY would a heavily pregnant woman stab her husband in front of her child halfway through dinner?
There can't be any more reasons than either she had gone mad or it was self/ child defence? So this is what the police would be looking at next. Sadly for Helen, evidence points more at the former.
Which is also a bad SL for those with MH issues- never get your issues put on record, because they can come back to bite you late on.