Martha wanted Jackson back at Kilronin because she could no longer keep the place going and generating money. She was going to end up broke and out on her butt. Martha wanted a grandson so she poked a hole in Helen's diaphragm because they didn't have plans on having a child. She set up the attack to scare Helen into talking Jackson into going back to his childhood home. When Helen only wanted the stay to be temporary she wanted Helen out of the picture because Jackson would definitely stay with Martha to help raise the baby. Martha said that she wanted things to work with Helen, but it was never established if that was true or not.
So in the end Helen would be dead, Kilronin would be up and running, money would start rolling in from the horses, Jackson would be back home for good with Jackson Jr. for Martha to raise with Jackson like

I think Martha was playing the situation to her own advantage as it developed. I don't think it is clear what her intentions at the beginning were. When she sent her poultry man to attack Helen in New York, it is not clear whether the man went too far on his own bat. So we never really know whether Martha wanted Helen's pregnancy to continue, at that point, or not.

Boy=family name & lineage

She had a boy, maybe she connects with boys.
She wanted a baby to take care of, just her and Jackson together. She's an odd mother.

Yea but what was her obsession with baby boys? I wonder why she wanted Helen to get pregnant in the first place.

Yup. Martha poked a hole in her diaphragm