Are you familiar with the JustNoMIL sub? So many of the women in that sub are angry with their MIL because they don't want to admit to themselves that their real problem is the father of their children. But to realize that you are in an abusive relationship means to face the fact that ypur SO is not the person you thought they were. It means to take steps to leave that person and to turn your life upside down, to move, to lose an income, to face the horror of custody battles. It's easier to direct your negative feelings towards a more suitable target - a person you can afford to be angry with, that you can afford to lose.

Abusive people have the tendency to show their true colours when they feel you are trapped, as in pregnant in this case. It's never a one-off with abuse. Your SM worries about you. If you thought your loved one was in danget, would you just stay quiet? She has no more betrayed you than the person who calls police when they witness their nephew breaking into the vicar's house hollding a gun betrayed their nephew. A crime has been commited, somebody could be in danger, you don't protect anybody by staying quiet.