The problem is that once this is done, they all act like Elly and run their mouths about how the children are monsters because the magic that's supposed to come along and make them loving mothers is all lies. Giving birth clearly did not magically remove Elly's habit of taking offense over non-events far too readily, it didn't remove her need to wail about how she's the Most Pathetic Victim In The Universe Whose Suffering Exceeds All Who Are, Were And To Be, it didn't remove her ability to be a stubborn jerk about a non-issue, it didn't remove her belief that being asked to get her facts straight before she rushed to judgment about something she didn't know much about was a command to never speak again and it surely didn't remove her volatile temper or psychological need to blame the people she runs over for their being in her way. Since she's the same paranoid, frustrated imbecile she was when she was single, it's obvious to her that somehow, her children somehow took away an ability to be happy she never had.
Were this not bad enough, Elly is convinced that Thérèse is an evil, anti-human monster because she saw through this and tried to avoid it. Then again, she's also an evil monster because she made a concerted mental effort to not experience the 'miracle' of having a surprise pregnancy ruin her plans because otherwise, Elly would have to admit that her children didn't conspire to be born so as to ruin her life and admitting that would make of her a thin-skinned jerk who is the paranoid asshole whining about nothing Jim thinks she is. That would be just awful so it's easier to demonize the woman she's busy screwing over to get her own way all the time.