Look at it from her point of view. She has two options - be open and honest, or hide.
If she's open and honest, worst case scenario is that they don't give her a chance and remove the child at birth. Without knowing her history or particularly knowing how SS judge these things we have no idea how likely this is.
Best case scenario is that they give her a chance but then she has to prove everything all the time and be monitored and there's always that thought hanging over her "If I fail, this child will get taken away too." What parent doesn't have moments where they feel they've failed or aren't cut out for this? And since she has experienced SS removing a child before that fear is all the more amplified. It might be the right choice, but it's not a nice one to make.
If she hides, worst case scenario is SS find out and she loses the child. No second chances. But there may be hope there that it gives her time and space to make the normal parenting mistakes any parent would and, over time, prove she is an adequate parent in the hope that if SS find out later down the line she can say "But look - I've coped fine for the last X years"
Best case scenario, of course, is that they never find out and she gets to live her life with no SS interference unless she fucks up REALLY badly.
Of course, this is all from the mother's perspective, from the "welfare of the child" perspective it could look quite different.