I feel differently about this case, but that's not to say I'm correct.
I just think that the elements of this crime -
(presumably - not backed up by charges yet) murdering someone who had been in her life for over half her life,
choosing a baby that could easily and quickly be identified, by sight and DNA,
and, very possibly, planning this since the day she heard Heidi was pregnant, and perhaps for no other reason than knowing Heidi was pregnant,
- make it appear to me as if the issue might not be as simple as proving she was pregnant.
How much less of a risk it would be to snatch a random stranger's baby - without the added complications of murder, association, and easy/quick identification of the baby.
I'm not saying I know what motivated MF, but I strongly lean towards this being a crime of some sort of retribution, in her twisted mind. To plan a pregnancy to run alongside Heidi's kind of cancels out the retrospective need to prove pregnancy. (IMO)
It makes sense that she was looking for a way to hook CG by having a baby, but choosing Heidi seems like a bigger factor when I weigh it up. (MOO)