Shades of “baby bump.” Once again, a singsongy term (that one alliterative and prenatal, this one rhyming and post-) much loved by the Daily Mail and other British tabloids has made its way to the U.S. Unlike the very popular “baby bump,” this new one doesn’t have much of a presence over here–where the preferred affectionate term is “mommy,” not “mummy”–but being featured in an NPR story a couple of days ago may change that.
Don’t get me wrong. It isn’t my place to comment on any exercise new mothers (or anyone else) choose to undertake. But let’s not take away their dignity. The NPR story says of this condition, “It turns out the jelly belly actually has a medical term: diastasis recti, which refers to a separation of the abdominal muscles.” So, rather than cutesy terms like “jelly belly” or “mummy tummy,” can’t we just call it what it is?
notoneoffbritishisms.com 
Diastasis recti is neither ‘baby bump’ nor ‘mummy tummy’. It’s a specific medical condition that CAN occur in pregnancy but isn’t inevitable. It can also occur in other people e.g. newborn babies.
Baby bump means your pregnant belly. Mummy tummy may mean that (I don’t read the Daily Mail, thank G-d), but I suspect it may mean the shape you get after you’ve had your baby when everyone in the media is urging you to re-acquire a flat stomach. It’s possible, therefore that ‘mummy tummy’ is somewhat pejorative while ‘baby bump’ isn’t.