When Jane Leeves was pregnant on Frasier, you could totally tell... Daphne Moon would always be sitting at a table, plus she was really showing in her face. (I thought the plot they came up with why she had gained so much weight was genius though!)
Same goes for Debra Messing as Grace Adler (Will & Grace)... I could totally tell she was pregnant (as I'm sure could most of the world).
Supposedly Claire Danes was pregnant this season Homeland though, and I totally didn't notice.
Do you notice when actresses are pregnant in real life but not on their TV show?