Oddly enough, the StarTrek.com guide lists _both_ Lursa and B'Etor as being pregnant in the "Firstborn" episode with the following text:
Circa SD 47779.4, she had just learned she was pregnant with a son; the father was not specified.
However, it does appear that Lursa was the one who was pregnant. A symbol on a dagger's hilt represented her son (which she didn't yet have). I'll make the changes on the page (subject to someone else providing better evidence otherwise of course) -- Sulfur 22:15, 9 May 2006 (UTC)
Here's the dialogue from the script:
These markings represent the members of our house...
This symbol represents our father...
Myself, my sister... and this... represents her son.
I was not aware you had a son.
I do not... But I am with child...
She found out only a few days ago... and she's told no one but me.
So it was Lursa who was pregnant. --Jörg 22:25, 9 May 2006 (UTC)