Black widows preggers?

[R3GG]

Hello, i have 4 black widows in my 39 gal, 3 of which have become extremely round around their stomachs over the last month or so. The 4th one isn't round at all and is slightly smaller. This has lead me to believe that the 3 fatter ones are female and the smaller one is male.

Has anybody got any experience of these fish breeding? They seem slightly off balance when swimming due their size aswell, im not over feeding them and havent noticed this happen before. The tank has been completely re-arranged fairly recently with the addition of plants - could this have brought on breeding?
Chars

 

[R3GG]

I see this is a topic of great interest

 

[exhibitx]

females have red in their lower fins, males don't.