When we had a few, several developed fatter bellies. Our ability to sex them was pretty poor, but we managed to work out that the males were smaller and that the porkers were the females.
As to whether they were preggers we were never quite sure, but once or twice we saw a couple doing an egg scattering dance.
Basically, the female lays her eggs on the substrate, and the male fertilises them when they are there (rather than when inside her), which gives way to the perculiar dance that they do.
Once there, the eggs lie on the substrate for days (?) before turning into small fry. I say question mark, as we have a lot of bottom feeders and shrimps and the danio eggs didn't make it to anything more than their dinner.
To increase the chance of getting baby danios you should look at isolating the male and female (and removing them shortly after they do their stuff), and laying something porus on the bottom of the tank, foam is a good one. The tank will need slow filtration.
Good Luck!
