My Fish Die After Giving Birth

Older females are more likely to die after giving birth - how old are the guppies that died?

And I would assume that a fit healthy guppy would cope all right, so I had better ask - do your guppies get a good varied diet?

I wonder how old she was when you got her 

Dried bloodworm is not as nutritious as live or frozen. You can also get other frozen food to try - I just bought a multi pack of bloodworm, brine shrimp, daphnia and mysis. Guppies also like veggies; try cooking a few peas, squeezing them out of the skins then chopping them till they are guppy mouth sized.

Dried bloodworm is one cause of constipation in bettas, so it may affect guppies the same way. With bettas it is always best to soak dried bloodworm until it is thoroughly rehydrated, something to try with your guppies if you don't already do this.

They could well pick on cucumber though they probably won't consume it quite as fast as, say, a plec would.
Courgette is more nutritious that cucumber. Microwave a slice in a bit of water for a few seconds to soften it. If you don't eat courgette yourself, all you have to do is buy a small one, slice it and freeze it on a tray (then put it in a bag). All you need to do is take one slice out of the freezer, let it thaw and put it in the tank. Freezing & thawing has the same effect on the slice as microwaving it. The slice will probably float at first which is a good thing with guppies; since they don't normally feed off the bottom, you don't need to weight it down.

Whether you feed cucumber or courgette, remove any uneaten after 12 hours or it'll make the water go yucky.