Given the sheer lack of characters in Redwall that are actually pregnant during part of the entire 22 book series (Veil's mother was hardly covered in detail, and Deyna's mother had just given birth to him the night before), I'm not actually sure what sort of outfit pregnant females within the series are supposed to wear. Assuming of course that they do wear anything at all, since with the exception of the cartoon version of Winifred the Otter in Mattimeo, I'm pretty sure all characters wear something. And since I'm hardly familiar with the correct terms of clothing both ancient and modern, I'm not sure what to call the outfit this lady is wearing. But maybe as a society they universally agree that when females are getting large enough, there's no shame in just ditching clothes until after they can fit again? After all, Redwall critters tend to have lifespans closer to their Earth counterparts, so there's no way any of them are pregnant longer than 9 or 10 weeks (otters are pregnant for around 63 days, which is 9 weeks, so I just convert every season of their aging to be a year of ours).

One thing I do mentally with all my pregnancy pieces is I try to visualize the size and quantity of the offspring within the given belly so as to make sure I know what she's carrying. The disadvantage to the angle from which I drew her is that there's not an accurate sense of how far forward the abdomen sticks out. Over the years I've looked enough into the topic of realistic pregnancy bellies in humans and beasts, and determined at least three basic types:

Humans tend to enlarge forward more than sideways.

Many beasts, like horses and elephants, tend to grow more sideways than forward.

Hyenas and cats tend to go large in all directions.

Obviously these aren't all cut and dry, but I have observed those three belly times for full term pregnancies.

The reason I bother mentioning all this is because I think this otter lady of Redwall grew a bit wider than forward. Even so, based on the size of real newborn otters compared to their mothers, and adjusting them to be more like humans with a bit more prenatal development prior to birth, I'd say she has to be carrying twins.