Demand is phrased by stage, by function and by occasion rather than by garment. Stage queries ask what to wear at a point in the pregnancy. Function queries ask whether something is nursing friendly, whether it fits over a bump, whether it can be worn afterwards. Occasion queries name an event: a wedding, a shower, a family photo session, a return to the office. The pages that carry them are trimester collections with copy about what changes at that point, occasion collections filtered on the nursing attribute, and function collections such as nursing-friendly dresses that exist as real URLs rather than as filter parameters.
The sizing guide is the most valuable page a maternity brand can own. It should explain in plain language how the brand's maternity sizing relates to a pre-pregnancy size, what changes between the second and third trimesters, where the fit differs across the range, and what to do when someone falls between two sizes. Written well it ranks for a query nearly every customer types and it prevents the return that otherwise follows. A postpartum guide does the same job for the second cycle.
Technically, sizes and colours are variants of one product on one URL. Sold-out sizes stay on the page with a notification option rather than vanishing, since size availability is the most common reason a shopper leaves. Seasonal collections keep their URLs from year to year, and model height and stage belong in text rather than burnt into an image, because text is what search and assistants can actually read.