Maternity leggings need to actually stretch to fit a changing body rather than just being marketed that way, so I looked at MARION Maternity's Black Maternity Eco Leggings with Concealed Pocket, priced at $85.
These leggings are made from a 75% recycled nylon / 25% spandex blend, with a breathable, cool-touch fabric finish. A back-waistband phone pocket is built in — a small but genuinely useful detail for anyone who's dealt with an over-bump waistband with nowhere to stash a phone. They're cut with a 26-inch inseam, are machine washable, and are designed to fit across all three trimesters and into the postpartum period.
Using recycled nylon in the blend is a genuine sustainability detail rather than just a marketing label, and the concealed back-waistband pocket solves a real, common complaint with over-bump leggings. These currently hold 12 reviews at a 5-star rating. One important note: as of this review, all sizes are showing sold out on MARION's site, so check current availability before counting on a specific size being in stock.
For a sustainably-made maternity legging with a genuinely useful pocket design, this is worth watching for restock. Check out MARION Maternity for current availability and pricing. Browse MARION Maternity's full lineup for their other maternity wear.