One of the most helpful maternity sewing projects I’ve come across is how to sew your own bra and make adjustments to existing ones. The running joke is that while you are pregnant or nursing, you can wear any size bra and at some point during the day, it will fit you. The rest of the day is really annoying though. Here are several wonderful links for making your bras more adjustable, as well as constructing your own.
Bra Cup Adjustments from Cloth Habit: This is really for making pattern adjustments when sewing your own bra, but the information is useful for figuring out how to alter existing bras, too
Altering Bra Cup Sizes from Sushanna Bellydance Costume Tutorials: Includes a great idea for how to make your own custom cup pattern using duct tape
DIY Bra Extender from La Bouilloire Noire: This is a simple way to create a bra extender from an old bra. You can also buy a pack of extenders at any department store, or if you invest in bra making supplies, you can cut your own from the hook and eye strips.
TIP: How to make a cross-bra strap DIY from Anna Evers at Plan B: This is useful if you need a cross-bra strap for your outfit or to keep your straps from falling down as your band size need fluctuates throughout the day.
How to Convert Your Favorite Bra into a Nursing Bra by Beauty Through Imperfection: This tutorial is great because it uses hook and eye closures that you can salvage from an old bra rather than purchasing a nursing bra conversion kit.
DIY Pull-Down Nursing Top from in-lala-land: Modify a tank so it attaches to your nursing bra.
Faux Cami from me! at Yes This I Know: If your bra is not totally agreeing with your top, attach the faux cami to increase coverage, nurse more discreetly or add a color accent to your outfit.
Bra Making Sew Along from Cloth Habit: this is a step-by-step tutorial on making your own 2- or 3-pieced-fabric-cup bra (see link below for 1-piece and 2-piece foam cups) from a pattern you’ve purchased. The whole Cloth Habit blog is about sewing lingerie, and has some great tips as well as a list of places to buy specialty fabrics and notions.
Make a Foam Cup Bra from Cloth Habit: This is useful for making an entire foam cup bra, adding foam to an existing bra, or making foam inserts to use as removable modesty lining.
How to Sew a Built in Bra with Cups by Pretty Prudent: Useful for constructing new garments or adding support to existing ones