There's no steadfast rule, but in general, it’ll be time to say goodbye to your lacy lingerie and find a good maternity bra around week 18. And no, your current bra in three sizes bigger won't do the trick — maternity and nursing bras are specifically designed to provide the best support for growing, pregnant bodies. Forget those "room to grow" ideas — it’s important to have optimum support right from the start. Instead, save money by getting nursing bras, which you'll also be able to use after delivery. They offer the same support, but have an opening in the cup for feeding.
Professional bra fitters are your best option for finding the perfect fit, but if you’re shopping solo, keep these rules in mind: the cup should fully cover your breast, the band shouldn't ride up in the back, back closures should be thick and have four or more hooks, and straps should be wide and cushioned for comfort and support.
Some professionals recommend steering clear of underwire bras, because they can restrict milk ducts. Look for high cotton content material, which will let your skin breathe. Buy at least two or three — no need to spend your entire pregnancy doing laundry. Keep in mind, though, you'll probably have to make two purchases — one in your first or second trimester and another in the final weeks.