When cloning a bra make sure to remove the underwire first. You need to do this for a couple of reasons. One to ensure you can match the original wire against one of our wire charts. Two to truly get the shape of the frame. The frame may look very straight with the wires in but the true shape of the pattern should have a more “frowny” look to it. This Stella McCartney bra shows the difference quite well. On the left, the underwire has been removed. You can see that the left frowns and has a convex shape while on the right the band is straight except at the hook. The downward shape at the hook may have been caused by wear since many women pull the band down when hooking it up from behind. This shows the importance of springing the wire into the breast root and projecting the breast shape.
Now you can get away with cloning a bra without removing the underwire in case you want to still wear or return the bra. However, this does require more drafting since you will draft a new pattern using a basic bra block and measurements from the bra itself. Most home sewers clone their bra patterns by pin tracing or using press and seal wrap. When you do it that way you need to remove the underwire for best results otherwise you may find your wire won’t spring open correctly.
Here is the wire I extracted from that bra compared to two wire shapes that I stock. The demi and the shallow demi. The sample wire, however, is a much sturdier gauge about a 24 gauge vs the two 20 gauge wires shown for comparison. If I was to truly recreate the uplift of this bra I would need to have the factory make this wire style in that gauge. Because a lot of the forward projection of the breast in this particular bra comes from the rigidity of the wire pressing in at the side of the breast root. This gives the breast a round shape but maybe less forgiving in fit. It may not fit as a wide a range of customers as a bra with a more flexible wire. It may be less comfortable to wear. However, as this is a high-end fashion bra by Stella McCartney style takes precedence over comfort. Unfortunately.