Princess seams are nothing but manipulated darts. If you line up both pieces at the apex it visually creates the traditional bodice of shoulder- waist darts. Since most commercial patterns are mainly drafted with a “B” cup so many people need to make that adjustment. Based on the size of increments you can pick one of two quick methods.
First you must find the pattern apex and adjust to correspond to yours. You may need to shift the princess seam right or left on the front and side front pattern pieces to centre the seam over your apex position so that the curves of the seam can fit over your bust.
If your increase is fairly small like a ‘B to C cup’, you can simply extend the curve of the side front at the apex very slightly and ease the added length into the front near the apex as you sew the princess seams. This method works better on a soft and loosely woven fabric because you need to ease the extra fabric it in place.
For a larger than a “C” cup increase you must add enough length and width.
- Slash the front and side front pattern horizontally at the apex mark ( without going over the side seam).
- Slash the pattern vertically from the waist just to but not through the side neck.
- Spread the slashes on both patterns pieces in both directions, The guideline is 1/2 in per cup size increase. (in metric is aprox 1,27 cm).
- Redraw the cutting lines. Just reminder the guideline is just that and can be easily pinned out at the side seams during fitting.
If you are working with fabrics that are less “fitting” friendly you probably want to add a dart. Always start by correcting your apex. If the apex position is incorrect increasing your cup size won’t do you any favours.
- Slash and spread as mentioned but only cut thought the pattern horizontally at the side seam.
- Spread at the side seam, then draw a new underarm dart to within 1 inch (2.5 cm) of the bust apex on the princess seam to take up the added length of the side. The dart helps control the fullness of the side bust.
So many cute princess seams patterns out there. Do you have a favourite?