The apex — the most prominent part of the bust — on a sewing bodice pattern is designed to give shape to that fullness on your body. If the apex of the pattern doesn't match your apex, though, then the fullness of the fabric will not match your shape, creating an unflattering fit.
How to tell if the apex is a good fit
Make a muslin test garment . If your pattern includes an apex mark (typically denoted with a horizontal line or an X inside a circle). Transfer this marking to your muslin. Then, mark where your actual apex is. Note the difference between your apex and the one on the pattern.
If the bodice apex and your actual apex markings don't match up perfectly, you may need to make one of the following adjustments.
Lower the apex
On this bodice with both a horizontal bust dart and a waist dart, the pattern's apex is where the two darts intersect, marked in green. However, this is higher than desired and needs to be lowered. If left as is, the side seam bust dart would be too high and would release the fullness above the bust.
The solution here is to lower the horizontal bust dart so that the two darts intersect at the actual apex.
To start, draw a box around the side bust dart, using the center front line that's on the straight grain to draw the box, keeping it square with the grain.
Cut out the box, and shift it down so that the point of the dart aligns with the correct apex. Fill in the space with paper and true up the side seam — you've just lowered the bust dart without changing the armhole or bodice length
If your pattern also has a vertical waist dart like this one, you can lower it the same amount so that the point of that dart is also below the apex. Vertical waist darts that are too long don't look quite right — making them shorter and narrower will create a more pleasing look.
Raising the apex
Sometimes the apex on a sewing pattern is too low for the wearer. This can happen with a smaller bust, or if you're sewing a pattern for a teen who needs the height and circumference of a women's size pattern but doesn't have much fullness in the bust.
On the dressform above, the blue ribbon indicates the apex. The pattern's apex is about ¾" lower than the ribbon.
To raise the apex you can do the reverse of the process described above: Drawing a box around the horizontal dart and shift it up to align with the actual apex.
Another option is to shorted the pattern across the bodice front, which will move the apex upward: Draw a horizontal line perpendicular to the straight grain, which in this case is the center front fold line. Pleat out an equal amount at that straight horizontal line above the bust to shorten the pattern. This also removes the excess fabric in the chest area. Note if your garment has a sleeve you will need to remove a bit of the ease in the front of the sleeve for it to fit well in the armhole.
Raising or lowering the apex on a princess seam
A princess seam is a curved seam that contours over the bust. The roundest part of the curved seam should land over the bust apex.
The apex might be marked on a princess seam bodice. If it's not, you can place the side front and center front pattern pieces next to each other, aligning the notches, and note where the side piece is largest. On the pattern pieces shown below, the bust apex marked on the center front lines up with the largest part of the round edge of the side pattern piece.
To adjust, draw a box around the section of each pattern piece that includes the notches. (The notches on a princess seam pattern piece essential for accurately matching up the pieces.) Starting with the center front piece, draw a straight line parallel to the center front, and then add horizontal lines below and above the notches. I like to place my pattern piece side by side so that the boxes on both pieces line up.
Cut out the boxes. To lower the apex, slide the cut-out pieces down. Keep the horizontal lines of the box parallel to the cutting line, and measure carefully to make sure the boxes on both pattern pieces are lowered the same amount.
I placed colored paper behind the white pattern pieces to fill in the space. This way, I can easily tape the pattern pieces to that paper and keep everything aligned.
Next, true up the edges of your pattern pieces. The red lines above show the trued up cutting lines. Note that for the center front piece, you're taking away a bit of the original pattern, but that small amount of pattern is now added on the side front piece.
To raise the apex on a princess seam pattern, follow the same steps as above to draw a box around the notched sections. Then, cut out the boxes and shift them up. Again, measure how much you move them so that they are equal. True up the cut lines as shown by the pink paper filling in the spaces.
Raising and lowering the apex doesn't change the length of the pattern, so if the waist is in the right spot but the bust seems off, check the pattern apex — it might be just the adjustment you need.