We recommend making two pattern adjustments before constructing your first muslin. The first is the vertical length from your shoulder seam to the apex of your bust. This article walks through the adjustments needed to lengthen the pattern in this area.
Let’s take some measurements
In the photo of pattern piece A-Top Front, notice that the seam lines are marked with dashed lines on the pattern. I have placed a tape to measure straight up from the bust apex mark to the shoulder seam. Be sure to stop measuring at the seam line, as you’re looking for the length of the finished shirt from shoulder to bust point. The length here is 9-7/8 inches.
This is my dressform, standing in for you in this example. Measuring the same location on the model as I did on the pattern, and letting the tape rest against the body, I get a length of 10-3/8 inches from the bust point straight up to where I want the shoulder seam to fall. Since the difference between these two measurements is greater than 1/4 inch, I want to lengthen the pattern so the bust apex mark falls at the same level as the bust point on the model’s body.
The measurements we have for the example are:
1/2″ difference, with the body being longer than the pattern
Remember, if the difference between the measurements is 1/4 inch or less, you don’t really need to make this adjustment.
With our measurements in hand, we’re ready to adjust the pattern
Squaring up to the center front, draw a box closely around the horizontal bust dart. Cut out the box and set it aside.
Squaring up to the center front, draw a horizontal line through the bust apex mark, then cut along that line.
Tape the bottom half of the pattern to a piece of paper large enough to fill in the amount we need to lengthen the pattern, which in my example is 1/2 inch.
From the top edge where the pattern piece is taped to the paper, measure up the amount the length needs to be adjusted (1/2 inch in example). Draw a horizontal line at that point, squaring up to the center front. Tape the top half of the pattern to the paper along this line, making sure the center front of both halves align.
The lower half of the original bust apex mark is the new bust apex location, so highlight or circle that part for clarity.
Now we need to move the bust dart down so it’s pointing to the new bust apex location. Tape a piece of paper behind the top of the opening where the horizontal bust dart was cut out. From the top edge of the cut-out, measure down the same amount you lengthened the pattern (1/2 inch in this example), and draw a horizontal line at that point, squaring up to the center front. Tape the bust dart rectangle along this line, making sure the vertical cut edge stays aligned with where it was cut out of the pattern.
Close the horizontal bust dart, matching dart legs. Fold the dart down as you would with fabric, and check both sides to see whether paper needs to be added to reach the cutting line. Blend the cutting lines, and trim away the excess paper. Your shoulder to bust apex adjustment is done, and now you just need to lengthen the Top Back pattern piece by the same amount you lengthened the front.