Where are you in your pregnancy or how old is your baby? If you are in the last few weeks of your pregnancy or have a newborn you may need to drop an inch or so off of your above the bust chest measurement to compensate for expected weight loss.
Your height. This affects how long the carrier will be on your torso. Depending on torso length Very tall people can wear more than one size. Very short people may need to go smaller.
Your pre-pregnancy weight. This affects how you will bring the carrier up on your torso by filling out the carrier width wise. Also, do you carry most of your body weight up top or on the lower part of your body?
Do you have broad or narrow shoulders? If you have broad shoulders or a big bust or if you are dad with a big chest you might fill the carrier up a little more width wise. People with narrow shoulders may want to lean toward a smaller size.
Do you have a long or short torso? If you have a short torso you may need a smaller carrier. If you have a long torso you may have more options.
Fitted t-shirt size. Carrier size generally corresponds to your normal pre-pregnancy fitted t-shirt size in combination with the other factors mentioned above.
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