To make an egg warmer belly band/haramaki you will need a cotton jersey knit. I find that the 90% and up cotton mixes are best. I was never partial to polyester on my belly while I was pregnant.
You want to cut the fabric to your measurements. Measure around your true waist (belly button) and your hips. Split the difference. So say your waist is 28″ and your hips are 32″, make the belly band somewhere between 29″ and 30″. The best width for me has been a finished 9″ in hieght. After all that info, cut your rectangle to be 18″ by your custom length. make sure that the fabric is stretchy on the length. SO that you can pull it over your belly as it grows.
fold the fabric in half lengthwise, so that the right sides are together. You are essentially sewing a tube. I use my serger for a nice finished edge. But you can use a regular sewing machine. Make sure you are using a jersey or ball point needle, otherwise you are gonna get frustrated!
Turn the tube right side out and sew the ends together to make a complete band. Turn right side out and you’re done!
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