Baby sweaters are often knitted in pieces that have to be sewn together before the sweater is wearable. Just as when sewing clothing, seams that frequently need to be sewn are those beneath the sleeves, up the sides and across the shoulders. Mattress stitch is the hand-sewing technique most commonly used for seaming knitting, because it leaves a minimal line with virtually invisible stitches. Knitting seamed with mattress stitch will look as though the knitted stitches are continuous.
Sewing Vertical Seams
Block the pieces of your baby sweater to make seaming them easier. Fill a sink with cold water and add wool wash according to the directions on the bottle. Soak the pieces for 30 minutes, then rinse out the soap by running cold water through the knitting and gently squeezing it. Spread the pieces flat on a large towel laid over an ironing board. Use straight pins to pin the pieces through the towel and into the pad on the ironing board. Allow the pieces to dry.
Sew vertical seams, such as sleeve seams running from wrist to ankle, by first lining them up alongside each other and connecting them at the bottom. Cut a piece of yarn twice as long as the seam and thread it through the eye of a darning needle.
Hold the bottom edges of the piece together and sew the yarn through both of them from the front. Pull the yarn through until most of it is coming out from beneath the left side of the sweater, leaving a 3-inch tail coming out from under the right side. Tie the two pieces together in a knot.
Locate the first full vertical row of stitches after the side edge, counting in horizontally. In stockinette stitch, this will look like a row of Vs. Sew underneath and behind the first bar of the first V in that row on the right side. The yarn will come out on the back side of the sweater.
Sew underneath and behind the first bar of the first V in the corresponding vertical row on the left-hand side. Go back to the right-hand side, with the yarn coming from underneath, and sew underneath and behind the first bar of the third stitch up from the bottom. You are picking up the bars between the last and the edge stitch on the right and between between the edge and the first stitch on the left.
Sewing Horizontal Seams
Line up the edges of the horizontal seams on blocked pieces of knitting. Tie a knot at the right edge of the seams to join the pieces, just as you did for the vertical seams.
Slide your needle through both legs of the first V in the second horizontal row from the seam on the piece of knitting farthest from you. For example, if you were sewing a neck-to-shoulder seam, you would tie the pieces together at the neck edge and pick up the first stitch on the back panel of the sweater.
Pick up both stitches of the first V in the corresponding row on the piece of knitting closest to you. This would be the front panel of the sweater.
Pick up both stitches of the second V of the back row, then both stitches of the second V of the front row. Continue in this manner until you reach the end of the seam. Weave in the tails of the yarn with the darning needle or use them to sew another seam, depending on your pattern directions.
Use wool wash containing lanolin to make animal fibers feel softer against the baby's skin. Use the same yarn you used to knit the sweater to sew the seams. When blocking, set up your ironing board next to a fan or in a sunny spot to help the pieces dry faster. It can take up to two days for knitting to dry, depending on the fiber and the climate. Wash cotton and acrylic in the washing machine if desired. Place the pieces in a lingerie bag and wash them in baby detergent on the gentle cycle using warm or cold water. You can block cotton in the same way as wool. Acrylic pieces can go through the gentle cycle of the dryer and do not require blocking. If you lose your place seaming vertically, remember that you are always working one stitch in from the edge of the knitting.