I received this question about commercial sewing patterns:
Supposedly the XXS size on commercial patterns (McCalls, Simplicity, Butterick) is supposed to be for babies 7 lbs or less, but they swallow newborns up. If you read around the Internet, there are lots of frustrated grandmothers and mothers-to-be that want to sew for their new or expected baby, but can't find a pattern that will fit. I read that doll clothing patterns don't work because the neck is wrong for a human infant. I tried buying vintage layette patterns from Etsy and they were just as bad. I want to know how to downsize a commercial XXS dress or romper pattern so it fits a NEW 7-8 lb baby. I am sick of gowns and knit sleepers. - PamIt has been a long time since I have sewn children's clothing from commercial sewing patterns. What I do remember is they are big. Too big. The proportions are bit off too. I don't know why either. Those companies have their own pattern blocks and I suspect they have not really updated or checked them in a long time.
Design school does not spend all that much time on children's clothing. I know there are few pattern making manuals that really get the sizing right, though sizing (exact measurements) is less of an issue than you might think. Regardless of where you start, you will have to make adjustments for fit. Only one manual even addresses infant clothing, and that book is Metric Pattern Cutting for Children's Wear and Babywear by Winifred Aldrich. I have a previous edition, but it covers most of what you might need. For Americans, the book is in metric. I didn't really have a problem drafting basic blocks using metric. You just need a metric ruler, though I did convert back and forth to see if I was on target. But even after drafting your basic blocks, you will need to make adjustments to the pattern until you get the fit you want. I have learned quite a bit by trial and error over many years.
For the home sewist, this is probably not all that practical. There is not really an easy way to size down a home sewing pattern that already has proportion issues (even Burda, which I prefer over the others). The biggest problem with some of the sewing patterns are tops are too wide, pant legs are too short just as examples. You could begin by folding out some of the width. But I can't say how much as it would probably depend on the style and desired size. In other words, no matter what, there will be a bit of back and forth. I guess my advice would be don't be afraid to experiment. Measure your baby and the sewing pattern and reduce width and length as needed. It is a bit of a challenge to measure a baby, but it really is the only way to arrive at your end goal. I wish there was some other way. Perhaps there are some indie sewing patterns for newborns? Please leave suggestions if you know of any.