Before I can move on to the grading question, I must make a couple of comments on my previous post. The best time to determine a sizing standard is at the very begining of your company. If you have been in business for 30 years, for example, it will be very difficult to change things around. So please, please do your homework.
Another thing, you can make your sizing standard anyway you want. Even though I am suggesting a simplified sizing standard, it may not be appropriate for you. If your intended customer is a big box retailer, it may be too simple. In fact, they may not like it. They are accustomed to choosing from a lot of different sizes. If you become part of a private label program, they will tell you what the sizing standard should be anyway. If you have something similar already set-up, there will be less stress for your patternmaker. Again, the sizing standard suggested in the previous post would be more appropriate for a company targeting specialty boutiques and small retailers.
Ok. Now the grading questiong...
"Can't we use the 3mo size for our base size and grade everything up from there?"
The motivation for this question is to save some work. In essence you make your pattern only one time and let the grading take care of all of the other sizes. While the idea is good, it presents some problems.
First, it is important to know how a child grows. A very good description and diagrams are shown in the book Patternmaking for Fashion Design by Armstrong. In my second edition of the book, it is on page 674. Up to about age 3 children are cylindrical in shape. There is little differentiation between chest, waist, and hips. At about age 4-5, girls start to develop a waist. At age 8, there is more definition and curves. Boys and girls are similar in shape up to about age 8.
Because there is a change in body shape at regular intervals, a patternmaker will break-up the sizes like this:
0-3M to 24M, 2T-4T, 4-6x, 7-16
By breaking up the patterns into these size ranges, a patternmaker would then make base size patterns in a 12M, 3T, 5, and an 8 (or whatever sizes are chosen). You could sample your initial style in any size you prefer - hopefully a middle size. When the design is approved and read to go into pre-production, the patternmaker will then make patterns for your style in each of the other base sizes. Those patterns are then graded for each range.
Some manufacturers will go to the trouble of sampling their style in each range. Retailers like to look at more than one size range because they have to "see" how to merchandise their floor areas. It is possible you will end up making a lot of sales rep's samples if you plan on selling the whole range. These extra samples are called sisters or brothers.
Even with our simplified sizing standard, you would still break up your sizes like this:
0-3M to 3, 4-6, and 7-16
Now some design entrepeneur's may still insist on having only one base size and grading everything off of that. What will happen is you will introduce errors into your patterns in the extreme ends of the range. The grading will be more complicated. The fit will be off in the transitional sizes between infant to toddler, young child to older child. For organization sake, your patternmaker/grader will greatly appreciate having things broken down into simpler groups.
Also, you will be bucking industry standards and not playing by the rules. Walk into any department store and look at how the children's clothes are arranged. You will notice that the clothes are arranged together in the size ranges we talked about above - infant, toddler, young child (4-6x), and older child (7-16). Your order form should have your styles broken down similarly.
By this point you are probably thinking that it is still a lot of work to make that many patterns for each base size. If you have a CAD system with nifty pattern drafting and grading capabilities, it isn't really that big of a deal. Even if you had to do it by hand, it should only take a day or so. Your patternmaker will be just repeating the same drafting process over and over again. If you make dozens of styles every season, you may seriously consider investing in a CAD system.