Knitting needles question

I want to make a pair of baby booties. The pattern is in newborn size and I want to make it in 3month size and wonder how many needles up should I go? It says to make it on 3mm. I’m doing it on US 3 dpns.

I also have a pattern for a 9-12 month vest and want to bring it down to 3 month and wondering how many needles I should go down?

I think for the vest, you’d be better off reducing the number of stitches on the sides rather than trying to get it that much smaller through needle size for a 3-month old. It has 110 stitches around, with 5 stitches per inch, so about 22 inches.

Check out the measurements of a 3-month vest and subtract stitches according to the measurements of that one, by 5 stitches per inch.

As for the booties, I can’t imagine that newborn size is all that much smaller than 3-month size. I wouldn’t go up much more than one needle size with the same thickness yarn or they’d probably be too floppy. You could make the ankle bands longer.

You should be fine on the booties with the slightly larger needle and making them about a half inch longer - here’s a baby foot size chart.

For the vest, you could probably use the same yarn and needle size and the sts for the original pattern, but I think you could make a 6 mo size; most babies can wear that at about 3 mo old, which is when they’ve outgrown the 3m size. Here’s a chest size chart, you want it at least 2" bigger, 4" is better. If you want it in heavier yarn and larger needles, take out enough sts to get the chest measurement you need. The cable st pattern is 8 sts plus 3 between them, so you’d probably cut down by 11 or 22 sts to make it smaller.