With babies comes gear -- lots and lots of gear. Each new thing comes with a different set of heigh/weight restrictions for it safe use. Here's how I keep track of the different gear and when to stop using it.
(I'm an incessent Outlook task user - but I'm sure you can modify this for whatever tracking mechanism you use.)
First, you will need a good growth chart -- one that shows the trajectory for weight and height for different ages for different percentiles. I've been using the ones from the CDC. Most babies track ths trajectory amazingly well. So, once you know what percentile your baby is for weight and height, you can fairly accurately predict at what age they will be what weight and height.
I have a task called "Baby Gear". Every time I get a new piece of gear with a weight or height restriction, I look up in the growth chart for an approximation of what age she will outgrow it. Then, I make a new entry in the notes field of the task for this piece of gear, the height/weight restriction, and the age approximation based on the growth chart. I order the entries in the order of when she will outgrow it. The top item on this list is the next piece of gear she will outgrow. I set the "due date" on the task for 1-2 weeks before that item is set to expire. When the task comes due, I cycle the old item out (and cycle in the replacement item, if needed).
Here are some items I tracked in this list:
Car seat - changeover from infant seat to child seat, rear facing limit, etc.
Pack n' play - when to move from bassinet level to bottom level
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